Fraser Cain: This is a huge week in astronomy. I would say one of the biggest weeks we’ve had in a year, let’s say n years … it’s an enormous piece of news: the discovery of an Earth-sized planet orbiting another star inside the habitable zone. So it’s possible there’s liquid water on the planet. So, is that it? Are we really close to finding out another world out there with life on it? Pamela? Let’s start with talking about what was discovered this week.

Dr. Pamela Gay: There’s this little baby star called Gliese 581, it’s one of the hundred closest stars to the Sun, it’s about 20.5 light years away in the direction of the constellation Libra. It’s just a little red dwarf, about 30% the Sun’s mass and only about 1.3% the Sun’s luminosity. We’re talking little, tiny, faint, baby, cute star. One of the neat things about these little stars is because they’re so low mass, we can detect the gravitational effects of smaller planets tugging around these smaller stars. Scientists have started looking around red dwarfs to see if they can find Earth-like worlds. In this case, they got exactly what they were looking for.
 
Using a really neat set of instruments down in Chile, where it seems like nowadays all the really cool stuff is going to live, they were using HARPS, the High Accuracy Radial-velocity Planetary Searcher, and they found this planet, and in fact 11 of the 13 planets less than 20 Earth-masses have all been found by this instrument. Not only did they just find one Earth-like planet, they’ve actually found an entire solar system of worlds orbiting this other star. There are three different planets (at three different distances, obviously, from their Sun).
 
The first one they found was about 15 Earth-masses. It’s a Neptune-like planet that orbits it’s star every 5.4 days. Then, the next one they found is about 5 Earth-masses. It orbits the star every 13 or so days, and then they also think there’s a third planet that’s about 7.7 Earth-masses that is located out at a distance that allows it to orbit every 84 days.

Fraser: What was the methodology that they used to find it? Back in one of our earliest shows we talked about the search for extrasolar planets. What does this tool do?

Pamela: They were looking at the radial-velocity changes in the stars’ stellar spectra that were created by the gravitational pull of these different planets on the central star.

Fraser: So if I recall correctly, the radial velocity is the speed that the star is moving toward and away from us because it’s being yanked about with the gravity of it’s planet.

Pamela: Exactly.

Fraser: And this is a very accurate measurement – they’re measuring the Doppler shift of the light, right?

Pamela: This instrument is extraordinarily precise. It can detect velocity changes that are slower than some people walk. It detects velocity changes as low as one metre per second. So we can walk around at the same rate that these planets are pulling on their central star, and that’s kind of neat to think about.
 
So we have these three little tiny worlds, orbiting this little tiny star. When scientists sit down to model and ask “what do we think these planets might look like?” they do calculations and say “a planet this tiny can hold on to these elements, a planet this close to the star should be able to support these sorts of things.” They’re thinking the smallest of the three of these planets might be either a solid rocky world or perhaps it’s something that’s completely covered in oceans.
 
Now, we don’t currently have the ability to look and see if it’s a “water world” but it’s neat to think about and hopefully this sort of discovery will motivate NASA and the European Space Agency to invest the necessary dollars to build the missions that can look at this little tiny planet and say, “yes, we finally found a water world just waiting to have a bad movie filmed on it.”

Fraser: What do we know about this world itself then?

Pamela: Well, we know it’s there, we know it’s small

Fraser: How small? What’s it’s size compared to the Earth?

Pamela: Well, according to the science paper I’m looking at, they put a limit on it of 5.6 Earth-masses, and we can’t directly determine it’s radius or anything like that, but based on models of what this planet should look like, we’re guess-timating/estimating that it probably has a radius about one and a half times the size of the Earth’s radius, which makes this a really dense planet. When you’re standing on it’s surface, you’re going to want to sit down. Any life that could’ve developed on a world like this will have developed with a much stronger skeletal structure to support them underneath the weight of the heavier gravity.

Fraser: How long does it take to go around the Sun?

Pamela: It’s just sort of zipping around it’s star every 13 days and it’s really close in. It’s only about 1/14th of an AU away from that star. Now, it sounds like it’s zipping around at high velocities, and really close in, so it’s got to be a really hot planet.

Fraser: This would be in that Hot Jupiter category, right?

Pamela: But because this such a faint star, because this is a star that is 1.3% the Sun’s luminosity, this close a position still allows the planet to have relatively cool temperatures. The estimates range from an average temperature anywhere between 0 and 40 degrees Celsius. Potentially the planet’s environment is no different than where I am near St. Louis or where you are near Vancouver.

Fraser: So, if there’s water, based on how far away that planet is from it’s star, it would be liquid water.

Pamela: It would be liquid water. There’s the potential for seasons, there’s a lot of things waiting to be discovered. Now, that doesn’t mean we’ll find them. Our hopes could be completely dashed and this could be just another Mercury, another Venus, another dead world where life can’t exist. But at least now we have some place to dream about, and some place that we know we need to look at as soon as the technology exists to actually look and actually measure which of our wishful dreams are actually real and which chemistry has decided to deny us.

Fraser: If we’ve already gotten to the point that we can detect a planet around this other star, why can’t we just use more sensitive equipment and just start uncovering some of those additional things? It should just be like, that’s the place to look now.

Pamela: Well, it would be if we had the right cameras. The problem is we need something that’s capable of looking at immensely small angular resolutions. It’s sort of like if I look at the Moon with my naked eye from here on Earth, looking through our cruddy atmosphere, I can make out some small craters but I can’t look at grains of sand on the Moon. Now, we’re looking at wanting to make out oceans on a planet 20.5 light years away.
 
In order to look for those oceans, we need to have the ability to zoom in on little tiny features and block out the light of the star that this planet is orbiting. We know how to do it: you take a coronagraph, the same sort of thing we use to look at the outer atmosphere of the Sun. So you block out the main part of the light from the star, and then you detect the very faint light of the planet and you have to resolve it. The way you can do that is you take a set of telescopes and you put them in orbit around the Earth. The angular resolution that you’re able to achieve is directly related to the colour of the light you’re looking at and the diameter between the furthest two points on the telescopes.
 
If you have just one telescope, or just one eyeball, and you want to know what it’s resolution is, you measure the diameter of the opening the light can get in, the diameter of the mirror. Now, if you have two telescopes, you start at the furthest left side of one telescope and you measure all the way over to the furthest right edge of the other telescope and you use that diameter. As you move the telescopes further and further apart, you’re able to achieve higher and higher angular resolution; smaller and smaller objects can now be clearly seen.
 
To do this, we have to launch them into space, otherwise our atmosphere is going to take away all the advantages we’ve gained by moving our telescopes apart. So you have to spend the money to launch basically three Hubble Space Telescopes into orbit. They’re actually looking to put multiple systems that are multi-metre telescopes in orbit, working together to look at these planets.
 
It costs a lot of money, and NASA originally was planning to spend the money on something called the Terrestrial Planet Finder, but in the 2006 budget, the project was put an indefinite hold. We had the plans, we had the desire, at one point we had the budget and now it’s all been sort of set aside for other priorities: things like putting men on the Moon and men on Mars have been given budgetary line items that had to come out of something, and the money to do those things came out of our desire to explore other worlds (or at least NASA’s ability to use congressional funds to look for other worlds).

Fraser: So the problem that we have right now, for example with the – is it the HARPS instrument?

Pamela: Yeah

Fraser: –the HARPS instrument on the ESO’s telescope, is that it’s only looking at the velocities; its only able to measure that velocity back and forth. It can’t see any aspect of the planet. It only tells us, what, the orbital period and the mass?

Pamela: We’ll use the orbital period to calculate the mass. So basically we’re getting one piece of information. From that one piece of information we can do a lot of calculations and then we can look at a lot of models. At the end of the day, the only things we know with certainty are the period. Everything else, well, we’re estimating the mass, we’re estimating the orbital size because we don’t know the exact inclination, we don’t know if we’re looking at the orbit as though we were looking at a plate edge-on or at a plate held at 20 degrees relative to us.

Fraser: Have there been any examples where telescopes have been able to see atmospheres? If we pointed Hubble at Jupiter, or a mega-Jupiter, would we be able to see the atmosphere or is it still the same problem: even with those big planets, they’re whipping back and forth and that’s how we’re able to detect them.

Pamela: Here we’ve actually been able to get lucky. We can use spectroscopy again, we can again look very carefully at the constituent lines in the rainbow of light from the planet and the star. This was first done way back in November of 2001, where the Hubble Space Telescope looked at a planet orbiting a star with the wonderful name HD 209458.
 
This sun-like star, yellow, 7th magnitude (you can see it in any amateur telescope), is about 150 light years away toward the constellation Pegasus. Its planet happens to move straight across the disk so when we look at the star we can see the planet truck across the surface of the star and we don’t see the planet directly. What we actually see is the light from the star decreases, and we also see when the little planet tucks itself behind the star, we also see it decrease.
 
When we take multiple images, images with the planet behind the star, and images with the planet in front of the star, we can look for differences in what is visible. Those differences are going to be things that are coming strictly from the planet.
 
We’re able to actually make measurements of what’s in the atmosphere. So, for instance they look for things like sodium, water and we’re finding this things. In fact, back in January you reported on finding water but now we’re looking at it with a higher resolution.

Fraser: Right, but the technique here is this transit method, right? Where the planet moves across the front of the star and you get this additive effect for the duration that the light from the star dims, the chemical composition that we can detect has changed.

Pamela: Yeah

Fraser: And it’s almost like it’s the addition of the star with what’s in the planet; it’s almost like you subtracted and you get what’s in the planet and in this case Hubble saw water, right?

Pamela: So far, the only way we can find these atmospheres is when we just happen to be looking at a system that has the exact right alignment and the planet is big enough that we can see the eclipse of the planet going in front of the star and know when the planet is behind the star and then we can look for the differences in the two things. We’re finding all sorts of neat stuff. There’s another star, HD 189733b – that’s the planet’s name, without the ‘b’ is the star’s name. Looking at it, we’ve been able to figure out that there are probably silicates in this star’s atmosphere. There’s this dusty cloud layer surrounding the planets. So everyday we’re learning new and exciting things, but currently we’re only learning them about the big Jupiter planets.

Fraser: Would we be able to use (and by ‘we’ I mean ‘they’) be able to use those techniques to look at this new Earth-sized planet? Wouldn’t it be able to watch it move across the front of it’s parent star and be able to measure the constituents of its atmosphere?

Pamela: So here I have to admit that the discovery is so new that I haven’t really run the numbers on what’s possible in terms of using the large spectrographs that exist here on Earth.

Fraser: I guess it depends on if it’s moving across the face of the star or not.

Pamela: It also matters how much light we can get into our spectrographs. In order to look for the little tiny lines that come from the atmosphere of a planet, you have to spread the light out huge, huge amounts, in some cases you’re spreading things out such that if you made a single rainbow, that single rainbow would be metres in length. So you’re taking all the light from this really faint star, spreading it out over metres and metres, and then trying to make measurements.
 
We can do this with bright stars. With bright stars, it’s no big deal: you take their light, you turn it into a rainbow, it’s a bright rainbow – you can even see it with your eyes if you’re in a dark enough room. Here, we’re looking at such a faint star that I’m not sure that we have any telescopes big enough that it can gather the light from the star, spread it out, and still have enough light to be able to make a detection. I don’t know: I’m hoping, hoping, hoping that we’ll be able to do it, but I don’t know for certain.

