Ep. 573: Exoplanet Atmospheres

Not only have astronomers discovered thousands of exoplanets, but they’re even starting to study the atmospheres of worlds thousands of light-years away. What can we learn about these other worlds, and maybe even signs of life.

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Show Notes

NASA Exoplanet Archive

Spectroscopy (Swinburne University)

Spitzer Space Telescope (Caltech)

James Webb Space Telescope (NASA)

Transiting Exoplanet Survey Satellite (TESS) (NASA)

Very Large Telescope (ESO)

EXPRES instrument (Yale University)

MASCARA instrument (ESO)

MASCARA-2b / KELT-20b (Exoplanet.eu)

Hottest known exoplanet is so hot, it’s tearing its molecules apart (Space.com)

A faint resemblance of Sun and Earth (Max Planck Institute for Solar System Research)

Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) (ESA)

Habitable Exoplanet Observatory (HabEx) (NASA Jet Propulsion Laboratory)

ESO’s Extremely Large Telescope

Thirty Meter Telescope

The Giant Magellan Telescope

NASA Hubble Maps the Temperature and Water Vapor on an Extreme Exoplanet (NASA)

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Transcriptions provided by GMR Transcription Services

Fraser:                         Starting to cast episode 573, exoplanet atmospheres. Welcome to Astronomy Caster weekly facts-based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always is Dr. Pamela Gay a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how are you doing?

Pamela:                        I’m doing well. How are you doing, Fraser?

Fraser:                         I’m doing great. Again, normally I talk about how wonderful the weather is. But actually, here the weathers gotten really nasty again. It’s gotten cold and raining and it’s been a pretty mediocre springtime and we’re stuck in doors. Although, amazingly they’ve decided that the virus is pretty much wiped out on Vancouver Island. So, at this point now no cases in two weeks, nobody in the hospital. We have no issue.

Pamela:                        So, clearly what you need is absolutely no more travel, no guest, no visitors,

Fraser:                         Yes.

Pamela:                        Isolate the island, lock down the borders.

Fraser:                         Yeah, shutting off the island. No one else can come to our island until that vaccine gets created. So, it’s like a mini New Zealand.

Pamela:                        So, we’re not so lucky here. We’re dealing with the fact that in the United States the Memorial Day holiday weekend lead to lots of people getting together. We’re having resurgent’s nationwide. Here in the small town I live in, we’ve had seven new cases diagnosed in the past week. But to get tested, you have to be so sick they’re gonna hospitalize you.

Fraser:                         So, definitely remain inside.

Pamela:                        Yeah. People in my life will not let me leave my house or leave my yard is the case, maybe.

Fraser:                         I think it’s important for us to just mention, as a Canadian, I’m watching what’s going on in your country right now with shock and horror. Yet, this is clearly a long time coming and I just really hope that this time around the people in power listen to the desires of the individuals who have had various levels of oppression. It is gone on for too long; it’s ridiculous to see this happen again and again. It’s great to see both the amount of protest and just a showing of support that’s going on. To see, I hope, people taking this seriously and enacting significant change.

Pamela:                        To put things bluntly, black lives matter and for too long they have been considered disposable by the police here in the United States. Where we see white serial killers, people who shoot up schools, getting carried away in handcuffs and given hamburgers. White supremacist allowed to fill our capital buildings while wearing sub machine guns with nothing happening. We have watched as children get killed playing in parks with water guns. We have seen a teenager walking home with Twizzlers get killed for no reason. Black lives matter.

Fraser:                         Yeah. This has been going on. It was just with the most recent event, it was just such a stark obvious, well documented example of a man being killed by police in real time. You got to see it.

Pamela:                        One in one thousand black men and boys in this country can expect to be killed by the police.

Fraser:                         So, you have our support in Canada, everyone who’s protesting. We will continue, I think to keep this in the forefront of all of our minds as we move forward. That’s it, let’s get on with the astronomy. So, not only have astronomers discovered thousands of exoplanets. But they’re even starting to study the atmospheres of worlds thousands of light years away. What can we learn about these other worlds and maybe even signs of life?

So, at this point now, of course, we know of more than four thousand exoplanets. I’ve lost count; it’s too many to count. It’s in the thousands, which is such great news. But all we know about the vast majority of these planets is it exist. It goes around the star every X amount of days, weeks, months. It might be this size; it might be that size and it might have more or less mass than the earth.

