Astronomers have discovered thousands of exoplanets, revealing entirely new types of worlds that we don’t have in the Solar System. It is enough to start getting a rough sense of what kinds of planets are out there. What’s the big picture?
Show Notes
- Our World in Data has exoplanet data!
- Radial Velocity method is the OG way to find exoplanets
- Transits are now primary way to find planets
- Gravitational lensing also works to find planets
- Kepler and TESS are the current planet-finding awesomeness, but Roman is on the horizon
- Looks like most stars above a certain metallicity (10% the metallicity of the Sun or more) have planets!
- We are now measuring atmospheres of planets, but it’s early days… Need a 2nd JWST, please!
Transcript
Human transcription provided by GMR Transcription
Fraser Cain:
Astronomy Cast, Episode 723, Exoplanets by the Numbers. 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. I’m the 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 are you doing?
Dr. Pamela L. Gay:
I am doing well. It is the American Astronomical Society meeting this week. I am attending virtually and tweeting out as much news as I can on threads. I have no clue what that’s called.
Fraser Cain:
Threading.
Dr. Pamela L. Gay:
I’m loving the science. I have no idea.
Fraser Cain:
Yeah. Before the show, we were talking about just how crazy last week was with Starship and Starliner and Chang’e 6 –
Dr. Pamela L. Gay:
And Unity.
Fraser Cain:
— and Unity and Hubble. Then this week it’s the American Astronomical Society. So, we do not get a break. It is just mayhem last week, and mayhem this week, but hopefully we’re gonna be able to sort of get as much science and space news out as we can handle. So, if you wanna support the work that we all do across all of our platforms, definitely go and join our Patreons. We’ve got the Astronomy Cast Patreon, you’ve got the CosmoQuest Patreon, and of course the Universe Today Patreon. So, that’s a way for you to directly support all of this independent space news education that we do across our platforms.
Dr. Pamela L. Gay:
It’s true. We hope to see you there.
Fraser Cain:
Astronomers have discovered thousands of exoplanets revealing entirely new types of worlds that we just don’t have in the solar system. It’s enough to start getting a rough sense of what kinds of planets are out there. What’s the big picture? All right. How do you want to approach this? So, we’re gonna like statistically speaking, numerically speaking, let’s get a sense of what’s actually out there in the Milky Way as we understand it now.
Dr. Pamela L. Gay:
So, I think my favorite place to start is looking at how things were detected when we did our first episode of Exoplanets back in 2006.
Fraser Cain:
Wow.
Dr. Pamela L. Gay:
Then comparing that to today. I actually found it on Our World in Data of all places. I love this website. I was not expecting it to have this much exoplanet data.
Fraser Cain:
I didn’t know that either. That’s amazing.
Dr. Pamela L. Gay:
Yeah. It’s a great website.
Fraser Cain:
I use it for like COVID and things like the crime statistics.
Dr. Pamela L. Gay:
Right.
Fraser Cain:
Yeah, Okay.
Dr. Pamela L. Gay:
So, exoplanet data. Back in 2006, when we were first talking about this, the big thing was radial velocity detections –
Fraser Cain:
Yes.
Dr. Pamela L. Gay:
— where you’re looking at the gravitational pull and push in the line of sight of the observer by the planet going round and round and round of the star’s light. Back then there’d been 176 planets detected using the radial velocity method.
Fraser Cain:
Wow.
Dr. Pamela L. Gay:
We were just starting to talk about transits. At this point, 11 planets had been found via the transit method. That was it. Now, fast forward, there were only 203 total exoplanets known back then, 203.
Fraser Cain:
What are we now, 5,800, close to 6,000?
Dr. Pamela L. Gay:
So, the data I have is for the end of 2023, which I think at this point it’s fair to get it at the end of the year and just know it’s just growing exponentially. At the end of 2023, there were 5,438 exoplanets known of which 4,073 were found via the transit method. So, fully 4/5s are found via transits.
Fraser Cain:
Yeah, that’s the main way that we find planets now.