Fraser: Let’s go back and talk about what the future holds for this. This planet isn’t – we say it’s Earth-sized, but you say it’s what, five, six times the mass of the Earth –

Pamela: And only one and a half times the radius, so you’re definitely looking at a high gravity world.

Fraser: Right. So will this technique be able to find an actual Earth-sized planet?

Pamela: If we look around the smallest stars. That’s the neat thing: the smallest stars are actually the largest in number. If you look at the hundred nearest stars, 80ish of them are these little tiny red dwarfs. So we have lots of things to look at and a little planet can easily yank around a little star, so we’re going to keep finding more and more of these.
 
It’s a new day in planet discovery, and it’s a new day filled with the Earth-like planets and the potential exists to find a planet the size and density of Earth close in to one of these little stars, inside of it’s habitable zone, that just might be waiting to have life on it.

Fraser: So let’s talk about – how would you know if you see life on a world?

Pamela: The critters on our planet do neat things to the atmosphere. For instance, we take and trees output oxygen. People output all sorts of pollutants that aren’t normally found in nature. By looking for these special molecular and chemical signatures that we’re going to know there’s something there taking the raw atoms, the raw molecules that come out of solar system formation, and are transforming them into things that can only come from organic life.

Fraser: So there are no natural processes that could fill our air with oxygen?

Pamela: The oxygen could be there, but the ratios are going to be different. The carbon dioxide is getting broken up. It’s these changes in the form that you end up finding the oxygen in that are signatures of trees, signatures of algae. All these specific things that you just tweak the ratios as soon as you add single-celled bacteria into an atmosphere, into a terrestrial ocean, an entire ecosystem is modified by algae bacteria. Human beings, we just make a disaster of our environment and that’s really easy to see from space.

Fraser: I guess people always say, “maybe we’ll see light not as we know it, there might be some other chemical process that functions on those world that we don’t understand, like using silicon as a base as opposed to carbon” but I guess in those situations maybe we’ll find those as well, but I think that if we can see the free-floating oxygen in the atmosphere, at the levels that we would have here on Earth, there’s almost nothing else you can assume, then: there’s life.

Pamela: Right. Peter Ward wrote an excellent book called Life As We Do Not Know It. It’s all about the ways that life changes it’s atmosphere, and ways that we might look for life. It’s a good read, it’s a hard read, but the science is really juicy and it addresses all the questions about life that a lot of us just have rattling around in the back of our heads all of the time.

Fraser: So, we went into this a bit before but, right now there’s no observatory on Earth or in space that we could point at Gliese 581 and see if it has water or see if it’s atmosphere has oxygen. So what plans are in the works, then, to try and put some observatories up there?

Pamela: Well, the European Space Agency has a program called Darwin. They’re hoping to launch in about 2015. It’s going to consist of three, three metre telescopes – that’s bigger than Hubble for each of these three telescopes – as well as a fourth telescope that works as a communications hub. These are going to get put out at a point that is beyond the Earth, so you can go Sun, Earth, this point. It’s called L2, one of the Lagrange points. It will keep this family of satellites always with the Earth behind it and the Sun behind it and the Moon behind it, so they can look out and study the sky constantly, unhindered and unblocked. These satellites have the potential to start finding rocky worlds and being able to image them. These are what we need. Hopefully, Terrestrial Planet Finder will someday, under perhaps a different Congress, get resurrected.

Fraser: Terrestrial Planet Finder is the NASA mission that would sort of kick Darwin up to the next level, right?

Pamela: Exactly. It’s the little shiny star that will find planets and allow us to study them directly. Hopefully it will come back, hopefully the NASA budget will become friendlier to science someday in the future.

Fraser: It seems amazing to me. I think this discovery of this world is ahead of its time. I don’t think that astronomers were expecting that they would turn up an Earth-sized world in the habitable zone, going around another star, for a decade or more from now.
 
I think this came as quite a surprise and I think that, with the timelines for Darwin and the Terrestrial Planet Finder, and some of these others, they were sort of more synced up so they’d come online to help pitch in with the science. Now, with this discovery (and I’m almost ready to write the next stories on Universe Today), I’m sure within months, within years, it’s just going to be one after the other: smallest rocky world discovered so far. There’s going to be a lot of demand, a lot of pressure to get those visible observing missions going so we can start to confirm.
 
All it takes is one: you just get the Terrestrial Planet Finder, get it in space, turn it and you look at Gliese 581 or any that are about to be discovered and you’ll know whether there’s life in the Universe outside our solar system or not. What more fundamental question could we answer?

Pamela: I’m right there with you. Unfortunately NASA has been tasked by forces outside of them, to put people on other worlds we’ve already visited in our own solar system. I’m cool with that.

Fraser: Don’t make me choose. Don’t make me choose between space exploration and astronomy.

Pamela: But the commercial sector can pay for the people. Tourism will produce revenue. I’m all for exploring our solar system with human beings, I just want to see the commercial space flight do it. This is the type of place that can be self-funding. I don’t see the Terrestrial Planet Finder being self-funding. I think it needs NASA, it needs educational centres and universities behind it.

Fraser: I just think it’s crazy that the two sides, science and exploration are forced to compete with one another. I find that kind of infuriating because it’s almost like it’s unfair and it sets those two different purposes at odds with each other. I personally think the science side should fall under the general (in the US, anyway) under the general science and the National Science Foundation and let NASA’s exploration be completely separate. They shouldn’t be battling over budgets, it drives me—I don’t want to be forced to choose, and I think it’s unfair.

Pamela: It’s complicated where NASA controls all of our launch vehicles, so right now we live in times where a lot of stuff doesn’t make sense, there’s war going on, there’s budget constraints, but now we’ve found a little planet that could potentially hold life and maybe we have something to dream about.

Fraser: I think, within the next couple of months as well, I wouldn’t be surprised when people start to hear that a mission like the Terrestrial Planet Finder, which was ready to confirm these discoveries, has been shelved, I think there’s going to be a lot of demand and a lot of pressure to try and get that mission specifically back online. So if anyone knows of any signature site to sign or any letters we can write, let us know and we’d be happy to pitch in because let’s get the Terrestrial Planet Finder back and rolling.

Pamela: And for now, let’s be thankful that the European Space Agency is out there shining the way, they have COROT up there looking for planetary transits and Darwin is scheduled for launch in 2015. There is still a plan to be able to see these things directly.

Fraser: All right. Well, I hope that gives everyone some information about this discovery and we’ll stay on top of it as new stuff unfolds and try to explain that as well.

Fraser Cain: All right, so last week we talked about the Drake equation, which is an attempt by Frank Drake to nail down the variables that help potentially calculate the number of intelligent societies in our galaxy.
 
There’s a pretty strong counter-argument: if there are intelligent societies in the Universe, where are they? How come we’re not part of some galactic federation? Where’s my warp drive? Pamela?
 

[laughter]

Dr. Pamela Gay: Well, this is –

Fraser: Where are they?

Pamela: This is actually a fairly old question. It was first brought up by physicist Enrico Fermi back in 1950 over lunch where he basically sat, stopped and went “wait, where are they?” It seems that if we live in a Universe that’s populated (or just, for that matter, live in a galaxy – our own Milky Way galaxy) that’s populated by other intelligent societies, civilizations with advanced technologies, they should have (if they’re old enough) been given the time to spread out and conquer all the habitable worlds and just, sort of like Rome spread out across Europe, it seems that those alien civilizations should’ve spilled out across our little barbarian planet and incorporated us into some great federation… and they haven’t.

Fraser: It’s not a question that we ask here on Earth. You might say “where are all the other human beings?” well, they’re everywhere. Wherever you look, if you go outside, anywhere you look there’s other humans because as human beings we’ve gone and really explored the Earth to every last corner. Why hasn’t that happened in the Universe?

Pamela: This is a question that we’ve been trying to come up with good rationales for, and in this case by “we” I actually don’t mean just the scientists, but here it actually starts to call in on the philosophers, on the animal behaviourists and all of the people who work on the psychology behind exploring the entire Universe.
 
We Earthlings like to go out and pretend that we’re a virus and dominate every bit of land that can possibly support us, and even a few pieces of land that can’t. But what’s going on outside of our Earth?

Fraser: I think that we’ve got an interest in doing so; if we could we would colonize the whole galaxy. If we have the technology to do so – I don’t think there’s any question that we would have necessarily a will to do it. I think for most people watching Star Trek and Star Wars, it seems like a completely natural solution. Eventually we’ll get the technology, and eventually we’ll hit up the stars, and eventually this galaxy (and eventually all galaxies) will be filled with human beings who are our ancestors or, well, the opposite of our ancestors.

[laughter]

Pamela: And in that Star Trek Universe, as soon as we step foot off of our planet, we run into the Vulcans. Where are those real life Vulcans? We know there’s a planet around Eta Erandi, which in the Star Trek universe is where Vulcan’s located. But have they come out and explored? This raises the question that we approached last week of, are there other societies out there – is it just that we live in such an inhospitable galaxy that life is extraordinarily rare. Could it be that life is just starting to emerge throughout the galaxy? Could it be that other societies have already had a chance to kill themselves off? All these questions are things that we have to think about.

Fraser: Well I’m hoping we can teach a little astronomy ehre, so let’s sort of start at the beginning. What are some possible reasons why there might not be any aliens out there?

Pamela: The first argument, scientifically, is that maybe life is just starting to emerge. We live on a planet around a fairly metal-rich star. There’s lots of other planets out there that are metal-rich, but when exactly did planets start forming? It takes a certain amount of time, once a planet is formed, for life to be able to evolve to any sort of advanced state. We’re not quite sure what sort of interplay you need between extinction events and lack of extinction events to get from a whole lot of bacteria to plants to dinosaurs to dead dinosaurs and mammals to big mammals to smaller humans taking over the planet one phase at a time.

Fraser: So we could treat this like a race, where the whole universe has been running the same race the same amount of time and we happen to have won the race so far.

Pamela: That’s one possible argument. One of the problems with this is one of the basic tenets that we have is that we don’t live in a special time or special place or special anything else. This is the Principle of Mediocrity.
 
To get past the mediocrity principle, we have to say “well, maybe its not that we’re so much at a special place as we just got lucky.” But lucky isn’t mediocre, so we’re sort of in this catch-22, of perhaps because of a different principle, the Anthropic principle (that says because there’s life, there’s life), we get around the mediocrity principle through an escape door. It’s sort of this weird circular reasoning of “because there’s life, there must be life, but we’re not supposed to be in a special place but because there’s life maybe we are in a special place.’ This is where the philosophers get involved.

Fraser: Yeah, let them handle that.

[laughter]
 
Okay, so I guess the point being that we’re here, therefore there’s life, yet where’s all the rest of it? What are some reasons why? Maybe life can’t get a foothold or intelligent life can’t rise?

Pamela: Intelligent life is something that’s hard to get to. It requires, first of all, life to start. That, I personally don’t think is too difficult a thing, at least at the single-cell, nano-bacteria, single-bacteria sort of level. But to get to intelligent life, you have to have all the right conditions, you have to have evolution taking place, you have to have evolution that doesn’t lead to just Bass. The world would be awfully sad if the most advanced thing on the planet was a Bass (that’s a type of fish for people who are listening in a foreign language).
 