But we don’t know much and that’s kind of boring. What we really wanna know is there atmosphere, is there liquid water? Are there inhabitable zones, does it look like the kind of world that we could find life on? Finding and actually examining atmospheres is next level. So, what does it take for some kind of telescope to be able to examine the atmosphere of an exoplanet?

Pamela:                        What it takes quite simply is catching a planet in the process of passing in front of the star it orbits. Being able to get sufficiently high-resolution spectra both with the planet in front of the star and not in front of the star. That we can see what the planet’s atmosphere is when we subtract the stars light from the star plus planet’s light.

Fraser:                         Let’s talk about that idea of the spectra of the star and the spectra of the planet. So, what are you talking about there?

Pamela:                        So, anything that is giving off light is giving it off in some portion of a rainbow. This means that if we recreate the Pink Floyd cover art and shine light through a prism, we see it spread out into a rainbow. Now, it turns out different atoms will absorb light and admit light in very specific colors that are determined through quantomechanics and the exact temperature and energy configuration of that atom.

So, when we look at a spectrum and we see super imposed, on this rainbow, dark lines and bright lines, absorption, and emission lines. We can say this rainbow captured the absorption and emission of light from this specific set of atoms. With our sun, this allows up to get the composition of it’s atmosphere. With our own atmosphere we see lines that are getting absorbed out of the sunlight by water vapor in the atmosphere. Water vapors one of the most annoying things in our atmosphere that plagues spectroscopist. So, we can measure our own atmosphere with our telescopes.

Fraser:                         Right. So, you take the light from anything, a star, a galaxy, a planet, whatever you want. You break that light Pink Floyd style into a rainbow. You then blow that rainbow up so that it’s really big and you will see dark lines and bright lines in that rainbow. From that, that will tell you the presence of various chemicals. You’ll say, “Oh, there’s oxygen there.” “Oh, there’s water there.” “Oh, there’s uranium there.” Each element, each atom will provide a very specific signature.

Pamela:                        Yes.

Fraser:                         Now, you don’t know how much there is. You just know that it exists, right?

Pamela:                        Well, you can actually start to get in how much based on how much absorption or emission is going on. Each atom is contributing its own number of photons, but it can only do so much by itself. It’s through atom, upon atom, upon atom working together to absorb that light.

That we’re able to see the bright and dark bands that we see. Now, if you only have the smallest amount of oxygen in a planet’s atmosphere. We may never see absorption lines from that because there’s just not enough atoms to do the absorption. But if a planet that has an atmosphere rich in oxygen passes in front of it’s star. We will see that as deeper lines in the spectrum. So, we can actually start to get at ratios of how much of one atom or molecule is present versus another atom or molecule.

Fraser:                         So, then let’s talk about how this relates to studying the atmosphere of a plant. So, you’re taking this rainbow, you’re watching the star. You’re getting one rainbow and then the plant passes in front of the star and then you get a second rainbow?

Pamela:                        Yes.

Fraser:                         What does that tell you?

Pamela:                        Well, it tells you the two of them aren’t identical in their velocities and their compositions. So, all the little variations in the placement and depth or height of those absorption and emission lines. They tell us something different about the story. So, for instance we’ve all seen what happens when you turn on a blue light and a red light and you partially overlap the light. You see purple in the middle, you see the combination of the light from the two sources.

Well, this is exactly what happens when you have a planet passing in front the star. You have the light that isn’t going through the atmosphere and you have the light that is going through the atmosphere super imposed together. If you’re able to say, “Okay, I know what this light from the star is, let me subtract that.” That lets you see just what the planet and the planet alone is. It’s this secondary rainbow, which interestingly because the star and the planet have different velocities.

Is actually going to be blue shifted or red shifted. So, the individual, let’s say there’s carbon dioxide in the atmosphere of that planet. We’ll see all the little carbon dioxide lines shifted from any carbon dioxide. If it’s a super cool star, in the star’s light.

Fraser:                         Wow. So, that way you may know. Like if you see carbon dioxide you may say, “Okay, well we know there’s carbon dioxide. But we don’t know whether it’s coming from the star or the planet.” But it turns out, you can tell just by how that light is shifted. So, what are the instruments? What are the tools that astronomers use to study the atmospheres? Again, it’s blowing my mind that I’m even saying these words. What methods, what tools? In terms of telescopes, what observatories is doing the heavy lifting in studying the atmospheres of these other planets?

