204 | John Asher Johnson on Hunting for Exoplanets

0:00:00.2 Sean Carroll: 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 sub-fields 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 of it had actually been discovered. These days, over 5000 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. 15:41 33:16 48:20

0:01:00.7 SC: 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, 'cause 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 telescope spaced here on Earth, but also missions on satellites that are dedicated to finding new planets. As I'm recording this, we're in the process of ramping up the James Webb Space Telescope, JWST, which will be really, really able to examine exoplanets with much higher precision than we've ever done before. Finding them is one thing, studying them is yet another one.

0:02:06.2 SC: So I talked 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 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. To be fair to us, we only have the solar system as our data point, so there is a menagerie of different kinds of planetary systems, and they're all over the place. We're just beginning, even though we have 5000 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 in 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 wanna learn more about it, go to bold.org, B-O-L-D.org/scholarships/mindscape.

0:03:17.7 SC: And so the idea is we are crowdsourcing funding, so you can contribute. I've contributed. And what's gonna happen is every year, at least this year, hopefully in years to come, we're gonna pick one person who wants to have a little bit of help going to college to study the biggest question, not applied stuff, not things that are gonna 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 wanna 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.

0:04:21.1 SC: So please, if you have any interest, go to bold.org/scholarships/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.

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0:05:05.7 SC: John Johnson, welcome to Mindscape Podcast.

0:05:07.7 John Asher Johnson: Oh, thank you for having me.

0:05:09.1 SC: So I gotta say, when I was in grad school, we didn't have any exoplanets, 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, 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 mumblety mumble of years since I was a graduate...

0:05:33.1 JJ: Yeah. Yeah, yeah. Yeah. So I think, things have evolved rapidly and often quite wildly [0:05:41.2] ____ when you're studying exoplanets, full of surprises. And so I guess, maybe I can put this in the context of what the paradigm was back then when planets were first discovered, and then think about 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 hundreds of years of noticing that planet... The planets are coplanar and they move in almost perfectly circular orbits made it natural to believe that like, okay, yeah, all must have formed out of flatten dust and gas, and that the big planets are further out and the little planets were closer in... That also made sense for that formation scenario.

0:06:25.0 JJ: And so everybody expected to what... When you go out looking for planets, if you ever did look for planets, you'd expect to find things like our solar system.

0:06:33.6 SC: Right.

0:06:33.6 JJ: And in 1995, that 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 three-day orbit. And that was instantaneously like, who ordered that? Where did that come from? I guess we better go to the drawing board. And I think it's a science that grew up. So try changing the way that we think about planets, and then ultimately, our own planet and ourselves.

0:07:05.4 SC: And so where are we now? How many planets that we found roughly?

0:07:08.4 JJ: I think we just crossed the 5000 mark...

0:07:10.3 SC: 5000 exoplanets, so and is that steady progress or there's gonna be sort of leaps when we get new technologies or new satellites or whatever?

0:07:18.7 JJ: Historically, there have been leaps, with new technology. And I think what's really interesting with the field of exoplanets in particular, is that it... When you think of astronomy, you think about like getting to bigger and bigger and bigger telescopes since you know 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, as astronomers have grown better at what they do, they're finding that we actually need smaller telescopes. Let's get like 10 centimeters...

0:08:00.7 SC: Okay.

0:08:00.8 JJ: That sounds about right. And it's... But what 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.

0:08:33.5 SC: Does that imply that we kind of could have done it earlier if we had just put our minds to it?

0:08:39.2 JJ: Yeah, yeah. Possibly. I think, I guess the caveat there is that there was a certain threshold that needed to be crossed in detector technology...

0:08:49.1 SC: Oh, okay. That makes sense.

0:08:50.1 JJ: Once that happened, around 2000-ish, then it was pretty easy to relatively see.

0:08:55.4 SC: And you mentioned that the first planet, the Jupiter that has such a short orbital period was a little bit of a surprise. So now 5000 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?

0:09:14.5 JJ: Yeah, that's a great question. I mean, it is absolutely is a 3-day orbit than a 12 year orbit. You don't have to wait 12 years [0:09:20.0] ____.

0:09:21.0 SC: There you go.

0:09:23.4 JJ: That if nothing else, but there's other reasons as well, like the signal is just larger? . So yeah, it's definitely true that they are easier to detect, but they also had to be there. And the fact that they were... Their occurrence rate was not zero. Instantly, like, as a surprise, but yeah, as the years went on, I think for the first five years, Jupiters were the kind of planet that were discovered. And there was a large collection of them. 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 orbits.