You have to have the right chain of events to get to intelligence. That intelligent life has to also be capable of the technology. So there’s always the paradox. What if life evolved on a planet with a completely cloudy atmosphere and they never saw beyond their cloudy atmosphere to imagine a Universe beyond? What if life evolved under the ocean so you had a completely watery existence? Say you were trapped in the lower parts of the ocean, where life clung to the vents at the bottom of the ocean (the thermal heat vents like we have in our own oceans), but never reached the surface and never saw the sun. What would cause that sort of intelligent life to want to even try to go to outer space that it couldn’t even see?
 
You have this problem where you have to not only get intelligent life, but you have to get intelligent life that’s interested in exploring the cosmos. You can always have an intelligent society that, for philosophical or religious reasons decides they’re just going to stay at home, or they’re not going to let our electronic technology (or whatever surrogate they have for our modern internet age) take over. What if they decide that it is better to have a simpler life, without their moral equivalent of the cell phone and laptop computer? All these things can prevent whatever life might be out there from stepping out and spreading across the Universe.

Fraser: I guess that a more negative possibility is that intelligent just wipes itself out. We stand now, with the capacity to strike a pretty big blow to our whole civilization with nuclear weapons, with some kind of ecological catastrophe. Who knows what nano-technology is going to do? I think that’s a possibility as well – you could get life that just gets to the point of high technology and then wipes itself off the Earth or off the planet and then the bacteria have to start again.

Pamela: That’s exactly right. Imagine the worst case scenario of every paranoid person yelling on a street corner’s worst nightmare of bird flu takes over, becomes airborne, global warming proves true, we lose large segments of our population to viruses and then lose all of the fisheries, the ocean levels rise and we’re basically reduced to the point of society collapsing as everyone moves inward from the oceans.
 
If society truly collapsed on our planet, it would be very difficult for it to rebuild. All the easily accessible fuel reserves have been used up. The coal and oil we have left is hard to get to, the mineral deposits that we have left are hard to get to, the biggest cities would end up underwater. It would be very hard for society on Earth to start from square one and build itself up to where it is today simply because if its easy to get to, we’ve already used it up completely.

Fraser: It’s hard to go from campfires to off-shore oil platforms –

Pamela: Exactly

Fraser: –to get at the hard-to-extract oil. For sure, I think that’s the one that, I think makes me pretty nervous. That there’s some technological or ecological disaster that maybe is almost inevitable for life – you get to a certain point, you master the energy, the forces of nature, but you make a mistake and then that’s that.
 
But, even so I think someone out there had to have figured out a way around that – had to have avoided that, had to have prevented something long enough to hit space. Once you hit space, I think it eases up your chances again, right? Then you’ve got hundreds of worlds where you can have an ecological disaster on one and it doesn’t really matter.
 

Pamela: Exactly. So here you are, you and your family have piled into your spacecraft and you’ve travelled for 20, 30 years to get to the nearest habitable world at a somewhat-sane (but still far beyond our current capability) speed. After travelling for for all of this time, you find yourself a nice rock to settle down on and you start agriculture, you start building cities, you start mining… is the very first thing you’re going to do, to set up a massive industrial complex to start building new spacecraft so that as soon as your grandchildren are born you can pile them into a new spaceship to go take over the next planet that’s 30 years away?
 
It seems to me that there’d be a certain delay time naturally built in to all but the most courageous explorers where, when you find a new planet you’d want to settle down and build your cities and once you’ve fully established yourself then start sinking the resources into sending resources to other worlds. Sending a colony ship off isn’t an easy thing; you have to stock it, you have to put your best minds onto it and a very small colony isn’t going to have a lot of resources that it can share with future generations on a planet it may never communicate with.

Fraser: I guess one of the other issues is that we discussed with the Drake Equation last week, we talked about how depending on how far apart the civilizations are, it might just be too difficult for them to even communicate with one another. They could be around us, but as you said its too slow, too difficult to get from world to world. Maybe you are just stuck on the world you live on, but then its so far away there’s no really convenient way to communicate with any other world. There could be hundreds or thousands of intelligent civilizations in our galaxy; too hard to move, too hard to communicate.

Pamela: Some of those are just going to settle down and decide that they’re tired of colonizing and maybe we’ll get lucky and they’ll get tired of colonizing before they reach us (Or unlucky, depending on your point of view, I guess).

Fraser: What are some other reasons that could be used to explain? Let’s assume that on the one hand we talked about the fact that maybe there isn’t any life at all, or that the life is getting smashed or killing itself or is unwilling or unable to communicate. Let’s say that it is spreading around the galaxy. What could be some reasons why we’re not being talked to them?

Pamela: It’s basically the energy and endurance necessary to make it. Say that the other societies happened to have started off 180 degrees around the Milky Way from where we are. It takes time and energy to make it, and they might’ve decided “we’re going to very thoroughly settle our quadrant of the galaxy first.” The energy needed to get all the way over to where we are is simply something they haven’t expended yet.
 
We’ve had the technology to be able to go to Mars (if we pushed ourselves) for a long time. But we haven’t. We’ve had the technology to send people to the Moon for longer than I’ve been alive and we haven’t done it in my lifetime.
 
It takes energy and it takes national resources, planetary resources to explore space.

Fraser: That’s one of those assumptions, that it’s way more expensive to colonize space than we think. We already believe it to be tremendously expensive in terms of resources, in terms of what it would actually take to undertake such a journey, so it might be that it is in fact far less feasible than we can even imagine. It might be that it could take ten times the technology that we’re expecting to actually be able to make that journey.

Pamela: Exactly. We’re still trying to figure out how to get a human being to Mars and perhaps it’s a non-linear thing, and I suspect it is a non-linear thing. The further you try and go it gets not just twice as hard when you double the distance, but 20 times harder when you double the distance, or 1 thousand times harder when you try and get to the next star. With each one of these new leaps, you have to develop entire new strategies, not just for the technology but trying to sell it to your society. “Okay, everyone needs to tighten their belts and conserve their metal because we’re trying to send this thing off to a planet that it will be your children who hear if this machine’s successful.” That’s a hard thing to sell to a society.

Fraser: We’ve been looking for life so far using the SETI – Search for Extra-Terrestrial Intelligence project. Shouldn’t that have turned up something by now?

Pamela: One of the things that SETI pre-supposes is other societies are going to be leaking radio signals out their atmosphere much like we do. Well, what if people first developed communication using light instead, or what if they confine themselves to wired communication or to fibre-optic communication? What if their communications works in wavelengths that don’t penetrate through their atmosphere?
 
All these different things can prevent societies from leaking their radio waves into outer space such that we’d see them. You’d have to leak an awful lot of radio waves before you’re detectable. So if you have a mostly shielded society or – I keep returning to my underwater society because I don’t see why a society couldn’t emerge underwater – they just may not have the same technologies that we have that lead to the leaking of radio waves. It’s an awful big Universe. We struggle to see to the other side of the Milky Way. There’s always that problem that life could be too far away from us to easily detect.

Fraser: Or I suppose they’re using some kind of communication channel that it hasn’t even occurred to us yet. For us it seems quite natural – let’s communicate with radio waves, let’s communicate with light beams. Maybe there’s something else. It really makes a lot of sense to communicate with either some other exotic – like, let’s communicate with dark matter.

[laughter]

Pamela: We don’t even know the forces that we could use.

[more laughter]

Fraser: Right! We don’t even know what it is, and yet maybe in 20 years it will be just like “duh, we should’ve been communicating with dark matter all along – it’s great for communication.” So once we hook up the dark matter telescopes and receivers then it all makes a lot of sense.
 
That’s of course, insane for me to say that.

Pamela: Yes.

Fraser: The point being that something we don’t quite understand, using it as some form of communication – or let’s communicate with gravity.

Pamela: Or they’ve figured out how to control what neutrino masses stay and they can send neutrinos in packets where they look at what the three different species are. I don’t know, there’s all sorts of crazy things that we can speculate on figuring out how to control that currently we don’t even understand how it works. All these things – I’ve heard people talk about potentially creating solid state memory for computers by controlling the spins of atoms and then reading off what spin different things have. That would allow amazing amounts of storage, but we don’t have the technology to do that in a systematic (let alone a cheap) way right now.
 
There’s so many things that we’re still learning how to do. We’re just a baby society in technology.

Fraser: The other possibility is that the energy involved to actually communicate (from what we understand) is immense. For you to broadcast out at a level that people can receive within a reasonable range is enormous – more than we can muster. It might be that civilizations never want to be able to commit that much energy to transmitting signals just for the chance of “is anybody out there?”

Pamela: And there’s always the timescale problem of, “what if they communicate at such high frequencies that they can burst their entire codex of everything they know and have ever known – their entire history – in less than a second? What if they’re extremely long lived and their entire society moves significantly slower such that the word “is” takes a hundred years to say?

Fraser: We wouldn’t even notice the oscillations in the background of the radio waves.

Pamela: Exactly. So there’s so many different things, and then there’s always the problem – what if they’re out there and they know we’re here and we just happen to be trapped in some cosmic refuge where no one is allowed to trespass for fear of corrupting our society? What if we’re zoo animals? These are all things that people have contemplated as possibilities.

Fraser: Like the prime directive from Star Trek, where they have some rule where they’re not allowed to talk to us until we’ve achieved spaceflight on our own or (worse yet) they’re not allowed to talk to us ever and they’re not going to say why and it’s just too bad for us.

Pamela: Exactly

Fraser: But anytime we send a spacecraft outside of our solar system it gets shot down.

[laughter]

Pamela: That would be really bad!

Fraser: Yeah, that would be unfair.
 
This is going to be one of those depressing episodes.

Pamela: Yeah, this happens a lot. The Universe is just trying to kill us everyday in new and interesting ways – or in this case, aliens are trying to, but no, I don’t really think that’s happening.
 
There’s always the case – here on Earth we have different places that we have set aside where no one is allowed to go unless they’re with a guide and they’re specially trained because we’re trying to protect the few places that are left on our planet that are still unpolluted by humans.
 
What if we’re in the part of the galaxy that some galactic empire (that is still being dreamed up by a science-fiction writer yet-to-be-born) what if that unknown galactic empire has set aside our part of the galaxy as a place to be left alone and untouched because it’s the only place still left that hasn’t been corrupted by that society’s civilization?

Fraser: So we’re unpolluted by swarms of nanobots.

Pamela: So we’re being left alone to just destroy ourselves completely independently.

Fraser: With our nanobots of our own creation.

Pamela: Exactly.

Fraser: Right.
 
I guess another possibility then is that they’re here, but we just can’t, somehow, interact with them.

Pamela: Or perhaps we interact with them everyday and we don’t even know it. I don’t think that’s true either, but its still a possibility while we’re playing with all of parameter space.

[laughter]

Fraser: All of the possibilities that could be

Pamela: Exactly.

[laughter]

Fraser: Right.
 
So then, of all of that vast list of possibilities, which do you like?

