Ep. 595: Planet Hunting (Updated)

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This is going to be another one of those evergreen topics, where we come back to again and again. Finding planets. Every time we talk about this now, it seems like we’ve gained thousands of new planets. Well, buckle up, new techniques will grow that by tens of thousands and even millions.

Next episode: Universe’s Background Noise

Download MP3 | Show Notes | Transcript

Show Notes

How many exoplanets are there? (NASA)

Radial velocity: Reading the wobble (NASA)
Transits: Planets found in dips in light (NASA)
Gravitational microlensing (NASA)
The rise of direct imaging (NASA)

Kepler and K2 (NASA)

TESS Exoplanet Mission (NASA)

Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) (ESO)

Detecting Planets with Astrometry (ESA)

McDonald Observatory

Hobby-Eberly Telescope (McDonald Observatory)

High Accuracy Radial velocity Planet Searcher (HARPS) (ESO)

SPECULOOS Southern Observatory (ESO)

Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) (ESO)

The Anglo-Australian Planet Search (UNSW)

Nancy Grace Roman Space Telescope (NASA)

Extremely Large Telescope (ELT) (ESO)


Adaptive Optics (CSIRO)

Thirty Meter Telescope

Giant Magellan Telescope

Telescope Names (xkcd)

Kitt Peak National Observatory

Nicholas U. Mayall 4-meter Telescope (NOIRLab)

Lick Observatory

Mount Wilson Observatory

The 200-inch (5.1-meter) Hale Telescope (Caltech)

W. M. Keck Observatory

Unveiling Rogue Planets With NASA’s Roman Space Telescope (NASA)


Transcriptions provided by GMR Transcription Services

Fraser: Astronomy Cast Episode 595: Planet Hunting Revisited. Re-revisited? Welcome to Astronomy Cast, our 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 CosmoQuest. Hey Pamela, how you doing?

Dr. Pamela Gay: I’m doing cold. I’m out here in the American Midwest at that southern edge of this great cold blight that followed in all of the snow. But it’s February. And this means that the crocuses are starting up, –

Fraser: Mm-hmm.

Dr. Pamela Gay: – and when the ground thaws, it shall be muddy season followed by gardening. How about –

Fraser: Yeah.

Dr. Pamela Gay: – you?

Fraser: Same. Here, it’s way better. It’s finally turning the corner. We’re coming into our spring, I think. February can be hit or miss, but I’m now able to go for walks with just a couple of layers, and not my big warm coat. I’m in Canada, so –

Dr. Pamela Gay: Yeah.

Fraser: – we all have big warm coats, and I don’t have to wear gloves. It’s good. But yeah, exactly. Same thing. I’m seeing the crocuses come up, I put a zillion tulips and daffodils into the garden. And so, I’m looking forward to that all popping up in just a few weeks now. So, I’m pretty excited about that actually. This is gonna be another one of those evergreen topics, where we come back again and again.

Finding planets. Every time we talk about this now, it seems like we’ve gained thousands of new planets. Well, buckle up. New techniques will grow that by tens of thousands, and even millions. And we’ll talk about that in a second. But first, let’s have a break. Okay, and we’re back. How many planets are there? I haven’t even checked today’s number.

Dr. Pamela Gay: I know that it will change between when we record, –

Fraser: Yes.

Dr. Pamela Gay: – when it is edited, and when it gets published. There are thousands of known exoplanets. We’re gonna go with thousands, and just stop –

Fraser: Yeah.

Dr. Pamela Gay: – there.

Fraser: Yeah. Well, possibly, new planets will have been announced during the show.

Dr. Pamela Gay: Yeah. And it’s gotten to the point that I don’t know about you, but I see a new paper announcing that it has found 40 new planets, and I’m like, “Meh. It’s a –

Fraser: Yeah.

Dr. Pamela Gay: – day ending in Y.”

Fraser: Yeah, exactly. Eh. It’s a lot, but you know. Okay, we’re at 4,341 confirmed exoplanets, 5,742 candidate planets, and 3,216 of which are planetary systems.

Dr. Pamela Gay: And this is as of 2:14 p.m. –

Fraser: Right, yeah.

Dr. Pamela Gay: – GMT-6 on February 5th.

Fraser: Yeah, mark the time, because it is gonna change. But obviously, we’ve talked about planet hunting, various techniques, and stuff. But let’s give people a quick overview, and then we’ll move into I think a lot of the exciting new developments, the new spacecraft that have come up, and some of the new techniques that will take us to the next level. So, how do we find planets right now?

