Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 204 | John Asher Johnson on Hunting for Exoplanets

Episode Date: July 18, 2022

Recent years have seen a revolution in the study of exoplanets, planets that orbit stars other than the Sun (or don't orbit stars at all). After a few tentative detections in the 1990s, dedicated instruments in the 2000s have now pushed the number of known exoplanets into the thousands, enough to begin to categorize their distribution and properties. Today's guest is John Asher Johnson, one of the leaders in this field. We talk about the various different ways that exoplanets can be detected, what we know about them know, and what might happen in the future. Support Mindscape on Patreon. John Asher Johnson received his Ph.D. in astrophysics from the University of California, Berkeley. He is currently professor of astronomy at Harvard University. He is the founder and director of the Banneker Institute for summer undergraduate research. Among his awards are the Newton Lacy Pierce Prize from the American Astronomical Society. He is the author of How Do You Find an Exoplanet?  Web site Harvard web page Google Scholar publications Wikipedia Twitter

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I'm inviting you to join the best-sounding book club you've ever heard with my podcast, Earsay, the Audible and I-Heart Audio Book Club. Every episode, I nerd out with amazing guests and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and I-Heart Audio Book Club. On the I-Heart Radio app or wherever you get your podcasts. Hello, everyone. Welcome to the Mindscape Podcast.

I'm your host, Sean Carroll. The thing about science, as with many other intellectual areas, is that there are a whole bunch of interesting questions out there and the questions linger on, but the rate at which we make progress on different questions is highly variable. We can have a question sitting around for a very long time and not a lot of progress is made,

and then suddenly things change extremely rapidly. So there are fields of science, really tiny or really big subfields, that are undergoing tremendous revolutions, even as we speak. And one of these is the study of exoplanets, planets around stars other than our sun. When I was in graduate school, we didn't have any exoplanets. None had actually been discovered. These days, over 5,000 exoplanets have been discovered. So there's a lot to say about this whole science, not just what the exoplanets are, what their characteristics are, but of course we're going to care about the possibility of life on other planets. But let's not skip right

to the weird stuff about life and aliens and things like that. Let's get down and dirty. Let's ask, how do you go about finding exoplanets in the first place? Today's guest, John Asher Johnson, literally wrote the book on this subject. He's the author of How Do You Find an Exoplanet, which is sort of a semi-technical book. If you are happy with a couple of algebraic equations, you'll get a lot out of it. And in the book, he goes over all sorts of different ways because there's more than one method for finding planets around other stars. You can look at the wobbles of the stars, you can look at transits, little eclipses,

you can look at gravitational lensing, and so forth. We now have not just planets that we found with telescopes based here on Earth, but also missions on satellites that are dedicated to finding new planets. And as I'm recording this, we're in the process of ramping up the James Webb Space Telescope, JWSD, which will be really, really able to examine exoplanets with much higher precision than we've ever done before. So finding them is one thing. Studying them is yet another one. So I talk with John about where we are, how we got there, where we're going to go in the near future in the study of exoplanets. And it really is transformative. It really is both a combination of having better

technology to show things that we suspected were true all along. And also, as is a very typical in science, being surprised, finding out that what we expected was not exactly what is out there. There are a wider variety of planets out there than we initially guessed. I mean, to be fair to us, we only had the solar system as our data point. So there is a menagerie, a different kinds of planetary systems, and they're all over the place. We're just beginning, even though we have 5,000 planets, that's a very, very tiny fraction of all of them. So this is a new kind of science, a new science that we're beginning to learn at in an exciting new way. You can get it on the ground floor by listening to this podcast.

While I have you here, I will mention that we have recently launched a scholarship program, the Mindscape Big Picture Scholarship. If you want to learn more about it, go to bold.org, b-o-l-d-d-org slash scholarships slash mindscape. And so the idea is we're crowdsourcing funding, so you can. contribute. I've contributed. And what's going to happen is every year, at least this year, hopefully in years to come, we're going to pick one person who wants to have a little bit of help, going to college, to study the biggest question. So, you know, not applied stuff, not things that are going to be better in the short term for them and their families, although that's very important, the biggest, hardest questions of physics, philosophy, biology, neuroscience,

all of these different areas. So if you want to study that, you can apply for the big picture scholarship, and one winner will be chosen every year to get $10,000 to help defer their college tuition costs. Now, I say one winner will be chosen. That's only if we only get that much money, by the way of donations. We have made our first goal. We have 10K, so we will be giving out a scholarship this year. But we're continuing to raise money, so maybe we can help more than one person, or maybe we

can roll it over to future years. So please, if you have any interest, go to bold.org slash scholarships slash mindscape and contribute. I'd like to think that maybe somebody who is going to get this scholarship will be nudged towards asking these big picture questions. Maybe they'll discover a planet. Maybe they'll discover life on other planets. Maybe they'll figure out how life began in the first place.

I don't know, but we can at least put our money a little bit in a direction of making that happen. So with that, let's go. Don Johnson. Welcome to the Mindscape podcast. Oh, thank you for having me. So I got to say, when I was in grad school, we didn't have any exoplanets. You know, we knew that there was something we wanted to look for. And in fact, when I was in grad school was exactly when they first started saying, yeah, maybe there's some evidence out there.

But now there's thousands of them. So let's just put things in context. Give us the very broad overview of how far we've come in the last mumble-de-mumble years since I was at the rest of. Yeah. Yeah, yeah. Yeah, so I think things have evolved rapidly and often quite wildly by studying X-Fullo surprises. And so I guess maybe I can put this in the context of like what the paradigm was back then when planets were first discovered and then think about like how that has shifted. I think back then the idea of how of what star, sorry, what planets around other stars might look like was informed largely by our own. solar system. And so, you know, hundreds of years of noticing that the planets are a co-planar,

and they move in almost perfectly circular orbits, made it natural to believe that, like, okay, yeah, they almost have formed out of flattened, dust and gas. And that the big planets were further out and the little planets were closer in. That also made sense for that formation scenario. And so everybody expected to what, when you go, not looking for planets, if you ever did look for planets, you would expect to find things like our solar system. Right. And in 1995, it just turned everything on its head because the first planet that was found was about the same mass as Jupiter, but it had a three-day orbit, not a 12-year orbit, but a, you know, three-day orbit. And that was instantaneously, you know, like, who ordered that?

Where did that come from? I guess we better go to the drawing board. And I think it's just a science that grew up, changing the way that we think about planets and then ultimately our own planet and ourselves. And so where are we now? How many plants have we found roughly? I think we just crossed the 5,000 mark. 5,000 exoplanets. And is that steady progress or are there going to be sort of leaps when we get new technologies or new satellites or whatever?

Historically, there have been leaps when new technology. And I think it's really interesting with the field of exoplanets in particular is that it, you know, when you think of astronomy, you think about like getting to bigger and bigger and bigger telescopes. You're looking at things that are harder and harder and harder to see and study. And so, like, cutting edge of astrophysics has always been like, oh, what's the next big telescope? Oh, 30 meters. Oh, my God. But a lot of what's done with exoplanets is that as the field has matured and grown better at, you know, as astronomers have grown better at what they do, they're finding that we actually need smaller telescopes.