Dr. Pamela L. Gay:
There’s over 1,000 with the radial velocity method. However, the thing I didn’t know until I got to this was there’s been 199 microlensing detections.
Fraser Cain:
That’s pretty cool.
Dr. Pamela L. Gay: So, there’s like a lot of papers coming out very quietly that we just haven’t noticed as journalists.
Fraser Cain:
It’s funny because like I’ll look through the papers and I’ll see, you know, detection of a Neptune mass exoplanet. I’ll look at it and go, yeah, okay, fine. But what’s special?
Dr. Pamela L. Gay:
Right.
Fraser Cain:
Is it the puffiest Neptune? Is it a great explanation of something in the Neptune desert? No, no. Does it have thick hydrogen? No. Okay. You know what? I’m just moving on. I don’t have to… it’s not special.
Dr. Pamela L. Gay:
I don’t know about you, but if it’s not a press release, if it’s not in ADS and it’s not in Nature, I don’t notice it. It is accidentally dead to me because I can only look at so many places.
Fraser Cain:
So, I add Archive, right? And so I –
Dr. Pamela L. Gay:
That’s what I meant. ADS Archive.
Fraser Cain:
Yeah. Yeah. Yeah. So, you’re gonna get all of the planet announcements through those methods you just mentioned. The ones that are like considered really interesting by say the public relations officer will then get pulled from Archive or Nature or whatever. Then they’ll get a press release written about this planet, but there’s got to be something special about it. I have a slightly different preference, and perspective on what makes an interesting planet to me.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
So, I will layer that on top of what the press releases show. So, I’ll spot planets that are like, okay, that’s really weird. Like a circumbinary planet on a circumpolar orbit, right?
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
That’s weird. I don’t see a press release about, okay, that’s, you know, that’s the kind of thing that we’ll report on. However, a lot of them just like boring –
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
— another hot Jupiter found. Is it the fastest? No. Is it the most massive? No. Okay. You know, does it have the fastest winds? Nope. All right. Well, then we’ll just let that one slide.
Dr. Pamela L. Gay:
We’re at the point where there are papers that are like 144 new exoplanets confirmed.
Fraser Cain:
Yes.
Dr. Pamela L. Gay:
It’s like, okay, I’m not going to read your paper, but thank you very much. I’m glad to know. Well done.
Fraser Cain:
Did you find anything interesting in there? Yeah. What a –
Dr. Pamela L. Gay:
What a weird place to be compared to where we started.
Fraser Cain:
But we’re just getting started. I mean, that all this, as you said, it’s an exponential curve. So, we are moving up the exponential curve. The transit method is delivering tons of planets, but there are other methods that are just coming online now that are going to increase those –
Dr. Pamela L. Gay:
And telescopes.
Fraser Cain:
Yeah. So, let’s talk about that briefly. What are some of the methods and telescopes that you think, I know you’re gonna have to look briefly into the future.
Dr. Pamela L. Gay:
Well, so I’m gonna start by looking into the past.
Fraser Cain:
Okay.
Dr. Pamela L. Gay:
The thing that got me was Kepler had 2,774 confirmed planets. K2, which was a different… they made it work. It had 548 confirmed planets and there were another about 3,000 waiting to be confirmed. TESS currently has only 450 confirmed planets, but it’s a young mission. There are currently 7,000 candidates integrated into the archive –
Fraser Cain:
From TESS?
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
And 4,663 that are on their waiting to be confirmed right now list.
Fraser Cain:
Wow.
Dr. Pamela L. Gay:
So, this baby in the grand scheme of things mission is just out there chewing along and doing remarkable work. So, just TESS is going to more than double what we had after Kepler. Now we have Gaia adding in astrometric detections, which was something that I personally was like, well on you if we’re trying, I don’t think you’re going to succeed. That was my personal, like I’ve been listening to people talk about trying to find exoplanets around Bernard’s Star for decades. Now Gaia is out there are like, oh yeah, the star’s moving. We know there’s a planet.
Fraser Cain:
Mm-hmm. They’ve done a couple, but the thinking is that there could be thousands, maybe tens of thousands of exoplanet candidates in that data when they get the final data release for it.