0:10:00.6 SC: So hot Jupiters was the thing because... Hot just because they're close to the star, not anything intrinsic about Jupiter itself.

0:10:07.9 JJ: Yeah, that's right. Yeah, that's our very clever naming scheme for this class of planets. Is that, Oh, it's right next to it's star. It's hot. And it's the size of Jupiter.

0:10:15.9 SC: There you go. And so what do we know now about that distribution? Is the solar system kind of typical?

0:10:22.6 JJ: 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, that we're seeing that there's a decent size sample, but we've also seen systems that have their planets wildly out of orbit. So in some way, solar system fits, but there's a whole class of planets that didn't...

0:10:46.0 SC: 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?

0:10:56.1 JJ: That's right. Yeah. So like, you can think of it as like, the grooves on a record. And so that's what I mean by coplanar...

0:11:05.1 SC: Yeah.

0:11:05.5 JJ: Imagine if you like cut out the center portion of that record and then tilted it with respect to the rest of the... And those are two different orbits, there's examples of this...

0:11:14.1 SC: Okay.

0:11:15.1 JJ: And sometimes... And there's also examples of misalignment with the spin of central star. 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 planets, not the apparent retrograde that we see like Venus and Mercury, but actual retrograde that they're orbiting in the, "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 an entire populations of planets out there that look absolutely nothing like this. And so if you ask, like, how does it fit in, in the whole ensemble, it starts to look a little rare. A little bit rare.

0:12:00.0 SC: 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?

0:12:23.9 JJ: Yeah, I mean, it indicates that the revision needs to happen.

0:12:29.3 SC: Okay.

0:12:30.4 JJ: That is actually, we still don't really have any good explanation, like any solid, experimentally tested theory for how you can a retrograde planet.

0:12:41.2 SC: Okay.

0:12:42.2 JJ: I can wave my hands with the best of them and describe some scenarios, but we don't know that, like, that's not understood, it's guessed. And it's done after the fact, we saw the results of the experiment. We're like, "Hey, I have a great question for it." It's a little bit backwards. [laughter] But we tell good stories about how this things... And those stories are very different than the way that things happen in earth.

0:13:07.8 SC: We are not averse to the occasional waving of hands here on the Mindscape Podcast. I presume that some crazy gravitational interaction or something like that, or you capture by a fly-by, I don't know what would be your favorite hand waving scenario.

0:13:25.6 JJ: My favorite scenario and the one that I've done the most [0:13:28.8] ____ 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 pulls of all the other planets. And the way that works out for the earth is that is negligible compared to the pull of the sun, and it doesn't really affect the long-term evolution of... Depending on what you mean, like long term.

[laughter]

0:14:01.8 JJ: 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 sun, and then you have a companion star, 'cause 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, the other star, it starts getting longer and more eccentric, so it becomes less circular, it's more of an oval shape. And then it can oscillate between that and then a misalign state where it goes up over the poles of its [0:14:51.5] ____ and it stays perfectly circular. And so it can exchange inclination and eccentricity and the sort of harmonic oscillators. That's the first act of the story.

0:15:02.7 SC: Pretty good one. Okay, good.

0:15:03.7 JJ: Okay, and then the next act of the story, the planet starts feeling tidal forces. Central star when it goes on this highly eccentric orbit, it buzzes the stars 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.

0:15:38.7 SC: 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 you waited billions of years, various planets would get kicked out?

0:15:52.0 JJ: Yeah, I think that's still believed that, the fact that we even settled into the specific stable configuration was somewhat of a lucky draw. But yeah, and this all chaos theory type stuff.

[laughter]

0:16:07.8 JJ: These are simulated to different initial conditions that have led to the ongoing war.

0:16:11.8 SC: And I guess I'm just like you cooking this up after the fact, but the fact that there is a bias when we look for exoplanets for planets that are heavy, 'cause they have a big effect and close to the star, 'cause they're short periods, maybe those are the ones where it's more likely that some crazy orbital dynamics has gone on.

0:16:32.7 JJ: That's true, but that's countered by the fact that the sensitivity of the search methods is such that we're complete to Jupiter at being Jupiter. We are more sensitive to those oddball planets quite often, but that's not to the detriment of missing the rest of the population. We get a pretty holistic view.

0:16:56.8 SC: 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.

0:17:04.8 JJ: That's correct.

0:17:06.3 SC: 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, I know that.