Let's get like, let's get 10 centimeters. That sounds about right. But really, the technological advance was not necessarily the diameter of the telescope. It was just the focus and the dedication of that telescope's mission. And so I think what we're finding, that's the way exoplanets has evolved is that we're in this era of dedicated NASA missions that do nothing else but this one way of looking at it. And that's what's led to these big discontinuity number of planets here. Does that imply that we kind of could have done it earlier if we had just put our minds to it? Yeah, yeah.

Possibly, I think, I guess the caveat there is that there was a certain threshold that needed to be crossed into detector technology. Oh, okay. Once that happened around 2000-ish, then it was, yeah, it was pretty easy to. And you mentioned that the, that first planet that Jupiter that, that, that was, that first planet that Jupiter that, has such a short orbital period was a little bit of a surprise. So now 5,000 planets in. Are we still surprised? Is the solar system kind of an outlier?

Or was that just sort of a selection effect? It was easier to find a big, close planet. Yeah, that's a great question. I mean, it absolutely is in a three-day orbit than in a 12-year orbit. You don't have to wait 12 years. There you go to see the orbit go by. That, if nothing else. But there's other reasons as well.

Like, the signal is just larger. Yeah, so yeah, it's definitely true that they were easier to detect, but they also had to be there. And the fact that their occurrence rate was not zero. Yeah. Instantly, like, you know, as a surprise. But yeah, as the years went on, I think for the first five years, Jupiter's were the kind of planet that were discovered. And there was a large collection of them.

And but then as the surveys ran for longer, they were able to see longer orbits go by. And there are examples of gas giant planet, multi-year organs. So hot Jupiter's was the thing because hot, just because they're close to the star, not in the intrinsicity about Jupiter itself. Yeah, that's right. Yeah, that's our very clever naming scheme for this class of planet is that, oh, it's right next to its star, it's hot. And it's the size of Jupiter.

There you go. And so what do we know now about that distribution? Is the solar system kind of typical? Well, I think the answer to that is like a lot of answered a lot of scientific questions is it depends. If you're thinking about coplanarity, we're seeing that there's a decent size sample, but we've also seen systems that have the planets wildly out of orbit. So in some ways, the solar system fits, but there's a whole class of planets that did. So you mean we found, so there are systems where all of the orbits of the multiple, planets are in the same plane, but there's also systems where they're not in the same plane.

That's right. Yeah. So you can think of it as like the grooves on a record. And so that's what I mean by coplanar. Yeah. But imagine if you like cut out the center portion of that record and then tilted it with respect to the rest of the record. And those are two different orbits. There are examples of us. Okay. And sometimes and there's also examples of misalignment with the spin of central star. And in the solar system, the sun spins in the same direction that the Earth orbits and all the other planets. But we found examples of planets that go backwards with respect to this. So we have these retrograde bits, not the apparent retrograde that we see like Venus and Mercury,

but actual retrograde that they're orbiting in the quote wrong way. And so you can find examples of planetary systems that share characteristics with our solar system. So our solar system is not a complete outlier. But there's entire populations of planets out there that, look absolutely nothing like this. And so if you ask like, how does it fit in the whole ensemble, it starts to look a little rare? Okay. I mean, that's fascinating because the solar system, of course, is the data point we have the most familiarity with. So there's some bias there. But it also seems kind of natural if we think about how planets are supposed to be formed,

that they would be in the plane and moving in the same way as the star. So is this radically revising our theories of how planetary systems get formed? Yeah, I mean, it indicates that the revision needs to happen. Okay. That is actually, we still don't really have any good explanation, like any solid, experimentally tested theory for how you get a retrograde planet. Okay. I can wave my hands with the best of them in describes some scenarios, but we don't know that.

Like, that's not understood. It's guessed. And it's done after the fact. You know, we saw the result of the experiment. We're like, hey, I have a great question for it. It's a little bit backwards. So, but we tell good stories about how those things are. And those stories are very different than the way that things happen in Earth.

We are not averse to the occasional waving of hands here on the Mindscape podcast. So I presume that some crazy gravitational interaction or something like that, or you capture by a flyby? I don't know. What would be your favorite hand-weighty scenario? My favorite scenario in the one that I've done, like, the most to test, I suppose, would be there's a gravitational, well, let's just put it this way. First of all, the Earth going around the Sun, it does so because the Sun is tugging on. It feels the Sun's gravity.

But as the Earth goes around the Sun, it also feels really tiny nudges from the gravitational poles of all the other planets. And the way that works. out for the Earth is that it's negligible compared to the pull of the Sun, and it doesn't really affect the long-term evolution of it, depending on what you mean, like, long-term. So it's stable, it's fine, and the interactions are not significant, but you can construct an orbital system that has a planet orbiting, maybe, let's say, at the Earth's sun, and then you have a,

a companion star because maybe the sun of that solar system is in a binary and that companion star sits out beyond it's out there right and what that does

it can actually set up this set of gravitational interactions where the orbit of the planet in response to the gravitational tug of that outer companion that other star it starts getting longer and more eccentric

so that it becomes less circular more of an oval shape and then it can oscillate between that and then a misaline state where it goes up over the poles of its and it stays perfectly circular. And so it can exchange inclination and eccentricity and the sort of like harmonic oscillators. Okay, so that's the first act of the story. It's a pretty good one.

Okay, good. Okay. And then the next act of the story, the planet starts feeling tidal force. Central star when it goes in on this highly eccentric orbit, it just like buzzes the star's atmosphere and gets in real close. and then the star is stretching it. And that drains energy from the whole system. And the consequence of that for an orbital system

is that it takes that long, wide orbit and becomes more circular. And then you end up with a hot Jupiter that's going up over the poles of its star in the other direction because of these gravitational interactions. Start the spring season off right with a new pair of Tukovas Western boots. Hand crafted in over 200 steps from genuine leather.

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massive sci-fi adventure about survival and science. And what happens when you wake up alone very far from Earth? I really had to make a decision because I caught myself getting that frog in my throat and starting to get teary as I'm narrating some of these sections. And it's like, okay, yo, yeah, yeah, yo, is this indulgent? And I really thought about it. I was like, no, at this point it would kind of be betraying the trust, the author and the listener have in telling this story if I don't go through it. But there's places

in this book that deeply emotionally affected me. And I left it on the mic. That's great. Because it served the story. People will say like, oh my God, I cried at the end. It's like, yeah, dude, me too. Listen to Eursay, the Audible and IHeart Audio Book Club on the IHeart Radio app or wherever you get your podcasts. So I guess the, actually, before I say that, let me remind myself, is it correct that the solar system itself is thought to be unstable? If we waited billions of years, various planets would get kicked out? Yeah, I think that's still believed that, like, the fact that we even settled into the specific stable configuration as somewhat was somewhat of a lucky draw. But yeah, and it's all like, you know, chaos theory type stuff.