Dr. Pamela Gay: So, there’s three main techniques. The first one, which was actually the first way that we found exoplanets around regular stars, is when you look at a star and you spread its light out into a rainbow, there’s a bunch of bright and dark patterns in that rainbow of light. And those correspond to the emission and absorption lines of different atoms.

And they have very precise wavelengths they occur at. But if that star is moving, that’s going to cause the colors of those lines to shift to the red and the blue, depending on if the star’s getting blue-shifted towards you or red-shifted away. Well, it turns out, that the gravity of a planet going around and around that star will move the star ever-so-slightly at the same rate that a human being can walk.

Fraser: Right. Walking speed. You could out-run, you could out-walk the speed that the star is being moved by the planet.

Dr. Pamela Gay: But –

Fraser: But it’s enough.

Dr. Pamela Gay: – we can measure that. Just like the police out there measuring your car by how it shifts the radar, or the laser light bounced off of it, we can measure the speeds of stars spectroscopically. So, that is the OG way of finding planets around regular stars. Now, beyond that, we can also look at the planet directly passing between us and the star, whereas that planet goes in front of the star from our perspective.

It will block out the smallest portion of light. And this doesn’t require a huge telescope to detect, it requires an extremely accurate and precise telescope to detect. And so, now, we have folks with beautifully set-up equipment peering at segments of the sky from the ground and from space, looking at hundreds of stars at once in some cases. Tens of thousands of stars at once in other cases. Looking for these faint little dips caused by planets.

Fraser: And I think that’s the direction that all of this is going. And it’s a very similar direction that all science seems to go, is you start out by just testing out a new idea. Can we detect the light being dimmed as a planet passes in front of its star? Okay, great. Can we do that for an entire region of sky simultaneously? Turns out, yes. That was Kepler, that’s tests, and it’s a very powerful technique.

Dr. Pamela Gay: And the other technique, which in many ways is absolutely shocking with what it finds, is we’ve talked about gravitational lensing on this show before, which is where you have some sort of a mass pass in front of a background object. And this mass’s gravity is able to direct more light at us than we would normally see.

Essentially, that background object suddenly gets brighter if you’re dealing with a moving system. And there’s the cases where we’ve seen a star, and then additional blips of gravitational lensing that is caused by a planet. There was a case of a moon. And it looks like – and we’ll talk more about this in a few minutes – there has been dozens of Jupiter-mass objects found just hanging out by their lonesome, wandering our galaxy without –

Fraser: Right.

Dr. Pamela Gay: – a star.

Fraser: And so, with this technique, the gravitational microlensing, the upside is that you can detect the planet’s very cen – with great sensitivity, you can detect very small planets, even potentially moons. The downside is you only get one shot at it.

Dr. Pamela Gay: It’s true.

Fraser: Yeah. Okay, well I think that sets the stage for the main methods that we have for detecting planets today. And then, I think we’re gonna talk about the new techniques that we have, improvements on those methods, but then also some of the new techniques that are coming out that are gonna take us as I said into the tens of thousands, potentially even millions of planets. So, we’ll talk about that in a second. But first, let’s have a break. And we’re back. All right, so we’ve set the scene. We’ve got the radio velocity method, the gravity yanking the star back and forth from the planet.

We’ve got the transit method, where the planet passes in front of the star, dims it a little bit. We’ve got the gravitational microlensing, where the foreground star acts like a lens to a background object, and you can detect the planet in the foreground star. Crazy techniques. So, first I’d like to talk about how astronomers are pushing these techniques that already work to the next level. ‘Cause there’s a bunch of new missions that have come out recently and are just about to launch. So, let’s talk about that first.

Dr. Pamela Gay: So, it’s not just the spacecraft. I have to say that.

Fraser: True, yeah. No, absolutely, you’re right.

Dr. Pamela Gay: Ground-based stuff is amazing. And I think what the Very Large Telescope with their SPHERE instrument is doing is to me in some ways the most exciting. They have the capacity, with these giant telescopes they’re using down in Chile, to actually image directly planets going around stars. Now, this isn’t currently how we find them.

This is how we follow up on them most of the time. There’s accidental discoveries made of planets periodically. But with SPHERE, there’s the ability to see the planet all by its lonesome, ‘cause the images are that high a resolution. There is the ability to see them due to the gravitational effects, the Doppler shifting of the spectra, and to combine all of this data, including polarimetry data, to get a sense of how it all fits together, how everything’s aligned and moving within the system.