0:17:17.8 JJ: Yeah, if I were to say there was one planet for every two stars, I could express that as the planet occurrence 0.5 to half. If it's one out of three, I call it One-third, if every single star had a planet then it would be one. Okay, a study that I did with a collaborator a few years ago, we found out that number at five. There's five planets per star, which means that there's more planets in the Milky Way than there are stars in the Milky Way. And pretty much 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.

0:18:00.8 SC: 'Cause you could get a five to one ratio if some stars had 100 planets and most of them [0:18:06.9] ____.

0:18:07.0 JJ: That's true, yep.

0:18:07.3 SC: But you're saying it's not like that. It's more like most stars have a planet or a handful of planets.

0:18:13.2 JJ: That's actually an excellent observation. And we're able to look at the populations in other ways that show that it's not that skewed.

0:18:20.8 SC: Okay.

0:18:22.3 JJ: But it does have interesting ways of outcomes.

0:18:25.3 SC: And I remember that 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 planets. Was I too sanguine or other people too conservative?

0:18:44.3 JJ: No. I'm with you on that. I'm with you. I'm also a human and you're also a human, and it's possible for us to hold 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." Those two can act in intention. And I think that that's happened for the community, and it's that whole... There's also a part of... There's the paradigm before the first planets were found about what solar systems should look like.

0:19:12.7 JJ: But there's also just the way of placing value on certain aspects of astronomy and what should be done in order to do good astronomy. In the 1980s, it was not... It wasn't really considered possible to be a good astronomer and do something like looking for planets. Are you kidding? So that's something I think that scientists forget is that we do hard selection process on what is valued, and that comes about through a whole set of things, but just it's important to know back in the 80s, 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.

0:19:55.1 SC: 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 conservativism in the community, and all the stuff is expensive, so you can understand why, but it makes you wonder what other things we're missing.

0:20:22.0 JJ: Yeah, I think astronomers are all academics, and most academics they're self-liberal, but in their actual... In their actions they're quite conservative in the sense that there's an agreed upon... Not only an agreed upon answer to some set 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; not only what questions they're asking, but that, that question is now allowable. That question is okay.

0:21:13.3 SC: Yeah, 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 don't wanna give anyone the impression that I was out there saying we should go look for them.

0:21:28.8 JJ: You weren't publishing on this?

0:21:29.8 SC: I was not when I was an undergraduate. Again, I should have. This is a thought out there for current undergrads, what are the old folks missing? That's a good question.

[laughter]

0:21:40.0 JJ: Exactly. What are we missing now?

0:21:42.6 SC: And the other thing I wanna 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.

0:21:55.1 JJ: 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 solivagant, which means a lone wanderer. It's one of the scientists... Now, I 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 there are theories about what they're doing up there. But how did this thing get orphaned from its star, or do we not understand star formation well enough to understand that this might be one of the possible outcomes of making stars.

0:22:38.7 SC: Well, I think because a lot of people don't have the background knowledge about star formation, etcetera. 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?

0:22:58.3 JJ: Well, there was an undergraduate here, who looked at the possibility along with their advisor, James Guichon at Ingrema. They were looking at the question of whether you could at the center of the Milky Way, this giant, super massive black hole... And I promise this connects to planets. It sounds wild as a start, but okay, there's a super massive 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 or born there. We don't exactly know, but they're there.

0:23:33.1 JJ: And every once in a while, one of the stars 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 spaghettified star turn into little populations of Jupiters. And if so, if that was something that actually happened, then what would be the occurrence and that's a really fun project. It's a really out-there project. It doesn't fit neatly under what's accepted to study or what question's interesting to ask, but she showed it was at least plausible. So that question exists out there for any aspiring, young astronomers, who might get to graduate school and an advisor who will really run loose.

0:24:22.6 SC: Or if not, they can do it surreptitiously, while their advisors not looking. That's always possible.

0:24:26.4 JJ: Or the paradigm might shift by then.

0:24:29.0 SC: It could be the common thing. It could be the hot topic. You never know. My own experience was, I was the world's expert at the cosmological constant and dark energy before they found it. It was not very exciting before they found it, but suddenly it was exciting, through no fault of my own.

0:24:44.8 JJ: That's right. You just have to find the right question to ask.

0:24:48.5 SC: I benefitted.

0:24:49.5 JJ: It's not gonna be encouraged, but you actually can ask it.

0:24:51.7 SC: Yeah, yeah, exactly. Anyway, so let's think about how we detect them, 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. That's okay. But so how do you find them? Presumably, the big obstacle is that stars are bright and planets are dim. Is that an over-simplification?