Right. So these are simulated at different initial conditions that have led to unstable endpoints where we are now. And I guess I'm just like you, clicking this up after the fact, but the fact that there is a bias when we look for exoplanets for planets that are heavy because they have a big effect, and close to the star, because they're short periods, maybe those are the ones where it's more likely that some crazy orbital dynamics has been on. I think that's true, but I think that's countered by the fact that sensitivity of the search methods is such that we're complete to Jupiter's being Jupiter, you know, so it's not that we are more sensitive to those oddball planets quite often,

but that's not to the detriment of like missing the rest of the population. So we get a pretty holistic view. So when you talk about completeness, you mean that for the stars we've looked at, if there was a Jupiter out to a certain distance, we would definitely find it. Yeah. Okay, good to know. And the other obvious question is, roughly speaking what fraction of stars or what fraction of stars like the sun have planets it's a big fraction of that if i were to say like um there was uh one planet for every two stars i i could express that as like um the the planet occurrence point five it's half right so if it's one out of three i call it one third and if every single if every single star had a planet then it would be one

Okay, so the study that I did with a collaborator a few years ago, we found that that number is five. You know, there's five planets per star, which means that there's more planets in the Milky Way than there are stars in the middle. And pretty much, you know, every single planet, every single star that you look at, if you were to look at it in every possible way, you'd likely come up with not only one planet, but multiple planets. Because you could get a five to one ratio if some stars had a hundred planets. That's true. And most of them didn't. But you're saying it's not like that. It's more like most stars have a planet.

It's actually an excellent observation. And, you know, we're able to look at the populations in other ways that show that it's not, it's not that skewed. But it does have interesting ways of preferring certain outcomes. But yeah. And I remember in some circles, that fact that so many stars had planets was considered a bit surprising. Like, look at all those planets. But I was never surprised. On the basis of the one data point we have in the solar system, it makes perfect sense to me.

There should be plans. Was I too sanguine or other people too conservative? I'm with you on that. I'm with you. I'm also a human and you're also a human. And so it's like possible for us to hold, you know, like, oh, yeah, of course. On the one hand. And on the other hand, it's like, that's never been done.

And it just got done. Wow. So those two can like kind of act in intention. And I think that that's happened for the community. And it's that whole that there's also. part of, like, there's the paradigm before the first planets were found about, like, what, what solar systems should look like. But there's also just like the way of placing value on certain aspects of astronomy, what should be done in order to do good astronomy. And in the 1980s,

it was not, it was not, it was, it wasn't really considered possible to be a good astronomer and do something like, looking for planets. Are you kidding? Like, so, And that's something I think that scientists forget is that like we do hard selection process on like what is value. Yeah. And that comes about through a whole set of things. But just it's important to know back in the 80s like it just was not. It just didn't get funded. You couldn't get telescope time to do something like that. That's just what's the value? That is a great point, I think, because it happens with many things with supernova surveys, with gravitational waves, with various things where once you find them, It's obvious that you should be putting a huge amount of effort into doing this. But before the first example happens, there really is a lot of inertia and conservatism in the community.

And all the stuff is expensive. So you can understand why. But it makes you wonder what other things were missing. Yeah. I mean, I think astronomers are like all academics and most academics, like self-liberal. But in their actual, like in their actions, they're quite conservative in the sense that like there's an agreed upon, not only an agreed upon, like, answer to some suburb. of questions, but there's an agreed upon set of questions that one is allowed to ask. And asking what fraction of those stars out there have planets around them was not really

seen as a valid question. Even if you could go back in time and use the exact same appeals that we use based on the justification that we have now, it might not even go over so well. So in that sense, it was surprising that there was a planet there. It was surprising that that planet was so interesting. And oh, my God, all of a sudden you start seeing the shift in the way that people think, like, not only what questions they're asking, but like that that question is now allowable. That is, that question's okay. Yeah, I mean, I can absolutely verify your judgment in the 80s.

This was not considered respectable. And I do think that I would have voted that almost all stars have planets. But I'm not going to, I don't want to give anyone the impression that I was out there saying we should go look for them. Oh, you were publishing on this? I was not when I was an undergraduate. Again, I should have. This is a sort of thought out there for current undergrads. Like, what are the old folks missing?

That's a good question. Yeah, exactly. What are we missing now? And the other thing I want to get up on the table in terms of the population of exoplanets is that there are apparently planets that don't have any stars that are just in between the stars floating around. Yeah. Oh, man. I don't remember this firmly enough to know that I have it exactly right.

But I think the name that was given to this class of planet called Solivigant. which means a lone wanderer. I think one of the scientists, now we can't even remember which scientists it was so I can give them proper credit, but maybe they won't want the credit if I got the word wrong. But anyway, yeah, these lone wanderers

are out there in the Milky Way. And it's, there are theories. There are theories. But like, you know, how did this thing get orphaned from its star? Or do we not understand star formation well enough to understand that this might be like one of the possible outcomes of making stars? Well, I think it's because a lot of people don't have the background knowledge about star formation, et cetera. I mean, I would personally put a large credence that these planets used to be associated with stars at the moment of their formation and somehow got kicked out.

But is it possible that they just a planet formed in between the stars? Well, there was an undergraduate here who looked at the possibility, along with her advisor, James Guillajan, at In Gurma. They were looking at the question of whether you could, at the center of the Milky Way, this giant supermassive black hole. And I promise, this connects to plants. It sounds wild as a start. But, okay, there's this supermassive black hole that's a million times the mass. And around that black hole is a population of stars that are hanging out down there. form there or we don't exactly know, but they're there. And every once in a while, one of these

starts will get too close to the black hole and it'll get sort of spaghettified, stretched out into a long stream of hydrogen and star guts. And the question that Edin studied was, could any of the stretched out spaghetti-fied star turn into little populations of Jupiter's? Okay. And if so, if that was something that actually happened, then what would be the occurrence? And it's a really fun project. That's a really out there project. It doesn't fit neatly under what's accepted to this study

or what question's interesting to ask, but she showed it was at least plausible. Cool. So that question exists out there for any aspiring young astronomers who might get to graduate school and an advisor who really run loose. Or if not, they can do it surreptitiously whether advisor's not looking. That's always possible.

Or the paradigm might shift by that. Right, it could be the common thing. Could be the hot topic. You never know. I mean, my own experience was I was the world's expert at the cosmontical constant and dark energy before they found it. So that was not very exciting before they found it. But suddenly it was exciting through no fault of my own. That's right.

Yeah, you just had to find the right question to ask. I've been to be encouraged, but you actually can't ask it. Yeah, yeah, exactly. Anyway, okay, so let's think of. about how we detect them. I mean, that's really why we're here. You've written a book called How Do You Find an Exoplanet? I encourage anyone who is interested in checking it out, although it does have equations in it. We're not afraid of equations here on the Mindscape podcast, so that's okay. But so how do you find them? I mean, presumably the big obstacle is that stars are bright

and planets are dim. Is that an oversimplification? Yeah, I think that is an excellent way of putting it. And that difference in brightness also mirrored in the mass ratios. Like, Jupiter is huge by planet standards, but it's a little in 1,000th sun. Yeah. And so the, we're talking about very small signal in the presence of very high contrast. And so, like, let's take, for example, like, you want to do direct imaging of the planet. Yeah, perfect. The, The analogy that I once worked out, and I think it's the one I use in my book, is to, if you're trying to find the glow of a Jupiter-sized planet sitting next to its star from the Earth, and let's put that star at, like, proximate.