Pamela:                        Well, the old work horse that did this for the longest time and is sadly no longer with us is Spitzer. The first mission to really be able to say this world has an atmosphere and we’re looking at it. Was that telescope working in the infrared where the light from planets is brightest because planets are just warm little bodies. So, we emit the most light in infrared and stars are often fainter in the infrared. So, you’re dealing with less star light, more planet light and Spitzer was kind of ideal for this. JWST when and if it launches is gonna be even better.

Fraser:                         When it launches, not if.

Pamela:                        It’s 2020. You have to grant me the if in 2020.

Fraser:                         Possibly yeah, this is a year in flux. I’ll give you that.

Pamela:                        So, right now we don’t really have any big workhorses. Which is very frustrating for the test mission, which was designed to find these transiting planets and send the data to JWST for follow up. So, we’re having to get creative in what we do, and that creativity means working with a very large telescope and other telescopes here on the planet. To go as far into the infrared as our atmospheres going to allow us to go and use ground-based instruments.

One of the ones that’s proving itself best is EXPRES. EXPRES really earned a name for itself with the MASCARA-2 discovery. That’s an actual planet name, I have no idea what that acronym is. Where it started to be able to look at planetary atmospheres in detail to see what all is out there. Right now, we’re at this really cool stage where we’re able to see hot Jupiter’s that clearly have lead in their atmosphere.

We’re able to find white dwarfs that have anomalous chemicals around them that point to gas giants getting shredded apart. Now, what we haven’t found yet is that world that clearly has oxygen and other signs of life. But side by side with all the folks that are working and doing these atmospheric measurements are folks doing atmospheric models.

Fraser:                         Wow, yeah. So, I guess that’s the next step then. Is we’re starting to see enough of these measurements of these atmospheres that they’re starting to get some confirmation of the models. Which they’ve been working on for a long time. We’ve been seeing just some incredible models of exoplanets. So, how do they think that exoplanet atmospheres would work? Because I guess, the point is that you would be looking for thing which we don’t have in the solar system.

Pamela:                        At a certain point, a planet is a planet and it’s gonna have an atmosphere that is suited to the chemical composition and temperature structure of the solar system it formed in. So, this means that we are looking at super-heated at gas giants. KELT-9B comes to mind. This is a gas giant that is so close to its host star that it’s surface facing it’s star is the same temperature as some stars.

It’s getting roasted from the outside, it’s not generating that heat from the inside. But when you’re looking at a planet like that, you don’t expect to see nice, happy everyday hydrogen and helium because it’s been ionized completely. This is where we start looking for things that have a few more electrons that they hold onto a little better. This is where we’re doing things like looking for sulfur in these atmospheres.  

Fraser:                         Yeah. I know there was one planet, it was a hot Jupiter. That we were seeing things like molten iron in the atmosphere, molten aluminum. Places where it rains titanium. Imagine clouds, but the clouds are made of titanium. It’s that hot, it’s so hot that every possible metal turns into a gas and forms clouds. Cloudy with a chance of titanium rain.

Pamela:                        This is showing our bias in what planets that we’ve found so far. Now, we don’t actually know what the typical planetary system looks like. We don’t know how often you have hot Jupiter’s versus how often you have little tiny worlds like Mercury that we’re unable to detect at this point in time. Currently, because it is easiest to find massive planets really close to stars. Most of the planets we have found are these massive planets very close to their stars.

We’re getting better, today we had the announcement of a two Earth mass planet orbiting a regular star. So, we have finally found a fat Earth, is what I’m gonna call this. It is beautiful in it’s Earthiness. It’s three thousand and some odd light years away, so we’re not getting there anytime soon. But the fact that we have found the sucker. At 85% likelihood, there’s still error bars.

Fraser:                         Right. Of course, it’s crazy to think we’ve found Earth sized worlds. We’ve found Earth sized worlds orbiting in the habitable zone of their star, it’s just that star is a red dwarf.

Pamela:                        These worlds are tidally locked, which gives them a different temperature profile.

Fraser:                         So, I guess, what would we be wanting to see? So, as that next round. For example, there’s a new telescope coming out. It’s probably going to be launching in 2028. The European Space Agency’s ARIEL Telescope. The Atmospheric Remote Sensing Infrared Exoplanet Large Survey. That is gonna be a telescope designed to study the atmospheres of exoplanets. It is going to be able to study the atmosphere of Earth type worlds around other stars. What will astronomers be looking for?

