Almost all the planet hunting has been done from space. But there’s a new instrument installed on the European Southern Observatory’s 3.6 meter telescope called the High Accuracy Radial velocity Planet Searcher which has already turned up 130 planets. Is this the future? Searching for planets from the ground?
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 366, Finding Planets with HARPS. Welcome to Astronomy Cast, your weekly fact-based journey through the cosmos. We help you understand not only what we know, but how we know what we know.
My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and director of Cosmo Quest.
Pamela Gay: I’m doing well; how are you doing, Fraser?
Fraser Cain: I am doing great. And I just want to give people another reminder; if you are listening to the audio version of this podcast and if you’ve got some compelling question about space and astronomy, we would love to answer it. However, you’ve got to do us a solid, which is that you’ve got to go and take that question and put it into the event page for the next recording of Astronomy Cast.
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Fraser Cain: Yeah. Yeah. Because it’s hilarious when we make the questions harder.
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Fraser Cain: So almost the planet hunting has been done from space. But there’s a new instrument installed on the European Southern Observatory’s 3.6-meter telescope called the High Accuracy Radial Velocity Planet Searcher, or HARPS, which has already turned up 130 planets. So is this the future, searching for planets from the ground?
So yeah, this instrument has actually been rolling for quite a while. I said it was just installed, but actually it’s been cranking since what? 2003?
Pamela Gay: 2004.
Fraser Cain: 2004. Yeah. So we’re close to a decade of ground-based planet hunting with the HARPS instrument. But let’s go back then and explore a bit of the history of just the way we search for planets right now, ground and space, and we’ll talk about why the HARPS is such a great idea.
Pamela Gay: So, we actually started this way. Everything’s coming full circle as it seems to always do. The first planets that were found orbiting other stars were found actually around pulsars using pulsar timing delays, but those are oddballs that we’re just going to ignore for the time being. So we know they exist, we acknowledge that, you do not need to send us email.
Fraser Cain: Because we get it – those were the first planets –
Pamela Gay: Yes.
Fraser Cain: – discovered around pulsars.
Pamela Gay: Yes. But then the first people trying to find normal everyday planets around stars that were potentially capable of supporting life and solar systems and planets as we know them, versus planets that have been shredded in the formation of a pulsar, these were found using high-resolution spectrographs and looking at sun-like stars that were fairly nearby.
This work was done at Keck, it was done at the 107-inch at McDonald Observatory. It was done at a variety of different places, and it was always done using a high-resolution spectrograph looking at a bright star, spreading the light out as much as possible and comparing how those lines slowly drifted in velocity as compared to a cell of gas located in the spectrograph as a comparison source.
Fraser Cain: Right. And so we’re looking for – in this case the astronomers are looking for the backwards and forwards velocity of the star caused by the gravitational interactions of the planet.
Pamela Gay: And initially we were only finding really big planets snuggled up closer to their stars than Mercury is snuggled up to our sun. And this was because we needed to have really big stars – sorry, really regular stars that were getting yanked around by really big planets. The bigger the planet, the closer the planet, both of these factors cause more movement in the star as the two objects gravitationally move around their center of mass.
Fraser Cain: Right. And these are the hot Jupiters that turned up as a complete and total surprise.
Pamela Gay: Right. And this is where 51 Pegasus was the first one, and we just kept going from there.
Now, the problem with this technique is first of all, spectroscopy’s a bear. It requires so much careful calibration to make sure nothing in your system is moving, nothing in your system is flexing, and the data reduction process is not an easy one. It’s just hard. It’s way harder than looking for your transit, which also requires very careful calibration, but –
Fraser Cain: But we’ve seen like we’ve all – we’re all quite familiar with I think what a spectrograph looks like. It looks like a rainbow –
Pamela Gay: Yeah.
Fraser Cain: – with black lines in it. But I don’t know if people are really – really understand where that picture comes from. Like what instrument – what is producing this rainbow and how are astronomers like seeing this rainbow.
Pamela Gay: So, there’s a couple of different ways that you can end up creating this rainbow. The most straightforward and simple way that you can actually create at home is you have a prism – some sort of a piece of cut glass or similar cut solid material light goes through. And different colors of light get bent different amounts as they go from the air through this medium – this glass, or whatever it is.