It's as close as stars get. The analogy is that I'm on a lighthouse with a cigarette lighter that I turn on, and your job is to see the light from the lighter against the glare of the lighthouse. But let's put you, the observer in California. let's put the lighthouse in Hawaii, and now we got the scale right. Okay. All right. So it's a challenge.

It's the contrast is one aspect of it. You nailed that. And the other is just the signals are so small. Right. The actual signal that you're looking for. Well, so you mentioned direct imaging. I mean, we don't need to go in order of plausibility. So let's us ask the dumb question first.

Like, can't you just take a really big telescope, use the best detectors we have, pointed at a star, and take pictures of any planets that might be around it? Yeah. In principle, yes, you can do. But there are obstacles that you have to overcome pull that off. And the biggest obstacle is this big, annoying thing called the Earth's atmosphere. Sorry, it perspectives everything, but like, for an astronomer, the atmosphere in the way causes a lot of heartache and headaches. And so even with like a really great optical design and a huge telescope at her and a really wonderful detector, you still have to stare, like, based.

Basically, it's like trying to see a freckle on your friend's face, and you're at the bottom of a pool, and you're looking up through the rippling surface, and you're trying to discern whether there's that little speck on their cheek or something. So you have to overcome the motion of the atmosphere, the obscuring effects of the atmosphere. And there are technologies that are allowing us to do that. As technology advances, this actually involves changing the shape of the mirror of the telescope in exact same way that the atmosphere being deformed above the telescope, which is a really cool engineering speed.

And so that's changing on microsecond levels. You have to change the mirror shape, the actual, you know, are you literally putting some servo motors under the mirror to work? Yeah, there's little tiny servos sitting underneath what's actually a very small-scale-down version of the main mirror. But unless you're moving a mirror around fast enough to cope with the atmosphere. And that's just like a huge engineering problem that you have to overcome first. And you have to rely on that thing.

And you have to understand the system very well so that you can still barely pick out the spec there. So is that a future prospect kind of thing that will be a big deal? Or is it just we can do it in principle, but it's never going to be the leading procedure? It's that latter one because right now, detection sensitivity, well,

And let's say like when imaging first started detecting planets, all they were seeing were baby planets that had just formed. And because they had just formed, they were still gravitationally contracting and glowing on their own, like little miniature stars. So there the contrast is evened out a lot because the planet's shining bright. But the issue with that is there's only so many places that we can see young stars. and those places are not nearby. Okay. But we're still, nonetheless, we're able to find planets that way. More recent advances since those early detections have brought us to the extremely massive planet,

very widely separate from their star. But again, you're just, in both of these techniques, you're limited by the number of targets. You get down to a certain sensitivity and you're really excited. In principle, you can find, but now you're just kind of limited by the number of stars that are close enough for that technology to really work for you. Is there some high-tech version of putting my thumb up to block out the bright thing so I can see the dark thing? Yep, that technique right there is called coronography. It was brought over from the people who study the sun.

Basically, a little spot that blocks out the light from the sun so you can see the corona and the flares and things like that that's often behind a corona. And we can use a similar type of thing, blurring out the light or blocking out the light. It's much, it turns out like the technical. details of what goes on with how that work. It's way different than the way your thumb kind of blocks it up. But the end effect is the same. We're trying to just push the light from the star down so that we can see what's around it. Right.

Okay. So if that is an obvious thing to try, but it's not the most effective, what in your mind is the most effective way to find these exoplanets? Because you found some. How many planets have you found? Is there a number on your CV? How many planets you found? I once did it.

I once counted it. It was close to 100. 100 planets. It depends on how. you like assigned credit and you know like if we only yeah these are teamwork kind of things right yeah these are teamwork things i've been on teams that have found well over well over uh yeah so the method like most effective would kind of implies that like we we have all of the money in the world that we can pour into every different technique and then we can like sum up like which one found more

planets but like you know where where resources have been invested and where it's been most effective in terms of like the outcomes is transit spike because and that's the method where you're looking for the planet to eclipse the light of its central star so that it has its orbit aligned in space just right so it passes in front of its star and eclipses it and they're called transits if it's a planet it's an eclipse if it's a star but the technique is the transit technique and that's found the vast majority of known planets. And in part, because that's what's used by the Kepler satellite, right, which is in a lot of this heavy lifting.

Yeah. It's also the detection method that can make do with the smallest telescope, which makes it well suited for putting it on a satellite, getting up above their Earth's atmosphere, and being able to see things really clearly. Maybe tell us a little bit about Kepler because it's no longer working, right? These things are a finite lifetime. Yeah, so the Kepler mission was a... It was a space telescope that went up in 2009 and was launched into what's called an Earth trailing orbit.

So it orbited the sun. But its orbital separation was just a little bit longer than the Earth's. And so it just sat out there in cold space as the Earth drifted away. And what was great about Kepler is that it was single-minded in its science goals. It just there were basically like no movable or interchangeable parts on that thing. It was like you it popped off the dust cover from the front mirror from the mirror and it was just that it just stared at space. So there was the Kepler field that the telescope looked at and it stared at it for well, the original mission was three years, but it went years past past its original mission. So it didn't scan the sky.

It didn't scan. It just stared at one like one sector of the night sky. And there were millions of stars in that patch of sky that Kepler could see. And that gave it lots and lots of opportunities to see transits. And it turned out transits by the thousands by the time it was done. You know what's wild? We can video chat from space. Order groceries just by talking out loud.

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But I'm betting also that the point is it's waiting for rare events. So it makes just as much sense to just stay on one star as to keep collecting new stars. Yeah, yeah. If you take a whole box of hot Jupiters and you dump them out into the Milky Way, and they all tumble out of the box, then only one-tenth of all of those hot Jupiter systems would have the alignment necessary for a transit. So the random orientation of all these stars and planetary systems and space is similar to, like, dumping them out of the box.

They're just randomized all over the place. So it's only 10%. And that decreases really rapidly as you move away from the central star. It decreases. It's one over the distance away from the central star. And so by the time you get out to the Earth orbit, it's only like 1%,

and it just keeps decreasing as you go further away. So you need to look at lots and lots of stars to get this chance alignment to work out for you. Right. So the chance alignment reflects the fact that you need us, the planet, and the star to be on a line with each other. That's right.

If the star orbits perpendicular to our line of sight, it's invisible as far as this technique is concerned. That's right. Yeah. So the larger the sample that you're looking at, the greater, you know, you'll just find more planets. Right. You have to get above a certain threshold. And could you see something as tiny as the Earth, again, something as big as the Sun with this kind of technique? In principle, Kepler, I mean, Kepler was designed to be able to detect the Earth.

Earth around the Sun as viewed from far away. Okay. So we could do it. There are a number of systems that bore a lot of resemblance to the Earth's sun system in different ways. Somewhere orbited closer in. Sometimes the planet was a little bit larger. But, you know, basically those types of systems are confirmed to exist. But the majority of the planets that were found were closer to the star where they were more likely to pass in front of it.