Dr. Pamela L. Gay:
So, what this is doing is with the radial velocity and the transit, we can see things that are going around and around perpendicular to the plane of the sky. With the astrometric measurements, we’re suddenly able to see things that are going around and around in the plane of the sky. Between these two methods, sure, things at a 45-degree angle are going to be really darn difficult to spot. However, we’re essentially doubling the geometries that we’re able to start observing things at and that gets exciting.
Fraser Cain:
Yeah. I wouldn’t be surprised if when data release for from Gaia comes out, they announce the most planets ever seen. Since that data is in there, it’s just a matter of taking the time. In fact, Gaia, I think has finished its final observations or it’s finishing them up this year. So, it’s still gonna be a couple of years before that data actually gets released. It’s just a matter of using a lot of measurements to be able to get that. As you said, with the transit method, you’ve got to have your planet line up perfectly between you and the star.
Dr. Pamela L. Gay:
Right.
Fraser Cain:
Less than 1% of planets are in that. However, with astrometry, with Gaia, you just need the planet to be tugging at the star enough for it to make a little circle in the sky while it’s moving around. So, yeah, that lets you see the other 99% of the planets that are out there.
Dr. Pamela L. Gay:
They’re saying DR4, data release four is coming not before the end of 2025. So, we have at least a year and the final version is, so DR5 we’re looking at not before the end of 2030. So, there is still a lifetime of data coming.
Fraser Cain:
What about the direct measurement method? I mean, there have been planets discovered that way as well.
Dr. Pamela L. Gay:
Yeah. It’s not the easiest for many different reasons, although there is one planet, COCONUTS-2b, that amuses me to no end. It is a giant gas planet border lining on being a brown dwarf that is orbiting a little tiny red star. So, it just like is hanging out in the data being like, “Hi, I’m here. I’m a planet far, far away from my star. Take a good look at me.” It is six Jupiters in mass and was probably stolen by the star that it’s going around. It did not form there.
It definitely did not form there. So, it’s super far away from its star. So, because of this, it was discovered back in 2011. So, we have one super early example of an easy to see gas giant far, far from its star. Now we are starting to be able to see more regular, probably actually formed with the stars they’re going around, but it’s going to be the Nancy Grace Roman Telescope that is both my sadness and my hope.
Fraser Cain:
Why is there sadness?
Dr. Pamela L. Gay:
We didn’t get the chronograph we could have had.
Fraser Cain:
We’re getting a pretty great chronograph on it.
Dr. Pamela L. Gay:
We are but remember the sunshade that looked like the giant James Bond villain weapon that they were gonna orbit with it? That, had they launched it, would have allowed us to see Earth-like worlds. So, like I said, it’s my sadness and my great hope, because I remember what could have been. This is why I don’t look ahead of time at what missions are planning.
Fraser Cain:
It is going to be able to dim the light from the star to be able to reveal objects that are 1-100 millionth the brightness of the star. That corresponds to Jupiter around the sun.
Dr. Pamela L. Gay:
Yes.
Fraser Cain:
So, when they focus in on a star, they will be able to directly observe Jupiter’s in orbit around that star with Nancy Grace Roman.
Dr. Pamela L. Gay:
Yes.
Fraser Cain:
However, to be able to see an Earth, you need it to be 1-10 billionth. So, there are several orders of magnitude still required to be able to get there. However, with Nancy Grace Roman, this is the test platform to try out a bunch of the technology that will then get used in the habitable world observatory. So, I would say, you want to spacewalk before you space run.
Dr. Pamela L. Gay:
Yeah. Roman is the slowest-built telescope, it feels like. This is a telescope that it turned out the Air Force just kind of had hanging around. It’s no big deal. It was a spare and they handed it over to NASA.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
NASA has been chugging along, building instruments for it getting it set up. It’s not a Hubble replacement. We had initially hoped it would be a Hubble replacement. It’s working in the infrared, it’s designed as another survey scope. You probably know more than I do. You follow things that haven’t launched [inaudible] [00:15:08].