Red gets bent one direction, blue gets bent a different amount, but literally in the same direction. But with this bending, all of the light gets spread out. And then as it passes back into the air, you then send it on its way to the detector. In some cases you use multiple mirrors; you end up folding the light up in different ways, all of this to get it precisely focused on your detector.
Fraser Cain: And so really a spectrograph is a prism breaking up the light from a star as wide as they possibly can.
Pamela Gay: Exactly. And prisms are actually kind of a pain in the everything to use because they’re big, they’re bulky, you have to rotate them just right. And one way that we’ve worked to get away from using prisms, which can crack and get all sorts of issues, is we’ve moved to using what are called grisms. These are finely striated pieces of material that it’s the striations on them that, a lot like the surface of a CD or DVD will reflect the light out into a rainbow.
You can actually reflect sunlight off of a CD and end up with rainbows on your wall, or reflect a laser beam off of it and you’ll see a diffraction pattern on the wall.
Fraser Cain: And just do your own spectroscopy at home.
Pamela Gay: Exactly. It’s not quite as precise because it’s not like music creates a perfectly repeatable grism, but you can still get a sense of what’s happening. With most major instruments including HARPS what you’re looking at is you have some sort of a grism that spreads all of the light out. But the spreading of light out means you actually need often a room in the basement beneath the telescope.
So for low-resolution spectroscopy, some medium-resolution spectroscopy, you just send the light down the telescope like you normally would, and somewhere between the big mirror on the telescope and your camera, you spread the light out into that rainbow.
Fraser Cain: And so, is it literally there’s like some, you know some dark room somewhere where they’ve got this great big prism that’s being reflected on the wall in a – and a CCD that’s somehow picking up chunks of it, or –?
Pamela Gay: It’s not – that would be so much more fascinating. No. Actually, so for the highest res spectroscopes you take all of the light and you focus it into a fiber optic fiber, and you send it down into the basement, usually beneath the telescope – nice cool, temperature controlled room. And there you bounce it around in a way you cannot perceive with your eyeball because you’re taking the light from a star and you’re spreading it out over a large area. So, even for really, really bright stars you have to catch those photons for a lot longer than your eye would be catching them. So –
Fraser Cain: Right.
Pamela Gay: – we do that with a detector.
Fraser Cain: Right. You have long photons. Yeah.
Pamela Gay: But, yeah – so they spread them all – they spread them out all over kingdom come basically, in this room, mirror to mirror to mirror to mirror, lens to lens, grisms involved, prisms involved in some cases. These are very complex optical systems with dozens or more optical pieces, quite often.
Fraser Cain: Cool. Okay. So – okay, so that is – this is the planet hunter’s toolbox –
Pamela Gay: Yes.
Fraser Cain: – and the – this has been done from Earth and this has been done from space?
Pamela Gay: Not so much just because if you’re going to take up an entire basement on the planet Earth to do this, it’s not like the Hubble Space Telescope has a basement beneath the telescope that you can put your spectrograph in. So when we’re doing the high resolution, looking for planets using Doppler shift, we’re doing this from the ground.
Fraser Cain: Right. Okay. And so then of course the space-based method, you know a lot of it is – like thanks to Kepler is this transit method –
Pamela Gay: Right.
Fraser Cain: – and this is just this other way of finding planets. So this is – so I think this gives people a pretty good understanding of what the spectrograph is – how it works. And this is important because, you know this is what the HARPS instrument is.
So let’s talk a bit about the actual HARPS instrument and sort of where that came from.
Pamela Gay: Right. So the smaller the planet, the less its gravity – the mutual gravity between it and the star causes the star to perceptively move. When we first started looking for planets orbiting other stars using this Doppler shift, looking for the planets to pull – to mutually cause the star to go away from us as it orbited the system’s center of mass come towards it – towards us, towards the spectrograph as it orbited the system’s center of mass.
Originally we were looking for multiple meters per second kinds of motion. So imagine someone sprinting. The velocity of a human being sprinting was what we were expecting the stars to be doing in order for them to be detected here on the planet Earth. We would do a series of measurements, a whole bunch of stars would be involved, spectra, spectra, spectra, spectra, spectra, spectra, go back later, spectra, spectra, spectra, spectra, spectra, take repeated motions, and build up a light curve. Except it’s not a light curve in terms of changes in brightness, but rather changes in velocity.