Sure. And also, how long did Kepler? look you said about a decade? I can't remember the exact duration of K2, which was the mission that came after the original Kepler. So it went for three years, and then I got renewed for three years, and then I got renewed for three years.

And it just kept on chugging. It wasn't supposed to have enough fuel. It broke down. It just actually broke down because it has these three little reaction wheels that keep it pointed at that same place, These little gyroscopes, they keep it stable, its angular momentum.

And one of, they actually had four when it launched. The first one failed right away. The next one failed after about a year after the extended mission. And Kepler was just like starting to wobble. Wobble. So what they figured out that NASA engineers figured out is that they could, the solar panels on the back of it or like the roof on a house, they come up to a point.

And so what they did is they oriented Kepler, the telescope. into the stream of photons coming from the sun. And the exchange of momentum from all those photons breaking over the solar panels, like it was the bow of a boat, stabilized it in that third dimension. Wow. And then that was the K2 mission. And that just went on. Yeah, so I think it all told it must be about a decade.

They are clever, those NASA engineers sometimes. Yeah. But I'm glad you brought that up because it's a reminder, again, people who don't do this for a living, the precarity of being a scientist working on one of these missions. You can't go up and fix it. This is not in Earth orbit, right? Like you said, one wheel broke instantly,

and if two had broke instantly, we'd have been in trouble. That's right. Yeah. That's why space missions are so expensive, in part is just because of the precision that there's just no room for error. Not to mention that you have to put it on this gigantic

exploding tower and send it up into a room. sort of it. And there was a, there was a, the test mission. Is that a follow-up? So the test mission was the follow-up. And you asked, why don't you scan? And test answered that question and said, yeah, why not?

And so let's scan. And so what test does is it trades off the duration of the survey. So Kepler went for, you know, years and years. Test, all it, what it does is it scans a big stripe of the sky 30 days at a time. And then it moves over to the next stripe. until it kind of does, it paints out the globe around, you know, the night sky around the earth. The detectors, what's really nice about that, though, is that the detectors overlap near the pole as it's pirouretting. The parts that look up close to the center of its motion overlap from sector to sector.

And so you get up to, I think, a year near the poles. But almost for the rest of the sky, you get 30 days. And so what you do is you get to see lots more stars, but you see them for shorter duration. So it's a shallower, broader survey than Kepler was really narrow and deep. And did it, but it found a bunch of planets? Yeah, that's one that, you know, took it from the, you know, a couple thousand that came out of Kepler into the 5,000 today. Okay. And that was also the transit method.

They were looking for the transit method. They were looking for a little decrease in the brightness of the star. Yes, yes. They're using the transit method. And is this teamwork once you find the, planet candidate with one of these satellites? Do you follow up with telescopes here on Earth? Yeah, it is absolutely a team effort.

You know, NASA designs, flies, and maintains the mission. And they also do a lot of, you know, they're the ones that take the signals that are being sent from the satellite or the space telescope. And they translate that into actual usable data that we as astronomers can use. And so then they pass that on to us. And then that is just, that's just data, right? So you can collect a lot of data and you can get almost no information from it if you don't look at it well enough, right? Or if you don't use the right way of looking at it.

And so then usually where an astronomy graduate student steps in is figuring out how do I take the data that's handed in these files from NASA and how do I turn that into usable information. Now, for Kepler in particular, the engineers are very involved, even at that level. They're really helpful in helping us just really get a solid understanding of like, okay, when this thing is lighting up in this way, and you're getting this much, many counts in this little sector, you can trust that.

This means this and that. And, you know, they understood the instrument. And they, so they allowed us to make reliable transitions from just raw data into usable information. But then there was a whole extra step where you want to make sure that the thing that looks an awful lot like the signal caused by a planet, like that little dip in light for a transit, that can look like a planet but actually be a different astrophysical phenomenon. So, for example, you could have a star that's passing in front of its star, but only their tips graze. And so it's only a small fraction of the light that goes down. And this was a big concern going into the Kepler mission. And when I was at Caltech, I had a graduate student working with me named Tim Morton.

And he and I figured out that like at the precision that Kepler was getting, that the rate of those false positives would be hardly, it would be like a tenth of what was feared. And so, but that nonetheless, you have to go through that process of saying, Are we fooling ourselves? Or how can we feel very confident that we're looking at that? The whole planet's precision, its photometric precision, just traced out that shape with such clarity that it was almost like each Kepler light curve was a textbook version of what a transit should look like. And so, you know, it turned out that the fears came from an earlier era where that signal was very jagged and noisy. And you could hide a lot of stuff in noise.

And so we had so little, such little noise with Kepler. Nonetheless, it was kind of accepted protocol that you have to make sure that you take a different telescope, maybe on the ground, maybe with one of those systems that corrects for the Earth's atmosphere, and then it just takes a quick peek. Take a little snapshot and see if you can see another object nearby that could be the culprit that's causing a planet light signal. And so that follow-up, that's called ground-based follow-up. So you have the space-based mission that's producing signals in science, but the science, in order to mature fully, needs these ground-based assets. And so that's like where I really positioned myself with the Kepler mission, and the early Kepler mission, was doing a lot of the ground-based follow-up. Do I gather from this that we've become more confident in what Kepler itself isn't online anymore,

but these days are we less devoted to the idea of the need for a ground-based follow-up that the data are really good? Yeah, I mean, I think it, yeah, the lesson of, like, higher precision and dedicated experiment giving you that higher precision means that you have more confidence has really started settling in. But I think that the procedures that people go through to actually publish a planet as a new planet, still include that legacy of, like, doing all your due diligence to check all around it. I like it. I think that it's a big deal.

It's a big deal. It's a big deal. It's good to be careful with your science, even when you have a lot of confidence. That's perfectly fair. And even though Kepler and Tess have done amazing things, we have also found a bunch of planets just here on the ground. And not all from transits, right? There's this Doppler mechanism, which is an entirely different one. Yeah.

So there's the method of looking for the movements of the star in response to its planet. And so, you know, when we think about planetary systems, we often think about like a star orbiting its planet. and to a close approximation, that's exactly what's happening, is that the star is stationary and the planet is moving around it. But we know that every action has an equal and opposite reaction. So the same force that the star is exerting on the planet, the planet is exerting that force back on the star. And that sounds really impressive until you consider that the star is so much more massive.

So it doesn't move much in response to that force. But it does move. And it moves enough that for part of the planet's orbit, it can look like the star is moving towards us. And then as the planet moves around in its orbit, it tugs the star, the star responds, and now it starts moving away from us in its orbit. And that back and forth motion is the signal that we look for

using the radio velocity technique. So it is the doubler shift of the light from the star that we're looking at, even though we're detecting the planet. We're actually detecting motion of the star, just to check. Yeah, that's right. Yeah, every technique is dependent on a lot of fundamental understanding of the stars in order to understand your planets. Every technique requires that. And almost all techniques require measuring what the star is doing in response to the planet that we see the planet.