Fraser: So, you kinda jumped the gun there, which is that I was – that’s an entirely new technique, the direct imaging. There’s astrometry as well. But as you said, there are some ground-based missions that are doing really sensitive radio velocity work. There’s a ground-based with the – oh, was it the University of Arizona? Oh, isn’t it your old stomping ground up on the McDonald’s Observatory?

Dr. Pamela Gay: So, McDonald Observatory is the University of Texas, and they are using the Hobby-Eberly Telescope to look –

Fraser: Yeah, that’s right.

Dr. Pamela Gay: – at planets.

Fraser: That’s right, and they have a powerful new radio velocity system, that can detect down to potentially Earth-sized worlds.

Dr. Pamela Gay: And this is another one of the cool directions that’s being gone in, is we have the Hobby-Eberly, we have the HARPS instrument on the 3.5 meter La Silla Observatory, there’s the Anglo-Australian Planet Search. That’s on a 3.9-meter Anglo-Australian telescope. And these various systems are looking at stars for year after year after year, which is something you can’t do with a space telescope that doesn’t last as many years. And with systems like the Anglo-Australian telescope that started its search in 2000, they’re trying to find the changes in motion of a star that indicate there’s a Jupiter-mass object at a Jupiter-like distance, –

Fraser: Right.

Dr. Pamela Gay: – and it’s the ability to detect things on many many year-orbits where we really want to see the completion of three orbits before we actually believe something is there.

Fraser: Mm-hmm.

Dr. Pamela Gay: This is the next big thing that’s going to begin to happen, because finally, enough years have gone by since we detected planets to have enough data to find this new kind of planet.

Fraser: Right. And so, you’ve got those ground-based observatories. There’s also new – there’s CHEOPS, for example, which is a –

Dr. Pamela Gay: Yes.

Fraser: – European Space Agency mission which recently just released a whole bunch of data about six planets in residence around a star. It had been discovered by tests, and then CHEOPS went and just analyzed the entire system to the nth degree, which is just fantastic.

Dr. Pamela Gay: And we’re starting to get better statistics, because we have so many more instruments. Gaia is another one that is a quiet planet-finder working in the background as it surveys star after star after star after star, periodically picking up on those dips that indicate a transit, and in the process, is helping us understand what is the population of planets out there? And it’s turning out that you pretty much – if you are a star, you have a planet. It’s kinda looking that way, –

Fraser: Mm-hmm.

Dr. Pamela Gay: – as long as you have the components. So, there are stars out there that are hydrogen, helium, not much else. And the environment that they were created in didn’t have much else. And planets are made of heavier things than hydrogen and helium. So, to get planets, you do need to have more material around. So, our Sun for instance. When you look at its light, you start to see iron, technetium, carbon, oxygen. All these heavier elements in its atmosphere are reflective of the heavier elements that went into making our planets.

Fraser: Right. And then, we’ve got telescopes like Nancy Grace Roman, which is gonna take microlens – it can do to I guess transit, to microlensing. What Kepler did for transiting, Nancy Grace Roman is going to do for microlensing.

Dr. Pamela Gay: And this is extremely exciting, because we’re so far with microlensing technique at the tens of planets. More like 10-ish planets that have been found this way. And there’s multiple groups. There’s the OGLE group that’s been doing this, there’s MOA, which is Microlensing Observations in Astrophysics, there’s the PLANET survey which is Probing Lensing Anomalies NETwork. And these systems are pointed either at the wealth of stars in the center of our galaxy, or out towards the nearby Magellanic Clouds.

And they see the transits, and then they wait in hopes that as the two objects separate, we’ll be able to look at the object that has the planets that were observed with the transit. You can only see so much from the earth. Nancy Grace Roman from space – this is the mission formally called WFIRST – is gonna be able to follow up on many more stars without the complexities of an atmosphere to contend with, and hopefully rapidly increase the number that are found this way.

Fraser: All right. We’re gonna go a little more into some of these other techniques, as well as the one that you already started to talk a bit about, which is the direct imaging, which is incredibly exciting. But we’ll talk about that after the break. And we’re back. So, Nancy Grace Roman gets us probably thousands of planets using microlensing. Gaia which you mentioned, which is using the astrometry method, where you’re seeing the star – you’re seeing the radio velocity, but now you’re seeing it from the face on.

Dr. Pamela Gay: It also picks up on transits. It’s just harder to explain astrometry, so I’ll try.

Fraser: Yeah.