Pamela: I think that societies are just starting to spring up and that they’re very rare. I think that when intelligent societies are born they’re just as likely to kill themselves off during that coming-of-age industrial revolution period as they are to survive it and thrive. I think that it’s going to take, perhaps millennia, perhaps millions of years, before what few societies might be in this galaxy have a chance to actually correspond with one another. I don’t know if our society is going to be one of those that actually survives its industrial revolution period, but I think every generation thinks it’s the last generation. We’ll have to see what our planet has in store in the next 40-50 years.

Fraser: I think I’m the hopeful one then.

[laughter]
 
I think for me, logically, most of those don’t really fly for me. I think that – I read a calculation, someone said it would take a million years to colonize the entire Milky Way, once you got rolling. The time to set down new colonies and then send probes to other stars – once intelligent life evolves, it wouldn’t take long to colonize the entire galaxy with technologies that we kind of understand, not that we can create but we understand the physics involved and nothing rules it out.
 
So I’m kind of leaning to toward the rare Earth hypothesis, which is more that intelligent life, or complex life, is extremely rare. Maybe 1 per galaxy, or maybe one per galaxy cluster, or maybe just one.
 
I feel for me, logically every other possibility would end up in a Universe teeming with life.

Pamela: One thing that always gets me about this discussion is right now we don’t have the technology to find other planets like Earth around other stars. We’re almost there, but we’re not quite there yet. The civilizations that we could detect because of their radio emissions would have to be a whole lot more either radio-loud or technologically advanced than we are. Right now we’re trying to find life more advanced than our own, that lives on planets we can’t yet detect. It’s one of these places where we’re just on the very edge of understanding and just trying to grasp at what we can. I think, in our lifetimes, so much is going to change that if we have this same discussion 20 years from now, we’ll have a lot more answers than we have right now at least in terms of what planets are out there.

Fraser: I’ll be interested to hear what the listeners think because this is one of those topics that there is no evidence for. Without evidence, the mind is free to wander. So, I’d love to hear what people think, where do they stand? What’s your vote? Let us know and we can compile it and maybe we’ll mention it in a future show what most people – maybe we should have a vote somewhere or on the forum.

[laughter]
 
I’d be interested to know what people think. It’s a neat concept to go into and to logically think through each possibility and think “what are the implications of that, what would happen, what would the universe look like if this was the case?” I find that really interesting.

Pamela: Yeah, and unfortunately we’re currently working from a data set of one.
 
When the first solar systems other than our own were discovered, we realised our entire model of how solar systems formed was wrong. Not entirely wrong, but it had some major problems. I wonder what’s going to happen as we learn more and more. It’s interesting times.

Fraser: Yeah, okay. All right. That was good, I think this was one of our more philosophical episodes.

[laughter]
I hope you people enjoy it because its less of the facts and more of the philosophy, but don’t worry we’ll get more hard astronomy next week.

Fraser: So this week we’re going to talk about UFO’s – well, not exactly, but we’re going to talks about extra-terrestrials.
 
So we want to talk about how scientists think about the chances of finding other life in the universe. Our starting point is going to be the famous Drake Equation, which attempts to understand all the variables that are or could be involved in the rise of
extra-terrestrial life.
 
Alright Pamela, pick a starting point – do you want to talk about Drake or his equation?

Pamela: Well, why don’t we start with where Drake announced his equation?

Fraser: Okay, a little of both!

[laughter]

Pamela: Okay, so Frank Drake was an astronomer (in fact he still is an astronomer – he’s at the University of California, Santa Cruz) and back in 1961, he and a colleague pulled together the first conference on the search for extra terrestrial intelligence and they did
this at Greenbank radio telescope.
 
He put forward this idea that in trying to figure out how likely we are to find life, perhaps we should start by trying to figure out how likely life is to exist, what number of stars out in the galaxy potentially are homes for intelligent life.
 
He brainstormed all of the different things that you should have to factor together and came up with this really neat equation that people continue to use today in trying to figure out what probabilities we have for finding those little green men that are at the heart of every good sci-fi story.

Fraser: I guess the Drake equation has a real practical value, which is that if you can start to pin down some of those underlying variables, you can get a sense of how large a search you have to do in our galaxy to try and listen for signals from other worlds or maybe even be able to image some other worlds directly. If the number is really really small, then forget about it, but if the numbers are really good then maybe there’s a chance within our current equipment.

Pamela: That’s exactly right. A good way to think of it is if you need to roll a six, you know that with a six-sided die, you have a one in six chance of rolling what you need. However, if instead you have a 20-sided die, suddenly you have a 1 in 20 chance of rolling the six you need. The higher the number of sides of your dice, the harder it’s going to be for you to roll that six.
 
If we’re trying to find a planet out there and the chance is one in four stars are going to have the type of planet we’re looking for, it’s going to be easy to find. If it’s a 1 in 10 thousand chance, we’re going to have to use an awful lot of telescope time over an awful lot of telescopes and years to find those potentially civilized worlds out there.

Fraser: So what are some of the pieces that go into the Drake equation?

Pamela: The Drake equation as it was originally formulated starts off by asking, “what is the rate of star formation, and what is the lifetime of those stars?” If you factor those two things together you can get a sense of how many stars there are out there that we need to consider. The reason you need both of these things is the rate at which stars are formed in combination with the amount of time that they’re around gives us a constantly updating number.
 
If you go into a really sad little bakery that only has one oven and can just produce one loaf of bread every three hours, then you have bread being created at a rate of every three hours. Now, if you just think, “okay, the rate of bread is one every three hours” then you might think that several days later you’re going to have a gazillion loaves of bread. The truth is one loaf of bread will probably only last for 24 hours, so at most if you have one loaf of bread popping out of the oven every three hours, and they go stale every 24 hours, that little bakery is never going to have anymore than eight loaves for you to look at.

Fraser: So stars go stale like bread?

Pamela: Exactly. Our own Sun is going to basically go stale in about a billion years, and no longer be suitable for allowing life to exist on Earth. So we have to consider how often these stars form and how long are they useful.
 
So, when you factor these couple of different things together, you get 15 stars being formed a year, and any given star is probably only useful for about 6 billion years. That leaves you in a situation where you have about 90 billion stars at any given moment are hanging out being useful for life.

Fraser: Is that in the Milky Way?

Pamela: That’s in the Milky Way.

Fraser: So there are actually 15 stars every day? Being formed?

Pamela: Every year.

Fraser: Every year, sorry. 15 new stars every year being formed.

Pamela: Yes. Roughly. We guess.

Fraser: That’s cool. So that part of the equation is kind of nailed down/

Pamela: Yeah, it does have some large-scale averaging over sort of the whole life of the galaxy, so it’s a smoothed out thing. There are certain places where you look and you see gazillions of stars being formed simultaneously. But if you look around on the internet, this is the generally accepted number that people are plugging in right now.

Fraser: Okay, and what’s the next part of the equation?

Pamela: So the next thing we look at is what is the fraction of stars that are going to have planets? This is one of those things that we’re still working to figure out, but near as we can tell, if a star has enough metallicity, enough metals, rocks, the heavier mass atoms in the area where it forms, then it’s going to form with lots of heavy atoms in it, and the heavy atoms are going to be available for the planets to form as well. It looks like, in our galaxy, about 50% of the stars have the necessary amount of metals to allow planets to form.

Fraser: Okay, so let me see if I understand this one. Obviously we’ve found the big, hot Jupiters
and some of the other exotic planets, so – I guess we can’t directly observe those, those stars and say “aha, there’s all those planets let’s count them all up so we know” but there’s a kind of connection between the mineral content or the metal content in the stars to the way we’ve been finding planets – we’ve been able to connect that?

Pamela: Yes. So, when you go outside and look up, some of the stars you see are made of almost pure Hydrogen and Helium. Others are still on the grand-scheme of things primarily Hydrogen and Helium, but they have a little bit of the heavier elements in them. That little extra bit of the heavy elements allows them to have planets around them because those same elements that were available to enrich their atmospheres were also available to form planets.
 
On a logarithmic scale where we set our Sun as the zero-point, we find that within roughly plus or minus 0.75 on this log scale, in the fraction of iron to hydrogen, we’re able to find stars with planets. So, that was a lot of gobble-de-gook, but the moral of the story is things that have lots of iron also have planets. There’s some really neat tables of information available on Geoff Marcy’s website (he’s one of the big guys behind finding planets), that list the metal ratios, the iron to hydrogen ratios for all the nearby stars that have planets found around them. You can just look through and see pretty much all the stars are about the same temperature and about the same metallicity as our Sun.
 
Admittedly there’s selection effects. They’re looking for stuff that looks like the Sun, to look for planets that look like the Earth (or, actually that looks like Jupiter – we can’t see the Earth yet). But other groups have gone and looked at metal-poor clusters of stars, looking for stellar transits, looking for events where planets pass in front of the star. They haven’t been able to find any planets that way so far.
 
The more people look for planets around metal-poor stars, the more they don’t find them. So, it’s looking like you need to have stars with at least as much metal as the Sun (roughly) or more metal than our Sun, more metal than the Sun seems to be working just fine.

Fraser: Okay, so they’ve been able to kind of nail that number down. This is good so far.

[laughter]

Pamela: So far so good.

Fraser: I think we’re going to find life just around the corner! Okay, so what’s the next one?

Pamela: So the next one is how many planets might exist around those stars that are capable of having life. The way we multiply these numbers together is you look at the total number of stars and then you multiply that by a number between zero and one that indicates the number of those stars capable of having life. Then when we look at what’s the number that can potentially support life – if it’s like, every other star might have a planet that can support life, you plug in 0.5, but if you think every star might have two or three places that can support life, you plug in a two or three.
 
In our own solar system, you look around and you say “okay, Mercury – way too close to the Sun, too hot, can’t support life as we know it, doesn’t count.” Mars, Venus and Earth are all three in this habitable zone that allows them to, if they have enough atmosphere (or little enough atmosphere in the case of Venus – it’s greenhouse effect sorta killed it) if they have high enough gravity to hold on to its atmosphere (Mars’ gravity isn’t high enough to hold on to its atmosphere) all these worlds are in a habitable zone where maybe life could’ve existed.
 
So, here I look around and I figure there’s probably a good chance that there could be two to three moons in any given solar system, two to three Earths, Mars’, Venus’, when you add all these different planets and moons that might’ve had life together, probably something like two or three is possible in a given solar system.

Fraser: So there could be two to three places in the solar system where life could evolve if that’s how it works. If it has a chance to evolve, or if there’s a way that it moves from planet to planet the panspermia concept.

Pamela: Yeah.

Fraser: Yeah, okay. So, so let’s say then, that, and once again we’re starting to get fairly, um, hypothetical right? We’re not sure, we haven’t —

Pamela: We’re totally hypothetical.

Fraser: We haven’t seen any other worlds yet, we haven’t seen anything else in the habitable zone, but I think that’s within striking distance, I mean within the next 10-20 years, we’re going to start having some amazing instruments up there.

Pamela: Yeah, no there’s what’s called the Space Interferometry Mission that’s being planned, and it’s estimating that of the 2 thousand stars it’s planning to look at it could conceivably find planets that are only about three times greater than the mass of the Earth around 120 of those stars. They’re estimating that they’ll probably find earth-massed planets – planets just in size like the Earth – around 6 of those 2 thousand stars, which is a small number, but it still pretty impressive to think that they can find and they can only find planets that are in special types of orbits, so that’s pretty cool.