And how good are we at measuring? I mean, let's be very fair to some of the listeners who might not be astrophysically inclined at all. What is this Doppler effect of which you speak? and how do you measure it for a star? Yeah. So the Doppler effect is that feature of like if there's an ambulance coming towards you as you're walking on the sidewalk,

it sounds high pitch and as it moves away from you it goes lower pitch. It's like wee, we, we, woo, woo, as it moves away. And so that change of tone is the stretching of the sound waves or the compression of the sound waves as it comes towards you. And the same thing happens with light because light is particle and it's also a wave.

And so the wave-like behavior of light is that when the star is moving towards us, the light emitted from the star gets shifted to the blue. There's a contraction. And as it moves away, there's a stretching. And so it becomes a redening of the light. And so what we do is we look for these minute shifts in the features of the star, as we observed them through a spectrograph, which is like a giant prism.

So we spread out all the light. When you look at the sun through one of these prisms, through these spectrographs, you see dark patterns of dark lines, and those sort of form the DNA signal or the fingerprint of the star. And those positions of those dark lines are known to very high precision.

And the Doppler effect causes them to all shift together. And so what we can do is we can watch those absorption lines, those dark lions move back and forth. The downside is that they don't shift much at all. It sounds very tiny. Yeah, it's absolutely tiny. So, you know, we're looking at one of those lines move by maybe a one 10,000th to one one thousandth of its width. So it, and what it translates into is like on our physical detector, we're watching the, this picture of the star spectrum shift on the detector by an amount that is equal to about a hundred silicon atoms lined up next to each other?

The good thing is that the trick that we use is that every one of those thousands of absorption lines that we see in the spectrum of a star like the sun, all of them shift by the exact same amount. So even though they're all shifting by a tiny amount, all together, they can give us a signal large enough for us to measure. And that's where like a lot of like if you want the like inside review of what astronomy is about, it's not so much the looking up at the night sky really at all. And it's a lot of asking like, oh my God, how do I do that in software? Software and hardware, I guess, right? Yeah, and so, like, you asked about it being a team effort, is that, like, that's where a lot of work behind the scenes goes in is like, okay, like, in principle that can be done. And I can understand what you just described as being possible.

But then when you sit down and you stare at the data product that, you know, a telescope operator hands you, you're like, oh, okay. This is the nitty-degree. This is going to consume most of my life right here. My best skin ever at 45? Give me a theme song and a best skin care award because it feels like this. Right. That's farmhouse fresh skin, all right? I'm blowing.

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I mean, if there were a little bit out, perfect alignment, you can still maybe get a Doppler shift. That's right, yeah. The downside of, like, okay, with the transiting planetary system, you know that inclination because there's only a certain tiny range of inclination. that would give you that signal that you're seeing, right? Because it has to eclipse. So with the transit method, you get that inclination built in, even though you miss a huge fraction of the actual planets.

The Doppler technique is sensitive to almost all of those planets that are there. But what you don't get is a measure of that inclination. So the first consequence of this is that if you're, if the orbital plane, if you're like looking down on the orbital plane, then you're not going to get much of a signal. because the star is moving perpendicular to your line of sight. But the moment you start tilting it away from that, you start seeing a signal. The trick is that you can also change that signal by changing the mass of the planet.

So if I make the planet more massive, then I move to make the star move more. And moving more looks a lot like being edge on. Right. And if I start tilting it, then I start decreasing the signal, which is also analogous to shrinking the mass of the point. planet in the aligned arrangement. And so those two, the mass of the planet and the inclination of the orbit are not determinable on their own.

You get a mixture of those two things. So you're open to potentially discovering a wider variety of planets, but there's a piece of information about them that you don't have. Yeah, yeah. And it's a piece of information that, you know, given that it's missing is, it complicates things in ways that most astronomers missed for a long. period of the planet detection era. And it's just the way that the Bayesian probabilities work with, you know, inclinations

of orbits. And but yeah, but in the early days of finding planets, and actually the first planet around a sun-like star that was detected, we were done with this Doppler technique. And a part of the reason that it was successful is that it was so sensitive to a wide range of those inclinations, whereas transit surveys weren't given. to the point where they can look at enough stars until later in the game. But once they started showing success, and they were doing so with small telescopes, then that became the impetus to put them up into space.

And in principle, if a star had multiple planets around it, you could map out the intricate back-and-forth dance that the star was doing because of all these planets. Yeah. Yeah, yeah. The way that I helped my students understand, like, you know, they often students were like, how would you ever do that if you had two or three, or four or five of these back and forth signals all happening at different periods and at different

phases. It just seems like it would be a mess. And the trick is I ask, you know, my more musically talented students, one of them that maybe like hum in A and hold that. And then I have another one do a B and then another at C. And I just have everybody just pay attention. That's super complicated what's happening in the air. Those back and forth motions in the air, but your ear can pick them out just fine. And it becomes this nice segue into understanding like signal. processing. You can actually separate out those different periods very cleanly and very clearly from one another. It's all using technology that's similar to what's what our ears have evolved. Right. It's interesting to me that the Doppler technique and the transit technique and even to a lesser

extent, the direct imaging technique, they're all able to find planets without like many orders of magnitude difference between how many we find. It almost seems like a fine-tuning problem that these very different methods are giving us ballpark similar kind of results. Yeah, I mean, it's really encouraging. Like, yeah, each detection technique, like one of the techniques we didn't even talk about was this gravitational lensing technique. Oh, right, yeah. I did want to talk about that, so I'm glad you brought it up.

Yeah, and it takes advantage of the fact that massive body, Einstein gave us a different view of gravity than Newton. Einstein gave us the view that massive objects can toward or bend space and time. And a consequence of a massive body bending space in time is that if you had light traveling towards that object, that maybe would have gone by it and then, you know, we wouldn't have, sorry. Imagine a background star that you're looking at. Some of its light goes off into directions that you can't see. But if space is warped around a massive object between us and that star, then that can bend the light back into our line of sight in the same way that a limb. a curved piece of glass, bends light rays, and brings them into focus into our eye. And so this microlensing technique takes the situation,

this highly improbable seeming event where you have some background star sitting in the Milky Way on the other side of the, or towards the center of the Milky Way. And you're just observing it in a million or millions of other stars. but it just so happens that between you and that background star, maybe halfway across the galaxy, there's a star moving with respect to you in the background star that has a planet around it. And so the first thing that happens is that star's gravity warps the background light

from that background star and you're suddenly seeing more light than you would have seen otherwise. And so the star becomes brighter. The background star becomes brighter. it the background star appears to get brighter because the foreground star, it was light you don't even see, but its gravity is making it brighter, and then it passes through, and then that background star gets dimmer again. So it's this characteristic brightening and then dimming. Now, what's really fun is if you have a planet orbiting that star that's passing in front of the background star, and now you have two

massive objects. The first massive object causes its characteristic up and down, and then super imposed on top of that up and down signal of light is the little planets blip because the planet's gravity got in on the act. And so if you see that characteristic double or actually sometimes multi-peak signal in, you're just staring at a bunch of stars night after night after night after night. The microlensing folks have been able to measure an estimate for how many planets should be out at about three astronomical units, three times of the Earth. Earth's sun distance.