Fraser Cain:
I’m willing to pay attention to things that are gonna happen in the future.
Dr. Pamela L. Gay:
They just break my heart too many times.
Fraser Cain:
I know, I know, I know. Yeah, but at this point, I mean, I think we’re 2027, we’re still three years away from it potentially launching.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
However, not only is it going to find them, I mean, it’s going to be able to make these direct observations, but it will be finding planets through gravitational microlensing at a ludicrous scale.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
So, its primary job is to be staring at large swaths of the sky to help measure the concentrations of dark matter, dark energy, measuring positions, and the locations of galaxies. But because it’s gonna be staring at these things for so long, it’s gonna spot tiny variations in the brightness of all the little blips of the stars passing in front of other stars. Then the slight change is because of the planet that’s in tug with the star. So, you’re gonna get, again, as a just a side effect of the kinds of observations that it’s doing, probably tens of thousands of gravitational lensing planets. However, the problem is they’re one-time only.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
You get the observation and you’re like, well, that’s all. At least we can tell general rough categories.
Dr. Pamela L. Gay:
The LSST on the Vera Rubin telescope, so the large survey of space and time on the Vera Rubin telescope, is going to also catch lots more of these little blips. However, its cadence isn’t as good for microlensing. However, it’s going to be catching transits. So, building telescopes that do survey work that allow more people who don’t have to compete for time on the telescopes to do excellent science. We are also, by happenstance, creating situations that for those of us who love time series data, we’re gonna have time series data for days, years, and careers.
Fraser Cain:
By the exabyte, yeah.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
So, there was a paper that I had read and reported on that if you just take the growth curves of exoplanetary discovery and you just project that out into the future by say 2050, we should be in the millions of exoplanets known.
Dr. Pamela L. Gay:
It’s one of these things where we are increasing the distribution in how far they are away from their star that we’re able to see them. We’re increasing the allowed angles that we can detect them at. Also, with potentially the majority of stars above a given metallicity having planets, now not all stars. Globular clusters seem to be great voids of no stuff to build planets out of. However, it’s looking more and more like everything that’s within roughly a tenth the metallicity of the sun has the potential to have planets, and everything as metal-rich as the sun is likely to have a planet. Then you have all of the rogue planets and everything else. It’s planets, planets, planets, planets, some more planets. Would you like some planets?
Fraser Cain:
Yeah. Well, and those rogue planets, again, Nancy Grace Roman will do a great job in detecting the transits from the planets, or the gravitational microlensing from those rogue planets as they pass in front of stars. So, we’ll get a much better sense of how many of those rogue planets are actually out there and drifting around. Then you look at the missions like KEOPS. You look at KEOPS, which has been measuring the parameters of known exoplanets. So, we’re getting a much better sense of that. You’ve got these amazing ground-based observatories that are doing much better radio velocity measurements on exoplanets. It’s kind of believed that we’ll probably get almost into that Earth-based world orbiting on a sun-like star via the radio velocity method.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
We’re in orders of magnitude of that kind of velocity change.
Dr. Pamela L. Gay: There’s so many questions that I have. There is a known gap in mass around log 1.8 of Earth’s mass, so not quite 10 times the mass of the Earth. This gap, we think, is probably due to at a certain size you’re not big enough to hold on to a giant atmosphere and you just lose it over time. So, you might start with higher mass and then shrink. We don’t know for sure. We’ve talked about this in prior episodes, but this lack of super-Earths or tiny, tiny sub-Neptunes is like, is this real?
Everything we see seems to be real. We’re seeing the same phenomena in transit, in radial velocity, they’re just not there. So, hopefully, we’ll start to get these answers of, is this real? Is this just a fluke of what we’ve been able to find so far? Do they tend to lurk so far out that we haven’t been able to find them so far because they only exist where it’s cold? We don’t know.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
I want to know.
Fraser Cain:
Yeah. I mean, are super-Earths those mini Neptunes that have just had their outer layers blasted away?
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
Right?
Dr. Pamela L. Gay:
That’s what I expect the answer to be.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
I’m a stellar person, so I could be wrong.