Dr. Pamela Gay: So, the transiting method catches the planets that pass in front of a star and dim the light. Because planets aren’t just bright. So, you put them in front. But the amount that it dims the light is kind of like a moth in front of one of those giant spotlights they put out for movie premieres that you see up in the sky. That moth can only block so much light. That difference in the total output is hard to see, but it’s possible. Now, at the same time, if you have a star, and you have a planet going around it in the plane of the sky, we may not be able to see that planet directly. But as it goes around, it will actually ever-so-slightly cause the star to move in the sky relative to all the stars around it.

Fraser: Right.

Dr. Pamela Gay: Gaia can see that.

Fraser: And so, if you saw all the stars, they would be just making these tiny little circles in the sky.

Dr. Pamela Gay: And the more planets you put around a star, the more complex it is to see these kinds of motions. The bigger the star, the less of an effect you’re going to see. So, techniques like this are really good at finding small stars with a couple of massive planets, or just one massive planet, because that tug of war is easy to see in the sky.

Fraser: Right. And, I think so far, we’ve only had a couple of big updates from the European Space Agency’s Gaia mission, mostly focused on stars. But the instrument, it has been gathering data on planets potentially. Or I guess, the stars moving thanks to planets. And I think we will see eventually tens of thousands of planets pour out of Gaia just as a side-effect of this incredible mission.

Dr. Pamela Gay: And this is another one where every extra year the mission is in space, every extra year it’s gathering data, the more likely we are to see these events. And in order to say that we definitely saw an object, we ideally want to see three at least events, so that we have the time separation from event A to B, and separation from B to C. That starts to hint that there is something there.

Fraser: Mm-hmm.

Dr. Pamela Gay: Now, in addition to that, you want to either confirm it via another method, or just keep watching until you’ve seen so many events that you can no longer deny that it’s there. And with Gaia, it’s going to be finding things that we aren’t going to see any other way necessarily. If a planet is going around in the plane of the sky, we’re not gonna see the Doppler shifting in the spectra, we’re not going to see the eclipse of a transit. So, it’s going to have to be the year after year after year, catching multiple orbits, that allows us to finally say, “Yes. There’s a planet here.”

Fraser: And, the transit method and the radio velocity method, they only let you see about one percent of the planets that are out there. They have to be perfectly lined up. In theory, astrometry will show them all. It’s just the level of sensitivity that you need to – there are gonna be limits. But it doesn’t matter the angle. So, let’s move on to the direct imaging method. You talked about the SPHERE instrument. This is really just a prototype. What’s happening on SPHERE is a test run of the real machine which is coming with the Extremely Large Telescope.

Dr. Pamela Gay: And even before then, they’re gonna be updating SPHERE to the ZIMBOL/CHEOPS upgrade. And the idea is, with these massive systems, that have the ability to flex the mirror in reaction to what’s going on in the sky, so that you can essentially erase the atmospheric effects. With these instruments, you precisely focus, and you put some sort of a blocking plate in front of the star so that its light isn’t getting to your detectors, not reflecting around in your instrument. And with the star blocked out, in this extremely precise system, you can see the little planet off by its lonesome and doing its planetary thing.

And this is kind of amazing. And this is getting done especially in combination with radio and infrared observations in young planetary systems, allowing us to make out these hot planets still forming in the eddies of dusty systems that appear in OMA. It’s starting to become many telescopes working together to try and see how it is solar systems form, by looking at the ways all the materials mix. And each new discovery teaches us we really have no idea what’s –

Fraser: Mm-hmm.

Dr. Pamela Gay: – going on, which is exciting and awesome.

Fraser: Yeah. That’s the best. With the SPHERE, they also have a machine attached to the Very Large Telescope called the ESPRESSO.

Dr. Pamela Gay: Yes.

Fraser: And it’s job – I just wanna show the level of detail here. So, its job is to detect the polarization of the light passing through the atmosphere of an exoplanet. And so, essentially, the light bounces off the star, it gets polarized by the atmosphere, and it creates this signal that this ESPRESSO instrument attached to the Very Large Telescope, using SPHERE as a chronograph, can detect. And suddenly, the star disappears, and you’ve got these little planets orbiting around it. Just absolutely stunning. But the next generation of the telescopes, the big ones, the Thirty Meter Telescope, the Magellan Telescope, the 39 meter Extremely Large Telescope, this will be the era of directly observing planets.

Dr. Pamela Gay: With badly-named telescopes.

Fraser: With badly-named – no way! I love the Extremely Large Telescope. It’s just right on the nose. I like it.