Okay. Just, you know, just like where the asteroid belt is. And that region nicely intersects the tail of what the transit missions can see. And also where the radio velocities can see. And that the numbers are coming up and starting to match in that region is this wonderful. Like,

this is like, those like science moments where you're like, oh, this is how we learn to, this is how we start to believe something. Yeah. It's not just that we got the answer once, but we got the answer three times with three completely different techniques. And so I think that, you know, with the lengthening baselines and the increasing technology that allow us to see transit signals and radio velocity signals and direct gemogen signals, all of these are going to start overlapping with the microlensing signals.

And I think we're going to start getting a very clear picture of planet populations. Wow. Okay. And I think that's one of the more exciting things to come out of the whole enterprise of finding plants is like it's really cool that they're there. And sometimes the systems are very wacky and they have retrograde orbits to go over the poles or whatever. But the ensemble of all of those planets starts telling you, you start showing you a picture. It starts showing a picture like the truth in the universe that you couldn't get at any other way. And yeah, that's the very, very beautiful story that you just told.

I love it. And it takes us back. So now that we have in hand these different techniques and what they're good for, what kinds of planets they're good for, let's revisit this question of the population of planets around different kinds of stars. So you've already hinted, but maybe just it's worth reemphasizing how the collection of solar systems is a little bit more heterogeneous and varied than we would have expected before we actually looked. I mean, I think in one way that you look at the solar system is that it has the sun in the center and that the sun is this ball of hydrogen that is its mass.

You know, I always reference to all the other stars in solar masses. So it's this one solar mass object that's yellow. Its spectrum peaks in the yellow part of the spectrum. So it appears yellow to our eyes. But there's all of these other kinds of stars. There's less massive stars called the Red Dwarfs. And as you surveyed the entire galaxy and look at the distribution of star masses that are out there, you find is there's far more of the least massive stars than there are of the most massive stars.

It's very, it's very skewed towards the small, low mass stars. So that, you know, in the immediate solar neighborhood, you know, if there is, you get out to about 100 stars, about 80 of them are going to be red dwarfs, but mass is less than about half of a solar mass. Okay. And so what we found is that red dwarfs have planets in abundance. There's lots of lots of planets. And that statistic of like at least five planets per star comes from an analysis of planet occurrence around red dwarfs. And so it looks like red dwarfs by having planets at roughly the same rate as they're a higher mass stars, they now just automatically move into what is typical in the galaxy, because the most typical star is a red dwarf. So if you look at it through that lens, then our sun, looks very unusual and the solar system looks very unusual. And the red dwarfs last longer also, right? They have longer less than that.

Yeah, much longer. So the sun should live to be about 10 billion years old. That's a lot of years. But if we can just think of it as like 10 billion. Well, the red dwarfs are going to live anywhere from like 100 billion. out to like, you know, indeterminate, you know, unmeasurable beyond the life of the universe type of old. Okay. So the very last star that will be shining in the Milky Way will be one of these red dwarves.

And so if any, if there's any planetary system around them, those would be the oldest, like, planets and the last planets left. So, I mean, that's interesting to put it in perspective. We're still in the kind of young, vibrant, and in the solar system's case, short-lived phase of the universe's evolution. Yeah, we're in the very interesting, very bright phase. And as the universe gets holder, everything is going to start reaching as low as synergy state, and it's just going to get real dark, real dim, real gloomy. But I like to think of those little red dwarfs as the last thing shining in that cold, dark universe. Well, maybe it's fun just to talk about Proxima Centauri B because it's the closest exoplanet,

as far as I understand. But it's also just an illustration of the weirdness. It's very much unlike the solar system in a lot of ways. The planetary system around it. Yeah. In some ways, it, I mean, again, it all depends on how you want to look at it. So if you think, like, what's typical, like, how do we want to characterize the solar system as if we characterize it by coplanarity, like all of the planets orbiting in the same plane,

then it actually, I think proxen, the planetary system starts looking a little, you know, a lot more typical, a lot more like our solar system. But like the scale of it is just completely different. It's like in response to shrinking down the star, like the systemable planets around it overreacted and shrunk even faster. But, you know, and that's what we're finding around red dwarfs is that like in that population of stars,

what you find are solar systems that are extremely compact. So, you know, the whole system goes from like, Like, you know, there's a four-day planet and then there's an eight-day planet and then there's a 12-day planet. And then way, way, way, way out there. There's that 50-day planet. But if you look at the solar system, our closest planet Mercury is like 88 days. These are the orbital periods. These are the years for these planets.

Yeah, these are the years. It's orbital period. So, you know, a year on these planets, you know, around a, like, if you're in the habitable zone of a really low-mass red dwarf-like proxima centauri, your year is starting to look like a few weeks. weeks, it's not, everything just gets much more compact. Right. And Proxima is also part of a triple star system, right? I mean, Alpha Centauri would be bright in the sky as a double star. It is, yes, so there's Alpha Sin A and Alpha and B, which are stars that are much closer

in mass and brightness and color to our sun. And then a way away from that pair of proxan A and B is little, I'm sorry, not proxen, Alpha's than A&B. Way, way far away from that pair is Proxima C. Which is this little red dwarf that is associated gravitationally, but kind of on the outs with the other two. Yeah, it's something like half a million year orbital period around Alpha's. Very, very long. Yeah. They're barely.

That's just barely bound together. I love it. I mean, I think that that is a good reminder of how different the universe is are going to be. Yeah, yeah. And that right there is a way that the sun itself, just by being the sun is unusual, is that it's by itself. Like, most stars have other stars that they share, you know, an orbit together. And so that we don't have one also might make us unusual. We do know that there are planets that orbit in binary star systems and triple star systems like Proxin, but there's also planets that orbit around two stars at once.

So the two stars are closer together than the planet is to those two stars. And so those are circum binary planets. And those are very unusual, very wild. And we kind of caught those by luck, but we're still starting to get. handle on how common that system might be. So you're saying Star Wars was a documentary and Tattoine could be real. Yes, Tattoine's, yeah, the dual sunset of Tattoine is something that in principle can be observed in the galaxy.

So it's not been in the galaxy far far away. It might be our galaxy. You mentioned the habitable zone. Tell us what that means. So the habitable zone is more of an idea that we can attach like physical values to. But the idea is that in order to have. life on a planet, you have to have liquid water. And the reason we think that is, is reasoning that is uncomfortably close to that reasoning of like what we expected to find

based on looking at the solar system. All of the examples of life that we know of are on the earth. So all of the examples of life that are on Earth require water for their existence. And so the idea by extrapolation is that you also need that conditions for liquid water to exist on the surface of the planet for it to be habitable. And so that is going to be different for different stars, but you can think of it as this narrow range of distances where it's like the Goldilocks zone, where if it's too hot, if you're too close to the star,

things are too hot and the water boils off. But if you're too far from the star, things are too cold and the water freezes out. And so there's a just right distance. So it's a range of distances from the central star. It forms like a band around the star. And so if a planet is in that band, then we would consider it potentially habitable.