Fraser: So that may get answered within our lifetime. Well we’re halfway through the equation, this is easy!

[laughter]

Pamela: Yeah, but the hard ones to guess are the ones that are coming up.

Fraser: Okay, hit me with them!

Pamela: Okay, so once you’ve figured out how many planets could possibly have supported life (even bacterial life), well how many of those actually bothered that possibility into a reality.
 
So, here in our solar system where we have three possible planets, as far as we know only one of them actually supported life and that’s the one we’re standing on.

Fraser: Right but that’s why they’re doing all this work into extrema files and stuff, they’re starting to find life is – wherever there’s water, here on Earth, there’s life. I’ve heard that there’s an estimate there’s potentially more life underground inside the Earth, in terms of bacteria in deep vents and cracks of the Earth in sort of biomass than there is on top of the Earth.

Pamela: Exactly. But it doesn’t show a lot of intelligence.

Fraser: No, nonono, but you didn’t say it had to be smart!

[laughter]

Pamela: No, I didn’t

Fraser: You said it had to be alive.

Pamela: And, you know I really wouldn’t be surprised if we found some sort of single-cell, or perhaps even – they’re saying there’s types of life that aren’t necessarily what we’re used to. Things that don’t necessarily have the nucleus of the cell, the cell wall, the mitochondria. Instead there might be even simpler forms of life, and I wouldn’t be surprised if we found any of these extremely simple, border-on “is this life or is this a crystal that’s capable of reproducing” type of life on Mars.

Fraser: But even if we find life on Mars, that’s where that thing I was mentioning before, the panspermia comes into play, because there could be a chance that there’s some mechanism that life is getting to and from planet to planet in our own solar system, that you know, asteroids or meteors are smashing into one planet, it’s kicking up debris, the debris is making its way to another planet, and you know, can deposit again. So even if we find life across our whole solar system that still doesn’t rule out that life is just good at getting around inside a solar system.

Pamela: It doesn’t, and it’s quite possible if we find life on Mars, life other places, that it’s just ejecta from wherever in our solar system life first formed. But the thing is, as we talked about with Comet McNaught, we’re flinging things out of our solar system periodically, so the idea of panspermia written large, allows you to actually fling bits of life all about the Universe. Now I’m not saying that happens, I’m just saying the possibility happens. I think, in my heart of hearts, that the probability that life independently crops up in multiple locations is much higher.

Fraser: I had read a research report where they calculated (and I don’t remember the details), but essentially every planet, every solar system or every star system in a galaxy is leaving behind this trail of debris behind it, from collisions and particles are being ejected by the solar wind and all of that, and that those debris trails are kind of mixing so it’s entirely possible that over certain millions or tens of millions of years, another star system may kind of pass through that debris trail. If there is a mechanism for life getting around, then it could transfer from system to system.

Pamela: And while the solar wind does a really good job at pushing a lot of the small stuff out of the way and preventing it from moving into the solar system, who knows if it could have any effect at all on a rogue comet from a different star system that decided it wanted to visit our star system.

Fraser: So even if we find life in another star system, we might still be related. But I guess that’s where you can do like, genetic testing to see, you know, if they both shared DNA, then you could maybe even track back and find out when the common ancestor was and that starts to explain some things.
 
So I think if we find life on Mars, and we find life on Venus, at least that tells us what the origin is. If it’s completely alien from the way our life works then that’s important.

Pamela: And the problem is, since we don’t really know how life starts, we don’t know what its simplest form is and we can’t say that this little viral packet that has like, three genes and this little viral packet that has like three genes (I don’t even know if things can form that small, I’m not virologist or bacteriologist). If the smallest, simplest forms that we find, if they are almost identical, we don’t know if that’s because that’s just how they form.
 
It’s sort of like there’s many different life forms on Earth that branched in evolution millennia ago and then ended up forming much later on, the exact same features that they didn’t have in common before, because that was the most useful way to form something. So, it’s without knowing how life formed, we can’t say “well clearly, you’d expect things to be completely different”. Sure, it seems like a good idea, makes for great sci-fi, but we can’t scientifically say that it’s impossible to get the most simple life forms to be virtually identical within two different environments.

Fraser: Okay, well you know what? I’m going to put a question mark on this one and we’ll move
on.
[laughter]

Pamela: Okay, that sounds great. So we move from the fraction that developed life, and I’m going to say we define this simply as the fraction that allow life to exist on them for a little while, doesn’t necessarily have to have started there, but the life exists. So once we have life, how much of it develops an intelligence?
 
When we look around our own planet, we have millions of different types of life forms, but we’re the only ones that drive cars. There are other intelligent life forms on our planet, but they don’t form cities the same way that we do. Many forms of whales seem to have complex language and societies and family groups, but they don’t talk on cell phones. And so we have to ask what fraction do they have with intelligent life, and we have to be careful here – do we include whales? Where do we draw the line for intelligence?

Fraser: Does the Drake equation draw that line?

Pamela: It simply says what fraction develops intelligent life, and doesn’t define “intelligent” for us. We might get a clue from the next line, where it’s what fraction develop technology and are willing to communicate. So there, you need to get through these three parameters: You have to have civilization, you have to have technology, and then you have to be willing to communicate. These are the people we’re going to find. If there’s a society on another planet that decides “we’re going to use fiber-optics for everything we do” they might develop technology, be an advanced society, and because they don’t leak radio waves out of the planet, and they’re not beaming radio waves off of their planet, trying to communicate, we’re never going to find them. So it’s complicated here, trying to define what makes a civilization a civilization, how many civilizations that are out there are going to develop technology, and of those how many are going to try and communicate – whether it be willingly or unwillingly- I don’t think we’re trying to communicate with our TV signals, but we actually are trying to communicate with our TV signals just because they’re out there and they’re flying and they’re detectable.

Fraser: Now what are some of the best estimates that you’ve heard so far? I think that’s kind of a crazy question, because, you know there are so many variables in there that there could be one – we know there’s at least one intelligent life form in the Universe, willing to communicate – we’ve already sent out our televisions so I think we’re already set. And then you could have it so that they’re everywhere, though that’s when we move into the Fermi paradox – if they’re everywhere, then where are they? So are there some estimates?

Pamela: When I look around the Internet, I see everything from the most bleak “we’re it” type estimates, to “there are millions of other planets out there that should be thriving with societies that we can detect”. My own rath attempts to figure out what’s logical got me anywhere from 100 thousand to 100 million possible societies out in the Milky Way right now. Now, given that there’s roughly 45 billion stars that I think it’s likely that we could look at and find planets capable of having life, you’re looking for anywhere from 100 thousand to 100 million civilisations among 40 billion stars, and that’s kind of hard to find.

Fraser: But doesn’t 100 thousand make it impossible to find them – what was the big number? 100 million?

Pamela: 100 million.

Fraser: 100 million puts them in the neighbourhood.

Pamela: It starts to get reasonable, but it’s still going to take a lot of telescope time. We’ve only found 170 star systems that have planets so far and we’ve been looking for about 12 years. Our ability to find them is going to get better and better, but we’re eventually going to run out of nearby stars to search. Once you start searching more and more distant stars, you’re only going to be able to tell if they’ve had civilizations if those civilizations managed to form very early early on. There you run into a completely different problem that is actually not addressed in the Drake equation.
 
It took time for our solar system to build up metals. It took time for our stars to form and our planets to form, so the problem arises of – could it be that our society is within a few thousand years of being as young as a human race can be? In this case, as we start looking at things further and further away, we’re looking further and further back in time, because it takes light time to travel, and if someone looked at the planet Earth a thousand years ago, there’d be no signs that life existed.
 
So as we start looking at planets that are a thousand light years away, we can only find societies that are either around stars older than ours, or that somehow managed to miraculously evolve faster than ours – and that’s possible. Our planet had a rough start, we had lots of mass extinctions, collisions with asteroids, many bad things occurred.

 
But those bad things also allowed the human race to eventually evolve because the dinosaurs had to get wiped out at some point. So it’s unclear if as we look further and further away, we’re actually going to be able to start finding more things or if because we’re looking back in time, we’re doing a selfdefeating act of some sort.

Fraser: Oh, I see, so it’s like the further we look back in time, the older we see, and eventually even though there might be other civilisations near by, if everyone just gets off the starting point at the same time, no matter where we look, it’s before the life had gotten civilised enough to start communicating.

Pamela: Right.

Fraser: So do you have any improvements for the Drake equation, if you had a chance to offer some feedback on it?

Pamela: So some of the things we need to take into consideration are, clearly, when could life have started? When did the stars that had the correct metallicity to form planets start forming? We need to take that into consideration. At what point can we expect in those stars’ evolution that intelligent life could be expected to start forming? What special characteristics do solar systems need to have in order for there to be intelligent life? There are people that speculate that in order to get a planet that is stable enough, you need to have a moon that is roughly the same proportion in mass that our moon is. Our moon prevents our planet’s tilt from wobbling all over the place, and allows the north and south pole to stay the north and south pole, and the equator to stay the equator. These are useful things for life.
 
We also have Jupiter, which does a really good job at catching rogue comets and shredding them before they make it in to the solar system. So just having Jupiter where Jupiter exists, with the size that it has, and having a moon as large as it is around our planet, these things add up to helping allow life to exist. I think it’s easy to form planets. I think the probability that you’re going to form solar systems that have the exact same combination of planets located in habitable zone that are large enough to hold an atmosphere, small enough to not gravitationally crush things, and happens to have a moon of just the right size is going to be hard. Finding that in combination with a giant gassy planet that’s far away from its star, catching comets, makes it even harder. So we need to figure out how to factor in these characteristics of individual solar systems.

Fraser: Right, right. So you need to have a moon, you need to have a Jupiter, you need to have the right size planet – can’t be too heavy, can’t be too light? – There’s a lot of variables. I guess that’s the great thing about astronomy right now. The scientists are hard at work either directly or indirectly trying to figure out a lot of those numbers.

Pamela: I have complete respect for the people out at the SETI Institute that are working to figure out how to tie all these pieces of information together. I have to admit I’m not exactly sure I want to find civilized alien life during my lifetime, but I respect the science that they’re doing greatly. There are a lot of really smart people working on what are the best techniques for trying to find planets around other stars. And trying to define which stars do and don’t have planets, both in terms of the temperatures of the stars, the sizes of the stars, and the metallicities of the stars.

Fraser: I wonder how much research has been driven by some of the pieces of the Drake equation. I mean, if some scientist who’s quite into science fiction is saying “I’m going to try and figure out this part of the equation” I think it’s probably kind of like Star- Trek, you know? It’s really helped to drive certain astronomers forward on their research and their enthusiasm for the topic.

Pamela: One of the things that I think speaks the highest about this is the SETI Institute, out in California, is privately funded. They don’t take government money. They’re able to employ really good scientists, get telescope time and do all their work through donations and sponsorships. I can’t think of any other group of that prominence and notoriety – everyone’s heard of SETI@Home who’s interested in astronomy and extraterrestrials. They’ve managed to do everything they’re doing on their own. NASA takes government money, obviously, and most universities are running off of NSF grants, NASA money as well as tuition. People are willing to put their money where their imagination is and trying to define where can we look for intelligent life?