Dr. Pamela Gay: See, we used to have telescopes like the Mayall, the Lick –

Fraser: Lick, yeah.

Dr. Pamela Gay: Mount Wilson. And now it’s Very Large Telescope. Extremely Large Telescope. Overwhelmingly –

Fraser: Over – yeah.

Dr. Pamela Gay: – Large Telescope.

Fraser: Yeah, there is an X – we’ve talked about this – there is an XKCD cartoon that goes into this and makes fun of this idea. I just wanna go right to the big one, the –

Dr. Pamela Gay: Yeah.

Fraser: – Extremely Large Telescope. What will we be able to see in the Extremely Large Telescope?

Dr. Pamela Gay: If it works as everyone hopes, we will start to be able to see Earth-like planets around Sun-like stars. And currently, we can’t see Earth-like planets around stars that big. And it would be exciting to find our siblings out there among the stars.

Fraser: Yeah, so we will be looking at other Sun-like stars, we will be observing the atmospheres of the planets that are going around them, looking for those biosignatures, looking for those signs of life. It’s interesting, I read a paper about a year ago or so where they were estimating if you just run the curve of how many planets and how quickly we’re finding them, it’s this growth curve. And if you just continue on this exponential path, by 2050, we’ll probably know of about 30-50 million planets.

Dr. Pamela Gay: And one of the things that broke me is as I was reading about the results of free-floating, no star in sight, Jupiter-sized planets that are getting found with gravitational lenses, they estimate there are as many between the stars as around the stars.

Fraser: Yes. I’ve even heard maybe many many more.

Dr. Pamela Gay: Yeah.

Fraser: Ten times as many as there are stars? It totally changes our understanding of what the galaxy is made up of.

Dr. Pamela Gay: So, there are probably more free-floating planets – we don’t know how they formed. Anyone –

Fraser: Yeah.

Dr. Pamela Gay: – tells you they know how a planet formed, they’re lying.

Fraser: Yeah.

Dr. Pamela Gay: There are probably as many free-floating planets as there are planets around stars of this type, and there’s more planets than stars, because stars tend to gather up many at a time.

Fraser: It’s mind-blowing. Again, –

Dr. Pamela Gay: Yes.

Fraser: – it’s mind-blowing to think that in our lifetimes, we didn’t know of any planets, and now we know of thousands of planets, and we will eventually know of millions of planets. Just absolutely fascinating that we’re both bored and excited. Yeah, don’t show up unless you’ve got a thousand new planets to tell me about.

Dr. Pamela Gay: Or something weird. Do you have a favorite planet? I have a favorite planet.

Fraser: No. Earth.

Dr. Pamela Gay: So, for me, it’s KELT-9b. It’s a planet that was found by a team with Scott Gaudi around KELT-9. ‘Cause again, we’re boring in how we name things. And this particular star is super-hot and bright. And they looked, not expecting to see a planet. They saw a planet, and the outside of the planet is super-heated by the stars so that the outside of this planet is like the temperature of the sun.

Fraser: Yeah. It rains metal. That’s so metal.

Dr. Pamela Gay: It’s a bad place to be. But –

Fraser: Yeah.

Dr. Pamela Gay: – it’s awesome that a planet was able to form.

Fraser: Well, thank you Pamela! Very interesting! I can’t wait for us to dig into these, but at a thousand at a time now. We’ll have to definitely –

Dr. Pamela Gay: Yeah.

Fraser: – compress the show. Do you have some names for us this week?

Dr. Pamela Gay: I do. So, as always, we are brought to you by our wonderful patrons at patreon.com/astronomycast. While our show may be going out on television, we don’t have commercial sponsors yet. We don’t know when we will. And so, we rely on you to allow us to pay Ally, to pay Rich to get Beth putting show notes up, to get Nancy wrangling everything. She’s our project manager. And you guys make this possible.

And this week, I would like to thank Robert Wenger, Joshua Adams, Catherine McCabe, Jordan Young, Burry Gowen, Burko Roland, Jeannette Wink, Aurora Lipper, Joe Hook, David, ACUT-Patron, Andrew Poelestra, Brian Cagle, David Truog, Robert Wenger, Venkatesh Chary, TheGiantNothing, Ben Lieberman, William Baker, Laura Kittleson, Robert Palsma, William, Joe Hollstein, Jos Cunningham, Les Howard, Paul Jarman, and cacoseraph. Thank you all. Thank you so much.

Fraser: Thanks everybody! And we’ll see you all next week.

Dr. Pamela Gay: Buh-bye!

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