Pamela:                        Well, there are two teams that have put together atlases. We have a team from Berkeley, a team from Cornell. They have been working to basically map out the distribution of different kinds of planetary atmospheres we can expect. The group at Cornell in particular have been trying to map out what you can expect form Earth like planets at different stages in their evolution. So, here they’ve done things like look at our own historical record. Geological record of our planet Earth and said, “Okay, during the Cambrian period we were having a massive outgrowth of small forms of life. What did Earths atmosphere look like?”

“All right, so fine. What did it look like during pre-industrial civilization? What does it look like now?” Looking a myriad of different points in our planet’s history where we had methanogens. Where we had our normal now oxygen producing plants and saying, “Okay, here is at different stages in a planet’s evolution what you would expect it’s atmosphere to look like.” There are also massive astrobiology collaborations that are looking to say, “Okay, if your life has this technology, look for this.” So, if it has this technology look for this. Pollution is a great way to find life.

Fraser:                         Yeah. I like that idea that if you look back at the history of the Earth. You said there were what, six major moments or so?

Pamela:                        Yes.

Fraser:                         Each of which has a dramatic – like the time at the beginning when it was just being smashed by asteroids. A time when it was a carbon dioxide atmosphere similar to say, Mars. Then a time when the oxygen started to take over. You can see a very different signature in the atmosphere of planet Earth at each one of those times. Now, as we are able to actually examine these other planets.

You just map them over and go, “Okay, that is a desolate world that is being hammered by asteroids. That is a world that has cooled down but doesn’t seem to have any life on it. Well, what’s this? That’s interesting, we’re seeing a planet with oxygen in it.” It’s really important work to start doing. Setting up these models now, so that when we know of the atmospheres of thousands of planets, we can start matching them up.  

Pamela:                        It’s got to be so frustrating for some of these scientist that we’re working really hard to have their atlases come out coincident with when JWST would be going up. We’re expecting rapid fire return on investment as world after world was measured as they were found by test, and nope. So, now, we’re in this wait and see game where it’s just like, “We know how to do this. We could be doing this.” We have the publications on what to look for.

We have test finding the worlds. But at least we have VLT, the Very Large Telescope system of four-millimeter telescopes and a bunch of one-meter side dishes. It’s doing the best it can from here on the surface of our planet. Scientist are showing over and over that if you don’t give them the toys they want, they’re gonna repurpose the toys they have to do things no one expected. We are measuring iron ray.

Fraser:                         Yes, exactly. Obviously, there are like I said, there’s the ARIEL telescope that’s gonna be coming and it’s job is going to be to do that. There’s the HabEx telescope, which could be one of the next great observatories from NASA. Which will do that at an even greater scale. Then there’s the whole next round of the mega telescopes. The Extremely Large Telescope, the Thirty Meter Telescope, the Magellan Telescopes. Each one of these could be brought on board. Of course, James Webb. So, really, we’re really gonna shift that emphasis from, “Let’s see if we can find planets at all.” To, “Let’s study planets and see how similar they are to familiar planets.”  

Pamela:                        What I’m really looking forward to is right now we’re at this point where when we find an atmosphere it’s usually quite extreme. There’s actually been cases with these tidally locked worlds where we have been able to measure the differences between the day and night side temperatures and atmospheric compositions. Because as the small round world orbits in front of it’s host star, we’re actually seeing more of the day side as it goes into the transit. Then more of the day side again when it comes out of transit.

That effectively shows us sunset plus nighttime, just nighttime, and then sunrise plus nighttime. These minor variations allow us to do even more complex math where we’re now subtracting the stars light. Subtracting the light from the nighttime side of the planet and we’re getting left with morning or evening to be able to do complex, essentially meteorology of extreme worlds.

But right now, we’re only at the extreme worlds case. We’re only able to do this with these worlds around red dwarfs with these massive worlds snuggled up to any kind of star they feel like snuggling up to. We can’t yet do this with that fat Earth we just found, but we’re getting there.  

Fraser:                         Yeah, we’ll get there. Also, in terms of technique. When you are trying to look at the planet in front of the sun. You’re essentially back lighting the planet to be able to observe the atmosphere. You’ve got a very powerful light source.

Pamela:                        Yes.

Fraser:                         So, if those planets are farther away from the sun and you are actually attempting to block the light from the sun. It becomes a much more challenging job of doing spectroscopy on that planet when you aren’t observing the star.  

Pamela:                        That’s actually way less of a problem.

Fraser:                         Really? Okay.