Yeah. So just to be super duper clear, that does not mean it is inhabited. That's right. Those are two different. Yeah. Habitable speaks to potential, not actuality.

And an obvious question to ask is, are there ways of gathering data about all these 5,000 planets we found that would indicate whether any of them had life on that? Oh. That is tricky. In principle, you could. And we're going to start maybe testing that notion that's in principle.

Now, we might start practicing it soon with the Web Space Telescope, JWST, which recently launched. And it has the capability of looking closely enough at the light from the central star, of a system that has a planet that is known to eclipse it. star and it can look during the eclipse and it can see the star spectrum so clearly that I can tell that the starlight is passing through the atmosphere of the planet and so you get this tiny tiny tiny little bit of filtered starlight during the eclipse that contains information about the atmosphere of the of the of the planet and if everything lines up just right and we find just

the right planetary system and all things work on jw is expected and the data analysis is done extraordinarily carefully and then checked and then rechecked, we'll maybe have just enough signal to start arguing about whether that was a biosignagement. Okay. And I think that those types of arguments will just like rage on in the astronomy community from some point in the near future for some amount of time. And then eventually we might have like a large enough sample to draw a statistical conclusion that there's got to be like there somewhere.

But like these are very, very tenuous signals based upon assumptions that may or may not be present on other planetary, other planet surfaces, other life forms and things like that. So we're making, I guess, at what spectroscopic signature we're going to see. And we're really crossing our fingers that we understand chemistry and biology well enough that we can predict that that signature will be there, that it was detected, and that we're not fooling ourselves somehow. Well, I mean, we're near the end of the podcast now, so we're allowed to be a little bit more speculative. So have your feelings about the existence of life, either primitive or advanced on other planets, changed that much because you've collected all this data? I think what it's done is it's opened the door wider to the possibility that we'll find those biosignatures, the evidence of life.

The signatures of biology is what I mean by biosignature. And what I mean is that if two decades of looking for planets left us with the conclusion that Earth-sized planets with the Earth's composition existing in the habitable zone of its star just don't exist. Let's just say that that was the conclusion. Two decades of planet searching and just we never seen it, no hope of seeing it, not even a statistical sniff of it. It's not happening. Well, then that would just close the door on the possibility of finding those biosignatures. we don't even have the initial conditions right now. We haven't found the evidence that the initial conditions exist. But I think what we've done is we've expanded our knowledge of planets in the galaxy

to the point where we say, yeah, this is feasible. If the life exists, it's likely that we'll be able to observe it in just the right way to detect it. It's leaving the door open for the possibility. But what sits on the other side of that door just tons of technical challenges. But at least we don't have to foreclose on the on the possibility and do you have any favorite thoughts about why we haven't found them yet the fermi paradox yeah um i i wonder if are some of the assumptions that go into that the existence of that paradox hold like the idea that like advanced civilizations necessarily have to get to the point of like spaceflight is one of their seminal of teeth achievements or are unnecessary precondition for other achievements.

Like, what is there, there's the possibility of life arising where the beings start forming a society in which everybody's needs are met just fine. And there's, like, no real, like, big push because there's no such thing as a Cold War. There's, like, they, everybody's just like, everything we ever want is right here. And the nice sky is so beautiful to look at. And everything's taken care of to the point where we have the luxury of, like, pretty much everyone engaging and looking at the sky? Who's to say that their first priority would be to beam a signal of their existence at some other star?

They just be like, maybe they just go through their entire existence as sentinel beings with that that impulse. And I don't know how to assess ahead of time the likelihood of that outcome versus our outcome, right? So who's to say, like, we're not this weird outlier where our civilization developed in such a way that scarcity was a thing and that, We couldn't figure out the distribution of things right. So we ended up going to war all the time. Then we had a big old space race, basically just to piss the other side off. And it had the side effect of like sending telescopes up there. And then we started wondering, like, well, I wonder if somebody out there's doing the same thing.

Well, maybe they're not. Maybe they're just not doing the same thing at all. Maybe this impulse was weird. We just happened to live in that realization that it happened. Well, or I suppose you could imagine that humanity could change a little bit along those lines. I mean, yeah, yeah, like maybe we're not there yet. Yeah, yeah. I mean, do you, what are your feelings about human exploration of space?

Do you think that that's just kind of a distraction or is that something that you look forward to down the road? I look forward to living in a world where I feel like that's one of the most important things to do. But there's just too many needs left on addressed here on Earth for me to really think too much about that beyond like hoping for like the conditions. conditions where that would just feel right. So I think that space exploration right now is not being conducted among nation states, is being conducted among wealthy oligarchs. And I don't trust them to make the decisions that will be beneficial to all the rest of us who live under completely different conditions.

It's neat that they're flying up there, but it's not something that gets my heart pounding in terms of exploring things, asking really interesting questions. and having those questions be the most important things governing the lives of most people on Earth. But I have a different take than most astronomers, so I recognize that. But I think it's good to have a variety of those ideas out there. Good to have a variety of takes. But I guess then for the final question, even though human space exploration, like you say, is being largely driven by individuals now, governments are still doing most of the science in space.

And so what is your, you know, what gets you excited? What is the thing that is going to happen down the line? Let's limit it to our lifetimes. But what should the audience be looking for as a big next step in this field? I think it's not, it's not flashy. But I do think it's the most important thing. And it's the thing that gets me most excited is the next set of major advances and understanding stars.

There's a whole. subfield of astronomy called stellar astrophysics. And it hasn't been the flashiest or most well-funded area of astrophysics in a long time. And it's largely been ironically supplanted by the field of exoplanets. Exoplanets are actually piggybacking on a lot of the technology and techniques that were developed to study stars. And there's going to be a moment where we reach a limit, the fundamental, we're already there largely, that the limit is that we don't have a good enough understanding of stars to really press forward our understanding of planets. Because it is impossible to know things physically, meaningfully know things about planets

without knowing the stars to great detail. And I think it's like one of the most impressive feats of the science of astronomy that we happen to know anything about stars, much less that we know them to like some cases to within a percent. It's just bonkers what we do. Nonetheless, a percent's not good enough sometimes. And so I think whichever graduate student does the unfortunately not very flashy thesis, but the most important thesis of like figuring out new ways of incorporating new physics and our understanding of stellar interiors and spectra, or or understanding the ways that stars, these big fluffy balls of hydrogen vibrate and move around and obscure the signals that

we're looking for. Once that breakthrough happens, that's where you're going to really see the floodgates open up. And so that's where a lot of my interest is these days is just like, you know, how do we advance our understanding of stars? And I often joke that, like, I'm an exoplanetary scientist by day, but by night, I'm a stellar astrophysicist. And so it's, this is another example of a, I think that we, it's really important to evaluate, what we prioritize or also might start undercutting the thing that we say that we're prioritizing. And so, I don't know, it's not, again, it's not the most standard take, but it is something that gets me really excited.

I like it. I like it because it's not the standard take. It's a perfect way to end. So John Johnson, thanks so much for being on the Mindscape podcast. Thank you for having me. It's been fun.