Fraser: It’s kind of like the most important scientific question out there, I think. Are we alone in the Universe? If we find that answer, either way (we’re alone or there’s others out there) would be one of the most important things we’d know about our state in the Universe.

Pamela: It’s definitely a question that’s a thrill to pursue. But I have to ask you this (just to turn things around): how do you see our society reacting if we actually do find signs of intelligent life?

Fraser: I think, because it’s so far way, people will go crazy in the beginning and then it will kind of be sort of in the background. I think you can almost get used to anything, so in the beginning, for the first little while, it’ll be big, big news and seep into culture in many different ways. Then I think it’ll be something you’re just kind of used to, like watching television. I think people will get numb to it pretty quick.

[laughter]
As amazing, as exciting, as deep and meaningful a discovery it would be, I think we would get numb to that and want something more. So, yeah, I don’t think it would sort of come in on everyday, all days, all we’d be thinking about. Life would go on. I think part of it, is it’s just so impractical to talk to them and to get out and visit them, but maybe that can be solved too.
 
Alright, well see – this wasn’t one of our hopeless episodes about a grim future!

[laughter]

Pamela: It was definitely filled with hope!

Fraser Cain: Last week we talked about the various ways that astronomers have been finding extrasolar planets using the different kinds of techniques. This week we wanted to talk about the actual kinds of planets that have been discovered. Back in the day, astronomers were expecting that the kinds of solar systems they would be finding would be very similar to the solar systems that we live in, and that hasn’t been the case.

Dr. Pamela Gay: It’s actually one of these things where, strange as it may sound, our modern solar system formation model dates all the way back to 1755 and Immauel Kant, who came up with the nebular hypothesis. Up until fairly recently (with modifications over time), everyone said to form a solar system, take a giant cloud of gas and disturb it somehow (say, a nearby supernova goes off and nudges it), it begins to collapse, it begins to spin, you end up with a flattened dust disk in which planets form. The dust disk is warmest near the star, and that’s where the rocky planets form because all the gases are getting evaporated out there. Once you get past a certain distance, there’s what’s called a frost line. Past that line you’re able to get the gaseous planets.
 
So we had this very distinct theory-based, using tests of our own solar system, model for what we thought solar systems should look like: you have a star, rocky planets on the inside, gaseous planets on the outside and we keep all the chunks of ice in the outer most parts of the solar system.
 
Then we started finding planets around other stars, and they don’t match our models, necessarily. We’ve been working to modify that model to account for the hot Jupiters that are being found near stars, to account for the fact that there’s planets around pulsars.

Fraser: What kinds of planets have been turned up so far?

Pamela: The very first planet around another star was actually found in 1992 around a pulsar, by Alexander Wolszczan. He’s a radio astronomer at Penn State University. He was measuring the timings of pulses coming from a very fast spinning neutron star. He found that the pulses weren’t in perfect rhythm with one another. Sometimes they seemed to arrive a little bit too early, and sometimes they seemed to arrive a little bit too late. This implied that something was causing the pulsar’s distance from us to very slightly change so the pulsar was sometimes a little bit further away and those pulses had to travel further to get to us (and were thus slowed down), or sometimes the pulsar was a little bit closer.
 
The only way to explain this kind of tiny motion in the pulsar was to say there had to be a planet there that was yanking the pulsar around. Since then we’ve continued to find planets around pulsars. In fact, fairly recently a miniature system was again found by Alex Wolszczan in which there are four different planets that seem to form a half-size solar system model, where you have planets that are scaled down with spacings proportional to Mercury, Venus and Earth, and then there’s another planet out in the outer edges of this pulsar solar system. So we’re still finding these.

Fraser: When they say planet, they must be using the term pretty loosely.

Pamela: These are little tiny chunks. We can’t say if they specifically meet all of the criteria set down by the IAU. We don’t know if there’s even smaller chunks of stuff in the orbits they’re in. These little tiny, highly rocky things weren’t formed out of the original gas and dust that the star that eventually compressed down to the neutron star formed out of. Rather, we think they formed out of the gas and dust that was expelled by the supernova that formed the pulsar.
 
Basically, you start out with a giant star. The giant star hits a critical point in its lifespan. It can no longer generate its own energy, so it explodes as a supernova. When it explodes as a supernova, it spews the outer layers of the star out all around. It appears that some of this gas and dust that is spewed out from the supernova coalesces into pulsar planets.
 
After the atmosphere has been spewed off, the core of that star collapses down and forms a neutron star, a star that is so dense that the protons and electrons can no longer stay separate, and coalesce into neutrons. So you have this highly compact object that is spinning super fast, and around it the remnants of the former star form new worlds.
 
Not exactly like you find in our solar system.

Fraser: No, but then a few years later people started to turn up what we might consider to be “real” planets.

Pamela: Exactly. In 1995 a Swiss team led by Michael Mayor and Diedler Queloz of Geneva announced that they’d found a rapidly orbiting planet going around the bright nearby star 51 Pegasus. This wasn’t like anything anyone ever expected to find. It was a planet similar to Jupiter in mass, but with an orbit that was smaller than Mercury’s. This was not something anyone had expected to ever find. Suddenly all of our solar system formations had to account for how to get planets to basically be on top of their stars when they’re gaseous planets.

Fraser: Right, astronomers were expecting it was going to be rocks and rocky planets like Earth and then gaseous planets, and then ice planets, all nice and calm, but no.

Pamela: Everything lined up politely, ordered by type distance from the sun, and no. The Universe doesn’t like to do what we expect.

Fraser: So if we were in 51 Pegasus what would be seeing? What would this planet look like?

Pamela: We’d be really hot, and probably melt – or at least evaporate. The sun would just completely fill the sky. Our Sun is huge, but it’s far away, so it appears to be about the size of a quarter held out at arm’s length. If instead, we were so much closer, it would just fill the entire sky most of the time.
 
For 51 Peg the planet is just 0.05 AU away from its central star. This makes the star (which is a lot like our Sun) fill basically 70 degrees of the sky. Imagine the Sun stretching almost from zenith to the horizon; it’s pretty spectacular to imagine.

Fraser: Do astronomers think this kind of situation is common, or is this just what they were able to find?

Pamela: We’re still trying to find out exact statistics. Currently the observational techniques that have been used the longest and the most are sensitive to hot Jupiters. They’re sensitive to giant planets very close to normal stars. That’s primarily what we’re choosing to look at, and that’s what our technique is most able to find. There’s selection effects.
 
We’re starting to use new techniques to find planets around other types of stars and to find other sizes of planets, and we’re getting more and more technology that is able to find smaller and smaller planets. Exact proportions, we’re not sure, but it will come in time.

Fraser: So in the beginning what we saw were lots and lots of gigantic planets really close to their home-star. Now as the techniques are getting better, we’re finding smaller planets, more distant and more what we would consider to be “normal” orbits.

Pamela: We’re finally starting to find rocky planets. That’s perhaps the most comforting thing of all. We’d been finding hot gassy planets for a long time, and now we’re finally starting to turn out icy planets, we’re starting to turn out rocky planets, there was a system found that had three planets, again still in fairly close orbits.
 
Our orbital period takes us 365 days to go around the Sun. These planets have orbits of just nine days, 32 days, 197 days. They’re much closer into their sun, but they have masses that are only 10-18 times the mass of the Earth. These are things we can start to think about. We are finding rocky planets, and this is just a really nice thing to know… these things do exist, we just need to start looking for them and develop the technology to find them consistently.
 
We’re also starting to find things that we’re not quite sure how to account for. There was recently a planet found that was about the density of cork. This is not something any planet formation model can account for, and it’s not alone: there’s two of them so far that have been found. To get the density of cork, you have to be really hot – hotter than being close to a star can account for. Somehow these planets seem to be generating their own heat, which is something a planet is not supposed to do in large amounts.

Fraser: So when they say the density of cork, they’re saying it would be say, the mass of Jupiter but much larger, so it would have that mass spread over a larger area? I remember looking at old books, like my old astronomy books when I was a kid, and they always had pictures of Saturn floating in water?
 
Is Saturn the consistency of cork?

Pamela: No – that’s the confusing part. Yes, Saturn is less dense than water – it will float, it’s a giant gas ball. But there’s a difference between cork and just being less dense than water. These planets are, in the case of Hat P1 (an ever-so non-imaginatively named planet), this world is 1.38 times the radius of Jupiter, so it’s one and a third times wider than Jupiter is, but it’s only half the mass. That’s a pretty dramatic difference in how much mass you have crammed into how large an area. This is an extremely low density world.
 
To put some numbers on that, Saturn’s density is 70% the density of water, and Jupiter’s density is about 30% greater than water. This world is just 25% the density of water. That shows you it’s a lot less dense than Saturn.

Fraser: What seem to be the limits for a terrestrial world like earth?

Pamela: We’re still trying to figure that out. Getting down to the lower masses around normal stars (not around pulsars) is something that we’re still trying to figure out how to do. The smallest planets currently being found are being found through microlensing events, where the planet passes in front of a background star and its gravitational pull affects the light from that background object.
 
So we’re starting to find things that are several Earth-masses in size, but we haven’t quite made it all the way down to an Earth-mass yet. What we are finding is actually asteroid belts around other stars. We can’t find Earths, but we can at least find asteroids, and that’s another pretty cool thing.

Fraser: how would we be able to see an asteroid belt? The individual asteroids are a lot smaller than a planet like Earth.

Pamela: It’s all about heat. Dust, gas, rocks… they all (when they get heated up by a star) radiate heat. We can see that heat as infrared light. When you get a large dust belt, a large asteroid belt around a star, it gets heated up by the star and then the Spitzer Space Telescope can discover it.
 
The Spitzer Space Telescope has been systematically finding asteroid belts and even icy belts reminiscent of the Kuiper Belt around distant stars. All you’re doing is looking for reflected light coming off of a large belt of objects. What’s neat is some of these look very similar to our own belt. They appear around stars similar in age to our own Sun, and they have very defined edges.
 
For instance, the star HD69830, it’s a Sun-like star, it has an asteroid belt that is, admittedly about 25 times more massive than our own asteroid belt, but this asteroid belt is extremely well-defined, which tells us there’s probably a planet near it that is able to herd or shepherd the asteroids into staying in a nice, coherent, well-defined belt, just like the moons of Saturn are able to shepherd the rings into coherent rings.

Fraser: I guess that would be when they always have the science fiction shows where people are going through an asteroid belt, manoeuvring around all these asteroids. That wouldn’t really happen here in our solar system, but they’d be getting closer to that in a place like that with that many asteroids kicking around.

Pamela: Exactly.

Fraser: So the difference between a planet and a star is a planet is just an amount of mass. Especially with Jupiter, if you kept piling mass on Jupiter, it would eventually ignite as a star. How large can these planets get?

Pamela: This is where we start getting into great debates among astronomers. Coming up with a qualitative way to define what is a planet, and what is a star, requires us to start looking at things like are we going to look at energy generation mechanisms. Jupiter is actually generating more energy than it receives from the Sun. If you look at Jupiter, and you measure all the light that you get back at the planet Earth, and then try and account for all that light comes from, you first say, “Jupiter’s a gas ball, it reflects light from the Sun, we know how much sunlight is hitting it, we know its size, we know we should be getting a certain numerical amount of light reflected back at us,” and we get more.
 