So you look for the star’s velocity to be zero as it’s to the left or right in the plane of the sky of that center of mass to moving straight – sorry – as it’s directly in front of the center of mass, directly behind the center of mass, to be moving straight towards us, which is when it’s either to the left or the right, depending on the orientation of the orbit.
So you’d see this velocity change from moving towards us, moving away from us, not moving. Build that curve up over time. See it repeat over time. Look to see, are there spectral lines from another star in the system, is this a binary instead of a planet that’s causing this? Make sure that all the lines in the star are moving because if what you see instead – you have to worry about different chemical effects. This really doesn’t play too big of a role, but you still just double check everything’s moving.
With transits, you always have the fear, well this is actually the star pulsating, so what I’m seeing is some sort of an asteroseismology, some sort of an internal dynamics of the star. When you’re looking at spectral lines you don’t generally see one of them moving just because it can, you see all of them moving just because all of them are getting yanked around with the star. But you still want to check, just is there anything weird going on? Do you see changes in chemical composition?
That would be awesome because that would be a paper all by itself. But it’s a complicated process. I’m blathering.
Fraser Cain: Yeah, well, you – no. Now let’s get onto the question that I asked you, which was sort of how do, you know – the HARPS instrument; where did it come from and what is it?
Pamela Gay: So, HARPS came from the European Southern Observatory, it was built via a consortium and put down on the 3.6 meter at La Silla, so this is down in Chile. This is another one of those South American telescopes in amazingly dry high altitude locations. And 3.6 meters isn’t all of that grand of a telescope now that we live in this generation of 8-meter behemoths, but it’s big enough to catch – you’re starting to be able to do spectral work on fairly small size stars, which is actually important to what they’re doing.
And with the HARPS spectrograph, this is an echelle spectrograph, which basically means it folds the light up really nicely. Fiber optic system – with this particular system, they’re able to start measuring walking speeds in stars. So if you can imagine jogging through space, you can outpace the movement of the star because they’re looking at – well, basically one meter per second with effective precision down to once everything is calibrated and everything else – down to 30 centimeters per second kind of motion.
Fraser Cain: Wow. And so what is the capability then? I mean this is the motion, but what does that really let them discover?
Pamela Gay: Well, for looking at small stars it allows them to start finding super Earths around stars smaller than our sun. So HARPS is actually how the smallest planets have so far been found.
Fraser Cain: Just think about that, folks. Right? This is the instrument that’s found a lot of the smallest ones that we’ve seen. And do they typically have a name that people would recognize when they hear them?
Pamela Gay: See, that’s the problem is – is they’re going through and they’re looking at known stars, catalogued stars, so they’re looking at things out of the Henry Draper Catalogue, so it’s like HD 10180, and then you’ll see A, B, C, D, E, F. And that particular system goes all the way up to the letter H as you go through the planets. So you’re looking at things that have these existing catalogue numbers that just happen to have planets.
Fraser Cain: That’s like Eliza Catalogue and –
Pamela Gay: Right.
Fraser Cain: Yeah.
Pamela Gay: But to point out things that this system has done that people have probably heard about, HARPS is the system that confirmed back in October of 2012 that Alpha Centauri B actually had a planet. So this is where you get to Alpha Centauri BB.
Fraser Cain: Yeah. Let’s once again, folks, there’s a planet at Alpha Centauri B.
Pamela Gay: Yeah.
Fraser Cain: Closest – you know, one of the closest stars to the sun, they found a planet there.
Pamela Gay: And this is all made possible looking at the Doppler shifts. And what’s cool about the Doppler shifts is unlike what’s happening with Kepler, it’s not as reliant on very, very precise alignments. So when you’re looking for a planet to cross in front of its star, it either has to be snuggled up very close to the star so it has a high probability of crossing in front of that star’s face, or it has to be very precisely aligned so that its orbit just happens to nick across that star. That really limits what systems you’re able to see.