Fraser Cain:
Then you layer on top of this, I mean, yeah, we’re starting to count up the numbers of hot Jupiters and various size exoplanets and noticing these discrepancies.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
Then also there are upcoming telescopes like Aerial that are going to start to catalog and characterize their atmosphere. So, Aerial, for example, its only job is to study the atmospheres of known exoplanets. We only have really measured the atmospheres of, like we’re back to the beginning of Astronomy Cast, a few dozen exoplanets have actually had their atmosphere studied mostly by James Webb, a little bit by Hubble. Aerial is gonna come along and look at thousands of them.
Dr. Pamela L. Gay:
What I’m loving is when I was in grad school, we were still working on the very careful high-resolution measurement of the atmospheres of stars. I have a paper on magnesium hydride ratios and isotopic ratios in stellar atmospheres. Now we’re seeing the same techniques that were getting used on stars getting applied to planets. There was a graph this morning in one of the exoplanet press releases that was like, I remember making graphs like that of stellar atmospheres. Now we’re using it to measure carbon dioxide, water, and sulfur dioxide in the atmospheres of planets using JWST. It’s amazing to watch the evolution and where the techniques that we’ve known for decades are now getting applied.
Fraser Cain:
How far away are we from knowing kind of what’s average and expected out there that this is kind of what an average planetary system looks like, distribution of planets, sizes of planets, which ones are in the habitable zone, which ones have atmospheres and things like that. Are we in striking distance of that or is it still a long way off?
Dr. Pamela L. Gay:
We are in striking distance of being able to say this is what planetary systems around certain size stars, particularly the small stars, look like.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
So, what you want to do is, as a function of star type, what are the planet types? Now, when it comes to understanding what are all of the compositional varieties, that’s gonna come down almost entirely to the JWST selection committee. So, JWST is not a survey scope. It is a competitively time-allotted telescope. There are certain things like the, I believe it’s the JADES survey that is looking to collect data that is a survey and it’s dedicated time. That’s gonna be publicly released.
However, if you’re out there saying, I want to look at these five specific planets, that’s competed time. You’re also competing against the person studying dark matter, studying what was the first galaxy to turn on, studying planet formation. So, how long it takes depends on how much time is allocated. JWST is deeply, deeply oversubscribed. We need Starship. If Starship’s gonna do something, give us another JWST, please, and thank you.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
We now know how to do it, build us a second one. That’s never going to happen.
Fraser Cain:
You can make it heavier and not so simple so it could be theoretically a lot cheaper.
Dr. Pamela L. Gay:
You still need a sunshade, but yeah.
Fraser Cain:
Yes. Or put it on the moon but that’s a whole other –
Dr. Pamela L. Gay:
No, that won’t help because the moon gets illuminated on all sides.
Fraser Cain:
Not in the permanently shadowed craters, South Pole.
Dr. Pamela L. Gay:
Okay. So, you stick it in a pole and give it a very limited field of view. But hey, it’s cheaper.
Fraser Cain:
Exactly.
Dr. Pamela L. Gay:
So, why not?
Fraser Cain:
I don’t know if that would be cheaper. That would be trying to land something on the moon and build it up and manage it would be probably more complicated than being in space.
Dr. Pamela L. Gay:
That’s true. Don’t let us guess prices.
Fraser Cain:
Yeah. Right. Yeah. It is exciting. I mean, when you think about how long we’ve been doing Astronomy Cast, and yet you just consider the dramatic change in how well we understand the exoplanet landscape out there.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
I never would have anticipated this is what we would know and yet this is how exponential growth works. So, it’s gratifying to see it.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
It’s one of those things that there has been a revolution in the way a certain type of science has been done. Yet there’s no point that I can go, here’s where everything changed. Here’s where we learned everything. Nope. It’s just been day after day, week after week, month after month, year after year. We’re just getting more and more observations, and more data. We’re learning more about the universe.
Dr. Pamela L. Gay:
Astronomy progresses at the pace of technology and spending on technology.
Fraser Cain:
Yeah, yeah. Awesome, well, thanks, Pamela.