To account for where that more comes from, we think about things like it’s a giant ball of gas that’s slowly condensing. It’s getting smaller over time, and as this happens, as it gravitationally contracts, the gas that is getting squished smaller and smaller together is actually radiating away heat.
 
So gravitational contraction can produce heat. What else can we have an energy generation mechanism? If you make Jupiter bigger, the deuterium (the hydrogen in it that has a special added neutron), will actually start fusing, and we’ll get deuterium energy production. That’s a very short-lived phenomenon. If we want to get actual hydrogen burning like we have in the Sun, you have to make it even bigger to get enough pressure in the centre of the planet to get the hydrogen to burn and fuse.
 
At what satge do you start calling something a planet, and start calling something a brown dwarf star? These are things that are still being debated. In the end it’s probably going to come down to at what stage do stars begin to generate their own energy, and do we count it with when they’re just burning deuterium or do we wait and only count them when they start burning hydrogen in their cores.

Fraser: But as always it’s not a specific line you can draw, it’s a grey area that starts even with Jupiter all the way up to something becoming a brown dwarf.

Pamela: Yeah, it’s a complicated question and as everything with Pluto recently demonstrated, trying to come down with a concrete definition is something that gets everyone hot around the collar. Everyone wants to say, “my object is a ___” and if your object is on the boundary and you have a particular opinion it becomes a very emotional battle. You want everything to be logical, but astronomers are still humans and we want to have our own personal, “this is a planet, this is a star” and it’s hard to say, “well, this object is on the boundary”.

Fraser: There was an object recently that was discovered on the boundary.

Pamela: That’s right. A planet was recently found around the brown dwarf CHXR73. This maybe a planet object is just 12 times the size of Jupiter, and it doesn’t look like it was formed with the brown dwarf star. So this raises the question of if something isn’t formed alongside the star that it orbits, is it a planet? If these two objects each formed out of their own disks of gas and dust and ended up getting gravitationally bound together later, are they still a star and its attached world? We don’t know.
 
Currently, the Spitzer Space Telescope is going to take a look and see if it can find a disk of dust around the little 12 Jupiter-mass object, and see if perhaps it is quantitatively its own separate star-like, very tiny thing, that might have its own planets forming around it. It’s right on the boundary where we need to have a definition and we just don’t have one right now.

Fraser: One thing that’s been a bit of a controversy is, I know there’s been a discovery recently of something researchers were calling “planimos” which are, I guess, solitary planets not actually going around a star, but actually having their own little mini solar system, completely floating through space.

Pamela: These are very confusing objects. They get more confusing the more of them we find. Sometimes when you’re looking around you find these things that are clearly not big enough to be stars. They’re not orbiting anything, so where did they come from?
 
If you have multiple stars forming together, you get a lot of weird gravitational interactions going on. It’s been shown, initially by Victor Zebehay, when you get multiple objects gravitationally interacting, you can have a three-body problem where one of the objects gets radically flung out of the system. So it’s possible that when you have multiple stars forming, and planets forming around these multiple stars, that some of the planets can get ejected from the system and end up roaming the galaxy completely on their own.
 
This seemed like a perfectly reasonable model until recently when astronomers discovered a double system of these planimos, these planetary-mass objects. This was worked on by Ray Jayawardhana. Using ESA’s 3.5m New Technology telescope at La Silla in Chile, he found a pair of these double planetary mass objects floating freely through space. They were bound together gravitationally, but only barely.
 
It seems hard to imagine how these barely bound together planets could have survived a violent flinging from the parent system they might have been born in. So now we’re trying to figure out how to form loosely bound binary planimos that are freely floating through the galaxy… and we’re not quite sure. But that’s what makes astronomy interesting.

Fraser: But if you’ve got a star and you’ve got a disk of material around it, and in that disk various objects are able to come together… I guess the question is couldn’t you have a smaller cloud of gas and dust come together and just not have enough mass to turn into a star, but it could turn into something.

Pamela: And that’s the other argument. Can you collapse down a small disk and have it collapse down to the density of a planet? Models are still working to try and figure that out, and the answer could be yes, and we just need to find one of these things in the process of forming. That’s the neat thing about the Spitzer telescope. It can answer these questions as it looks through the nearby galaxy and looks at areas where stars and planets around them are forming.
 
Spitzer recently looked at the Orion cloud complex and found nearly 23 hundred planetary disks around stars. These are all places where planets could be forming. Now all we need to do is find a disk that isn’t exactly forming a star, but just might be forming planets instead (and only planets).

Fraser: Awesome. I hope we turn up some more of these planets in the next couple of years. I think the point is we’re in what I call the golden age of astronomy. We’re just getting started, there are so many cool telescopes, there are so many new space telescopes, and a lot of new techniques that are being developed.
 
Hopefully, five or ten years down the road, this conversation will be completely different. Hopefully we’ll have found a lot more planets that are more like our planet and maybe even we’ll have an idea if there’s life.

Pamela: We’re just getting started. We’ve found over 200 planets, and I’m sure there’s thousands and tens of thousands of them out there to be found yet. We are just now starting to have a firm, statistical understanding of what’s going on, and you need to have the clear observations before you can build clean models, but that’s happening today.
 
We’re finding things, we’re going to be able to start defining the models, new technologies are being built, being used. Direct detections of planets are going to be happening in the next months, not just the next years. It’s a great time to be.

Fraser Cain: Last week we speculated on how Pluto’s planethood would affect the definitions of extrasolar planets. That got us thinking that would be a good thing to talk about this week.
 
There are more than 200 extrasolar planets that have been discovered so far with new methods, techniques and observatories in the works, so those numbers should go up as we go through time. We wanted to spend this episode discussing how we detect those extrasolar planets, how we learned to detect them in the past, and what the future might hold.
 
So, Pamela. How do we search for planets going around other stars?

Dr. Pamela Gay: That is the key question that people have been asking since the 50s and 60s basically. As technology has improved, we’ve been trying to search for the elusive evidence that our solar system isn’t unique. People originally started looking for the stars to actually wobble in the sky. We looked for the gravitational tugs and pushes of planets in other solar systems to cause the pinpoints of light in the sky that we like to think aren’t moving to actually wobble to and fro.
 
For a while there were actually claims that Bernard’s star probably had a planet, because people very carefully measured its position relative to other stars, and thought they saw it moving. Since then, that has panned out. Bernard’s star does not have planets.
 
This raises the prospect of if we look for them to move up, down, left, right, north, south, east, west in the sky, what about if we instead look for motions such that the stars appear to be moving toward us or away from us. It’s really, really hard to make accurate measurements of the motions of stars relative to one another’s light in the sky. This is called astrometry; it’s really hard.

Fraser: So if I was looking up in the sky, and carefully, carefully measuring the position of a star, I might see it making a little circle in the sky as the planet that was going around it was tugging it side to side – I guess as the planet and star were commonly orbiting their centre of gravity.

Pamela: Yeah. Stars and planets and everything in a given solar system, move around the centre of all of that mass in the solar system. If a solar system is aligned in such a way that we look at it and the planets appear to be in the plane of the sky, circling around that star, if the planets were big enough, theory says we should be able to see that star moving around in little, itty, bitty, tiny circles about the centre of that entire solar system.
 
The problem is planets just don’t weigh that much compared to stars. The stars don’t move enough that we can presently see them moving north, south, east, west, in the sky. But we have other techniques. We can very carefully measure the velocities of the stars. Just like policemen can measure the changes in the rate of light that gets bounced off of a car moving 60mph versus a car moving 120km/h (just to mix up our units here), we can similarly measure how the light changes when it comes from a star that’s moving toward us versus a star that’s moving away from us.
 
If there’s planets around a star, we call this affect the Doppler shift. We can measure the Doppler shift as the planets force the star to wobble to and fro in the sky. We can measure this accurately enough that if a star is fairly near by, and we can get enough light from it, and it has a planet that’s Jupiter-sized, and not too far away from the star, we can measure those wobbles to and fro in the velocity of the star relative to our velocity.

Fraser: But the planet and the star have to be pretty well lined up in a specific way, don’t they?

Pamela: Yeah

Fraser: We have to be aligned on the plane of the ecliptic between the star and the planet so the planet is kind of pushing it away from us, pulling it toward us, pushing it away from us.

Pamela: Exactly.

Fraser: If we’re seeing it face-on, then we’re back to the little spiral or the little circle the star should be making, but that won’t really affect its velocity.

Pamela: Right, so let’s imagine that solar systems are basically badly made plates. They’re not entirely flat: parts of them where you have an orbit that’s tilted go up and down… it’s not an ideal plate, but it’s mostly a flat plate.
 
So, you can hold that plate out at arm’s length and look at the edge of it. If we have that situation, then as the objects go around, they move toward us as they come down the right side of the plate and then go up the left side of the plate. This moving closer to us and further away from us causes the star in the centre of that solar system to also move toward us and further away from us (I’m doing this with lots of hand gestures you can’t see, so I’m hoping you’re following this).
 
If we rotate that plate 90 degrees so we can see the pretty pattern on the plate, and you now imagine planets going around and around the centre of that plate, we can now see the motion of the planets, but they’re always at about the same distance from us as they go around and around that central star. While the star may also be moving around and around the centre point with very tiny, tiny motions, that motion doesn’t cause it to move toward us and away from us. We just don’t have the abilities to measure that north/south, east/west motion in the sky with current technologies.
 
Luckily the Universe is fairly random. When we look at other solar systems in the galaxy around us, some of them are those edge-on plates, some are the face-on plates, and most of them lie somewhere in between. We can statistically assume that it’s random orientations and then work to figure out how many solar systems we look at should have planets causing motions that are detectable. It all breaks down to statistics and being very grateful that we live in a random galaxy.

Fraser: The easiest planets to see are going to be the ones that are directly edge-on, and close.

Pamela: Yes

Fraser: The harder ones are going to be the ones that aren’t edge-on or are more at an angle away from us and further away. Eventually I’m sure there’s some point where it’s all impossible to see or you need bigger and bigger telescopes.

Pamela: Right. So, we’re building those bigger and bigger telescopes slowly.

Fraser: Okay, so we can measure the velocity of the planet as it swings back and forth from us. Why is that so accurate? Why is that easy to do while measuring the position in the sky is hard?

Pamela: Measuring the position in the sky, we have to look for changes in the angular separation between two stars. Imagine trying to see the motion of someone that is a kilometre away who decides to step sideways a millimetre. That is a bigger motion than a lot of these stars are going to have when their biggest planets yank them about their centre of mass. Measuring that type of little tiny motion on the sky, we get befuddled by things like our noisy atmosphere.
 
As light comes down through the atmosphere, our atmosphere distorts what we see. If you go outside with a really good telescope and look at a bright star, you can see the light from that star dance around in your field of view. It makes it really hard to measure where the centre of that star’s light is.
 
So fine, go out into orbit and now try it. If you look at a system with a star like the Sun and a planet like Jupiter, the motion of that central star is going to be 30 times smaller than what Hubble is able to see. So we’re not going to see those changes.
 