Well, with HARPS – well, yes, they are doing a lot of follow up on potential planetary systems that have been initially identified looking at transits. They can also go through and look at these nearby stars, like Alpha Centauri that we know don’t have transits, and say hey, we can see all of these other alignments where the planets are yanking around the star, but never crossing in front of them.
Fraser Cain: Right. And I kind of imagine the situation, right? I’m thinking about my engineering background now, I’m thinking about sort of the way we used to calculate various vectors and stresses on things, right? So in this case, you know you can imagine – yeah. If the star was – or the planet was directly in front of the star, very close to us, and it was pulling back and forth you could see that star really moving back and forth as the planet was going around it.
But if it’s at an angle, it still has part of its velocity that’s moving, I guess up, and part of its velocity is moving away from us because it’s, you know, it’s actually moving up and down at an angle, right? And so, with a really sensitive instrument, if you can calculate it, if you can catch that 30 centimeter per second motion, it might actually be moving 10 meters per second almost up and down.
But the fact is because you’re getting that little bit of motion that’s going backwards and forwards from our perspective, you’re going to see that in the star. And so it really just gives you a much wider range of stars. Theoretically, you could pretty much use that to find anything that’s not face on, right?
Pamela Gay: And what’s really amazing about stuff like this, is with transits you can sort of get at the we think that there’s multiple planets in this system based on the oddball periodicity of the orbiting planets, where there’s clearly effects of a secondary body that is yanking on it and changing the timing. But with the Doppler shift, it actually starts to become much more – once you’ve reduced all the data, it becomes much more readily apparent that, hey, this system has planets B, C, D, E, F, G, and H all lined up, all affecting the central star in highly repeatable ways that they can detangle with instruments like HARPS.
Fraser Cain: So HARPS is able to find the super Earths –
Pamela Gay: Yeah.
Fraser Cain: – going around some of the smallest – or around the red dwarf stars. And I know that one of the big concerns for a while was, so what? Red dwarfs are awful, awful places to live around. So you know, you don’t hope for life. But that’s not even necessarily sort of true anymore.
Pamela Gay: Well, it’s complicated as so many different things are. So if you have a planet around a red dwarf, what you have to worry about is when that red dwarf star is very young, it is an evil, nasty little x-ray flaring monster that is going to irradiate its entire solar system, and that’s just not pleasant to be around. And you also have to worry about – depending on where you are within that habitable zone, are you tidally locked to the star, which can lead to all sorts of weird convective cells and massive winds and all sorts of issues like that.
But the more we think about what does it take to get liquid water, the more that we learn about our own solar system, it’s possible that one of those planets that got completely irradiated early on had water and potentially other organic materials come to it later on via comets, via asteroids, via a variety of different means. It’s possible that you have things that are heated through binary planets that allow you to be further out so you’re not tidally locked, but still capable of supporting life.
There’s so many different ways to get that liquid solvent, which is what water is. And there’s so many different ways to get enough thermodynamic heat transfer to allow chemical reactions, which is what we rely on geothermal vents and the sun for. There’s more than one way to get at those things that are required for life.
Fraser Cain: Yeah. I mean one of the things that’s quite interesting is the mystery of hot Jupiters, right? How do you get an object that close to the star? And so there’s –
Pamela Gay: Without falling in.
Fraser Cain: Without falling in. Yeah.
Pamela Gay: Getting them to the star is easy, having them there and staying there is hard.
Fraser Cain: Yeah. Yeah. And so it’s one of these ideas that maybe there’s migration going on, and so you could have the planets out further in the solar system abandon the migration. And there’s an interesting piece of research that actually just came out this weekend. I’m not sure if you’re even aware of this, but people have done some research that actually – mini Neptune-sized worlds can have their outer atmosphere irradiated away by the star, such that it ends up being a super Earth when the process is done.
Pamela Gay: That would explain a lot of the planets that have had us confused lately.
Fraser Cain: Yeah. Exactly.
Pamela Gay: So that’s good work.
Fraser Cain: Yeah. And so it’s – so that’s one explanation for why you’ve got these super Earths around these red dwarf stars is because they used to be more ice giants, and they just had the outer layers blasted off. You would end up with one being tidally locked, so anyway – that’s the thing.