Pamela:                        Yeah. So, the distance between the planet and the star doesn’t matter at all as long as the planet lines up with the disk of the star. That’s actually the bigger problem. If you have a star with a little tiny planet close to it, passing in front of it. It can have a whole lot of different orbits that still put it in front of that star. As you get further away, the number of orbits.

The amount of shifts you can have and how far up or down your orbit is that keeps you in front of that star. Well, you’re eventually gonna fall off the disk in a lot of cases. So, it’s much more rare to find these distant planets transiting perfectly in front of their stars. But if we see that transit, it’s still just gonna be sunlight through an atmosphere.  

Fraser:                         But I can imagine this future where these more powerful telescopes like the Extremely Large Telescope or James Webb or ARIEL. Are just observing this planet directly. They’re blocking the light from the star and they’re just observing the planet no matter where it is on it’s orbit. Whether it orbits above or below. Whether it orbits face on. I mean, each one of those kinds of observations will be fascinating. It’s just we need to get really good at blocking the light from stars and really good at observing faint planets.

Pamela:                        It’s the faint planets that’s gonna be hard. The first thing we’re gonna be able to do is go that three pixels. Those three pixels right there, those are a planet. But taking all of the light from those three pixels and then spreading it out into a spectrum gives you essentially no detectable amount of light. So, that’s where it gets tricky.

Fraser:                         Right. So, if you have one photon it’s really hard to turn that single photon into a spectroscopic diagram. You’re just like, “I don’t know.” But you do need a lot of light and the way that you get a lot of light on the thing that’s very faint is you have a very powerful telescope. You gather light for a very long period of time.

Pamela:                        These suckers are moving.

Fraser:                         Yeah. On a moving target. So, we’re going to see these give up their secrets very slowly.

Pamela:                        It’s true. But they’re gonna give up those secrets. Those secrets will be ours.

Fraser:                         I love it, I love your confidence, good. When do you think we will start, where we will know about the atmospheres of thousands of other planets? What’s your gut tell you? The way we know of thousands of other planets. When will we know the atmospheric composition of thousands of other planets? We know probably ten right now, maybe five, right?

Pamela:                        I think we’re doing slightly better than that. If you count the ones that we can say, “Oh, it has this one element we can detect. But we can’t tell you a whole lot more.” So, it’s gonna take a dedicated planet observer that has spectroscopic capabilities.

Fraser:                         Right. That’s the ARIEL, so 2028. So, probably it’s gonna do a thousand planets and do a large-scale survey. So, I guess, starting 2028.

Pamela:                        Exactly. That’s gonna be our new Kepler just for atmospheres.

Fraser:                         That sounds amazing. Pamela, do you have some names for us this week?

Pamela:                        I do. As always, we are supported by the generous contributions of people like you. Fraser and I both have personal Patreon accounts that you can support to support our own work. I am patreon.com/starstrider. Fraser, you’re universetoday?

Fraser:                         Yeah, universetoday.

Pamela:                        Then this show is supported, we pay our web content producer Beth Johnson, our video engineer Ally Pelfry and our audio engineer Richard Drumm. Through Patreon –

Fraser:                         Slash astronomy cast.

Pamela:                        Yes, slash astronomy cast. So, the people I would like to thank today. Again, we could not do what we do without the generous contributions of these people. We’re even helping to start offering benefits, medical benefits soon to our part-time employees. Because now more than ever, everyone deserves healthcare. So, your contributions if your already donating, thank you. If your on the fence, now is the time to get off the fence and give and be part of providing medical benefits.

So, this month we would like to thank Les Howard, Adam Anise Brown, Emily Patterson, infinitesimal ripple in space time. Add loves science, Gordan Derry, Bill Hamilton, Si Ni, Joshua Pierson, Frank Tippin, Alexis, Richard Riviera, Thomas Sepstrup, Steven Shewalter, Silvan Wespi, Jeff Collins, Marek Vydareny, Articfox, Brian Peacocks, Nate Detwiler, Matt Rucker, Brian Gregory, Ron Thorson, Dave Lacky, Kevin Nitka, Phillip Walker, Chris Scherhaufer and G-force184. Thank you all so very much for everything that you do that allows us to do what we do.

Fraser:                         Thank you so much everybody. Pamela, we’ll see you next week.

Pamela:                        See you next week.

Female Speaker:          Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative comments attribution license. So, love it, share it, and remix it. But please, credit it to our hosts Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website astronomycast.com.

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