At the same time, when we look at radial velocity changes, here we can look at transitions in atomic lines. Atoms have electrons attached to them (unless you make them too hot and jettison all of the electrons). So your average, run-of-the-mill atom in the atmosphere of a star is going to have a bunch of electrons that, because of the high temperatures of the star, are transitioning back and forth between different energy levels. Every time they make a transition, they either absorb a photon or emit a photon.
 
The result is that when we look at the light from stars, we see bright lines and we see dark lines in this overall continuum of light. The positions of these lines are exact, due to the fact that an atom is an atom is an atom, and the transitions are always going to be at the same wavelengths. We know from laboratory data exactly where those atomic lines should be. Now, when a star moves, that motion causes these lines to get shifted back and forth in colour.
 
If an object is coming toward us, we see all those lines get a little bit bluer. If an object is moving away from us, we see all these lines get a little bit redder. Sound does the same thing: when we hear a fire truck coming toward us, you hear the pitch get much higher. When it gets past you and starts going away from you, you hear the pitch get lower. The lower pitches are the same concept as light getting redder: it’s all waves. These very fine transitions in atoms we can measure very precisely kilometres per second changes in the velocity of a star, looking at these little fine lines that don’t get distorted by the atmosphere affecting two different stars differently.
 
Because we can measure them so finely, it’s technologically easier than looking for motion on the sky of the actual star. We’re just looking for velocity – position and velocity are slightly different. It’s easier to measure velocity than position.

Fraser: Right. Now, most of the planets that have been discovered so far using Hubble and even ground-based telescopes have been discovered using this technique, right?

Pamela: Right. The majority of these planets were found using Doppler measurements of the radial velocities – the velocity of the star toward and way from us in the sky. This work was initially done ground-based using the McDonald Observatory 107 inch telescope (they didn’t do it first, but they did do it), Keck Observatory… a number of the big observatories have got on board doing this. These guys looked at stars that were near enough that the light from the stars appeared to be very bright, because they were going to take that light from the star and spread it out across metres so you can see all the fine gradiations in the spectrum of the star.
 
We’ve all seen (at one point or another) rainbows cast on walls by prisms or a glass of water at a weird angle next to a window. Imagine that rainbow of light you saw on your wall spread out across metres and metres so you can see all the fine gradiations in colour from the Sun. We do that same thing with light from other stars.
 
It’s easy to see a rainbow on your wall from the Sun, because the Sun is really bright (even when you spread its light out on the wall it’s still fairly bright). Imagine spreading out the light from a flashlight: that’s harder to see, the light is fainter. As you start getting to stars, you pretty much can only do this with a lot of the brightest stars in the sky because you’re spreading that light out over so much space that you run out of light pretty quick to spread out.
 
So we look at the brightest stars, spread their light out with a spectroscope, and look for these little tiny atomic transition lines and measure their radial velocities. We look at bright, near-by stars.

Fraser: So if I had this rainbow on my wall, and flipped between the two positions of the star, would I see the lines moving back and forth?

Pamela: Exactly.

Fraser: Okay. I think that gives a good understanding for how most of the planets are found. I know there are some other techniques as well. There’s a way we measure the dimming of a star as a planet moves in front of it, right?

Pamela: That’s called a planetary transit. The idea is that when a planet (which doesn’t emit light on its own), passes in front of a star, it’s going to block out some of the light from the star behind it. Planets in general are tiny compared to stars, so they don’t block out a lot of light, but we have abilities to measure the light coming from stars very, very accurately. If you can measure the dimming and brightening of a star as a planet first passes in front of it, and then passes out from in front of it repeatedly, and start to see this cycle of the star’s light dimming and re-brightening to its original magnitude, you can say, “Hah, there must be something here.” Based on the amount of dimming that’s seen, you can say, “this is a planet” or, “this is just a really small star that is pretending to be a planet.” With the biggest planets and the smallest stars, we start to get a fuzzy line. With small planets, compared to small stars, it’s fairly easy to say we have found a planet this way.We have seen a star get dim, brighten back up to where it was, and we know something is there causing this eclipsing behaviour.
 
Again, though, this only works for when we’re looking at solar systems edge-on. The planet has to pass in front of the star in our line of sight. If you go back to holding that plate at arm’s length again, put a speck of dirt in the centre of the plate and how many different angles can you hold the plate at, such that the edge of the plate blocks the speck of dirt? Those are the only angles we can see these transits.

Fraser: I guess the benefit is the two methods are complementary. You can find it with one and confirm it with the other.

Pamela: That’s what people are doing. It speeds up the amount of time spent discovering planets. You can get two different people on two different instruments: one finds it, the other confirms it. With science it’s always good to be able to confirm your discoveries in more than one way.

Fraser: There’s a third way that’s turned up a couple of planets, and that’s pretty fascinating, the gravitational micro-lensing technique.

Pamela: This is one of these neat things that comes out of Einstein thinking way too hard about gravity. Our Sun sends off light in all directions, so I can be anywhere 360 degrees around east/west, go up/down, anywhere I want in the solar system and I still see light from the Sun. The amount of light I see is just the light that came from the surface of the Sun and headed in my direction. There’s other light headed in all other directions and I don’t get to see that from where I am, unless I use something to bend that light my direction.
 
Gravity doesn’t just affect you, me and things that we drop on our feet. It also affects light. If I have a really high-mass object like another star, and I put it between me and the light from the background star, the gravity from that object that’s closer to me will bend the light from an object behind it, such that light that would normally shoot off in some other direction, now gets bent to come to my field of view, so that I can now see that light that was destined for somebody else.
 
This causes objects to appear to suddenly brighten. As the object passes in front of the object in the back, light that we weren’t normally going to see suddenly gets to us. We get an increase in the amount of light here on Earth.

Fraser: So we’ll see a star we were already looking at, brighten up?

Pamela: Exactly.

Fraser: Right. And what we were seeing behind it is something we might not even have been able to see, but it’s actually the light from a more distant star that’s being focussed like a telescope.

Pamela: Yes. Gravity acts like a lens, magnifying the light that reaches us. It’s just bending light that wasn’t supposed to reach us, to reach us in a new way. Now, the gravity of that foreground object, depending on the alignment, can cause an object to magnify in a lot of different ways. You can get a ring of light from a background object, you can get a cross where the object gets replicated in all different ways – it’s a geometry problem.
 
The neat thing about this geometry problem is it’s also affected by the shape of the object in the foreground. If you have a star with a planet next to it pass in front of a background object, the mass of that planet will also work to magnify the light, to redirect that light from the background object toward us here on Earth, allowing us to see this weird mass distribution of star + planet in the foreground as it changes the light of the background object.

Fraser: Right, and I guess we see the brightening, and that’s kind of the alert that there’s a transit going on, and then the astronomers can measure the shape of how that brightening goes over time, as the closer star moves in front of the more distant star, and you’ll be able to say, “oh, okay, there’s a planet affecting the lensing of the more distant star”

Pamela: This isn’t necessarily a really fast process. Objects in the foreground may pass very slowly in front of objects in the background. When this happens, the teams that are looking for these events can go, “ooo, here’s one starting – let’s get everyone watching to see what happens as this foreground star goes in front of this background star (or foreground object passes in front of background star)”.
 
It’s while we’re watching these slow events unfold, with eyes around the planet, that we start to see these planets show up and not all the eyes that are turned looking at these micro-lensing events are necessarily professional astronomers with PhDs sitting at multi-metre telescopes. One of the planetary transits was actually observed by a group of our southern hemisphere friends who were working on their amateur astronomy facilities, helping teams of professional observers keep round-the-clock coverage of these micro-lensing events.
 
This is the type of work that anyone with a telescope can get involved in. there’s an excellent website, transitsearch.org that works to explain how transits work. They get amateurs involved as well, and some of the gravitational lensing teams, such as MACHO are looking for amateurs to also work with them.

Fraser: I guess the ideal though, is you can take your big telescope, point it in the sky at a star, look at it and go, “there’s some planets!” You can see them brightly next to the star. Why isn’t that happening, and what’s that going to take?

Pamela: We just don’t have the technology. That’s first of all something you just can’t do from the planet, because we have this atmosphere that just blurs everything to bits. We’re working to find ways to overcome that. The Very Large Telescope (which is a topic for another day) has found some really good ways to do it. In general, what you want to do is get above the atmosphere.
 
There are some different missions that are looking to send telescopes into space to specifically look for planetary transits and to look directly for planets next to stars. Sometime in the next couple of months the European Space Agency is going to be launching a mission, COROT that is going to be looking at a bunch of different patches in the sky and looking to see if there’s any variations in what they see in the light from stars that are due to planets. They’re specifically looking at astro-seismology, and looking for star-quakes that send ripples across the surface of stars (we see this in our own Sun). While they’re making all these careful stellar-earthquake measurements (or stellar star-quake measurements) they’re also going to be looking to see if planets affect things. It’s still not quite a direct detection method.
 
The direct detection method is going to come later, when we start launching new technologies that we’re still starting to develop, such as giant coronagraphs. The idea here is, you take an image of a section of the sky, and use special optics to reduce the light that comes from the central star by, say a factor of a billion, then look for faint planets that would otherwise get lost in the light from the star.

Fraser: Right, if you had a firefly next to a searchlight, 20km away, even if you’ve got a telescope good enough to be able to see the firefly, it’ll get washed out by the searchlight. If you can block out the searchlight you might stand a chance to see the firefly.

Pamela: This is something that we all do at a certain level when we’re driving into the Sun. You’re driving along, facing west at sunset, you hold your fist up in front of your steering wheel and your eyes, drive with your other hand and use your fist to block out the light of the Sun so you can see the cars around you. That is, perhaps, the crudest coronagraph you could have. By building much more complicated optics, you can effectively block out the light from a star and look for faint things around it.
 
That’s one way – the other way to do it is when light comes toward us, it’s waves. Waves can either cancel one another out, or build one another up. We’ve probably all seen this in water where two waves of different velocities combine and you suddenly get a bigger wave for a moment.
 
It’s possible to collect the light from stars with multiple different mirrors, and then combine that light with slight delays in timing, such that the light waves for the star cancel one another out, whereas the light waves for the planet don’t. This is called a nulling interferometer. It’s something that’s still under development, and it’s another way to be able to see faint things around a bright object, just by using the special properties of light to bring out faint things we might not otherwise get to see.

Fraser: So you would use the nulling interferometer to silence the waves from the star, but that might still leave waves from other objects around it visible in the telescope.

Pamela: Exactly. The nulling part, the cancelling out of the light waves only affects the central star in a field, whereas the light waves from the planets which aren’t in the very centre of the field, are still allowed to combine in such a way that we can see these faint planets.

Fraser: Great, once again Pamela thank you very much for explaining this in depth. That gives a really good background on the search for extrasolar planets. We’re just getting started. In the next few years and decades, there’s going to be hundreds and thousands of planets discovered. I hope within a few years we’ll start seeing pictures of other planets. It’s an exciting time.

Pamela: There’s plans to launch a terrestrial planet finder (if NASA doesn’t cancel its budget), by 2020, that will be able to see these things.

Fraser: NASA if you’re listening, don’t cancel that project. Thank you.