So, we talked – so anyway, we talked about the limits of where these telescopes can go. Now I want to talk about the – sort of the future because it feels like, you know with things like the over – you know, the very large telescope and then the – what is it – the overwhelmingly large telescope, the extremely large telescope, the – you know?
Pamela Gay: Well, it’s all a matter of what instrumentation they put on these telescopes. And here actually with the VLT, the Very Large Telescope, which is an existing system of multiple – I believe it’s 8-meter telescopes, they are in the process of commissioning an instrument called ESPRESSO. And it’s spelled E-S, and it stands for the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations.
And this is something that would potentially be capable, if all goes well and above spec – and I mean that’s the thing is they’re always looking for things to be a little bit better than spec. It would be able to detect planets like Earth around suns like our sun. The Earth exerts a 9 centimeters per second movement on the sun.
Fraser Cain: Yeah, crawling speed.
Pamela Gay: Crawling speed. And so if you can imagine a very anxious toddler chasing down the sun, that’s a possibility. And ESPRESSO is potentially going to have the ability to do this. Its requirement is 10 centimeters per second, it’s felt that they’ll actually probably be able to obtain better than that, but the requirement on delivery is 10 centimeters per second.
And because this is attached to so much larger of a telescope – 8 meters – it’s going to be able to look at fainter stars for smaller planets and more distant stars that are brighter, so you have something that’s – or more luminous. So you have bigger stars at greater distances, nearby stars that we couldn’t look at before because they’re too faint. We’re extending the types of stars we can look at and the bubble of space that we’re able to explore.
And so this is a very exciting instrument that is nearing completion, and I’m just like itching for it to start joining in the discovery.
And so between the original HARPS we also now have HARPS North, which is an identical instrument that is in the Canary Islands, and this upcoming ESPRESSO. We’re starting to get so much better at zooming in on the characteristics of the stars that have planets, the variety of planets.
You combine all of this information, the Gaia Mission that’s going to give us the very precise astrometry motions, so now you have very precise in and out of the plane of the sky motion from HARPS and ESPRESSO. You have very precise left-right, up-down in the plane of the sky motion from Gaia. We’re getting at amazing information on how the stars are moving under the influence of planets.
Fraser Cain: And there’s one, I guess – I mean as you mentioned, right, it’s amazing – it’s all coming together. You’ve got transits, you’ve got the astrometry that lets – you can see it side-to-side. You’ve got the radial, the Doppler methods; you can see how it’s moving forward and back. The last piece of the puzzle is coronagraphs, right? And that’s –
Pamela Gay: Not working on that right now so much.
Fraser Cain: Not yet. But what – this is the thing, where you block out the light from the star and you can actually see the planets and start to image these things directly. And I think you put next week we’re going to be talking about finding planets with Spitzer, right?
Pamela Gay: Yes.
Fraser Cain: And so this is part of the job of actually imaging these planets directly. Because this is something that Spitzer has done.
Pamela Gay: Yes.
Fraser Cain: And so next week I think we’re going to be – we’re going to go deep into both the work that Hubble and Spitzer have done to image these planets directly, and the future of what that looks like with them.
Pamela Gay: And what I really love about this theme is it really requires people on the ground, spacecrafts in space, instruments everywhere, all working together to build a complete picture. And there’s really room even for people out on their driveway with a 4-inch telescope.
Fraser Cain: Are we going to have our terrestrial planet finder after all?
Pamela Gay: No. No.
Fraser Cain: All we just had to do was just wait for a whole bunch of separate instruments and telescopes to all be built –
Pamela Gay: What it was capable of doing –
Fraser Cain: – together like a great big board?
Pamela Gay: No. No. We’re not going to get there. We’re getting to a different future.
Fraser Cain: Yeah. Well, it is a different future. And I’m almost about to say that it might be a better one. Instead of one monolithic observatory that does everything, we’ve got a whole series of incremental improvements across a bunch of different instruments and spacecraft that’s going to get us to the place of being able to find that holy grail, right?
Pamela Gay: Yeah.
Fraser Cain: Being able to smell the atmospheres of other worlds.
Pamela Gay: Yes.
Fraser Cain: That’s what we need to do. Cool.
All right. Well, thanks a lot, Pamela. And we’ll continue this conversation next week.
Pamela Gay: Sounds great.
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