0:25:19.1 JJ: Yeah, I think that is a 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 all in 1000s against the sun. And so we're talking about very small signal in the presence of very high contrast. And so let's take it for example, like you wanna do direct imaging of the planet.

0:25:49.2 SC: Yeah. Perfect.

0:25:51.0 JJ: The analogy that I once worked out and I think it's the one I used in my book is to, if you're trying to find the glow of a Jupiter size planet sitting next to its star from the earth. And let's put that star at Proxima, it's as close 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.

[laughter]

0:26:23.3 SC: Alright. So it's a challenge.

0:26:25.7 JJ: So it's the contrast is one aspect of it and you nailed that and the other is just, the signals are so small there. The actual signal that you're looking for.

0:26:32.7 SC: Well, so you mentioned direct imaging. We don't need to go in order of plausibility. So let's just ask the dumb question first. Like can't you just take a really big telescope, use the best detectors we have point it at a star and take pictures of any planets that might be around it?

0:26:47.4 JJ: Yeah. In principally yes, you can do it. But there are obstacles that you have to overcome to pull that off. And the biggest obstacle is this big, annoying thing called the earth's atmosphere, sorry, perspective's everything. But for an astronomer, the atmosphere in the way causes a lot of heartache and headaches. And so even with a really great optical design and a huge telescope pattern, a really wonderful detector, you still have to stare, basically it's trying to see a freckle on your friend's face and you're at the bottom of a pool.

0:27:26.1 JJ: And you're looking up at the 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 or the obscuring effects of the atmosphere. And there are technologies that are allowing us to do that. And 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 feed [laughter] And so that that's changing on microsecond levels. You have to change the mirror shape the actual...

0:28:00.3 SC: Are you literally putting some, servo motors under the mirror to warp it?

0:28:05.7 JJ: Yeah. There's little tiny servos sitting underneath a, what's actually a very small, scaled down version of the main mirror, but nonetheless, you're moving a mirror around fast enough to cope with the atmosphere. And [chuckle] so that's just a huge engineering problem you have to overcome first and that you have to rely on that being done extremely well and very well so that you can still barely pick.

0:28:26.2 SC: 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 gonna be the leading procedure.

0:28:35.6 JJ: 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 on the... 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.

0:29:13.3 JJ: But we're still, nonetheless, we're able to find planets that way. More recent advances since those early detections have brought us to looking extremely massive planet, very widely separate from star. But again, you're just in both of these techniques you're limited by the number of targets you have. You get down to a certain sensitivity and you're really excited that in principle you can find, but now you're just kinda limited by the number of stars that are close enough for that technology to really work for you.

0:29:39.5 SC: Is there some high tech version of putting my thumb up to block out the bright thing so I can see the dark thing. [chuckle]

0:29:46.4 JJ: Yep. That technique right there is called coronagraphy. It is 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 the technical details of what goes on with how that works is 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...

0:30:22.4 SC: Okay. So if that is an obvious thing to try, but is not the most effective. What in your mind is the most effective way to find these exoplanets? 'Cause you found some, how many planets have you found? Do you have a... Is there a number on your CV? How many planets you found? [chuckle]

0:30:36.1 JJ: I once did it, I once counted it. It was close to a 100.

0:30:37.6 SC: 100 planets, it's pretty good.

0:30:38.7 JJ: Yeah. But it depends on how you like assign credit and you know, like if we only, yeah, so.

0:30:44.6 SC: These are teamwork kind of things, right?

0:30:46.5 JJ: Yeah. These are teamwork things. And I've been on teams that have found well over well over 100. So the method like most effective would kind of implies that, we have all of the money in the world that we can pour into every different technique. And then we can sum up which one found more planets. But where resources have been invested and where it's been most effective in terms of the outcomes is transits because and that's the method where you're looking for the planet to eclipse the light, a central star so that it has its orbit aligned in space just right so it passes. And they're called transits if it's a... But the technique is the transit technique and that's found the vast majority of us.

0:31:29.1 SC: And in part 'cause that's what's used by the Kepler satellite, right? Which is in a lot of this heavy lifting.

0:31:35.9 JJ: Yeah. And it's also the detection method that can make due with the smallest telescope which makes it well suited for putting it on a satellite, getting up above the earth atmosphere.

0:31:44.9 SC: Maybe tell us a little bit about Kepler because it's no longer working. These things have a finite life time.

0:31:50.0 JJ: Yeah. Yeah. So the Kepler mission was space telescope that went up in 2009 and was launched into what's called an earth trailing orbit. So it orbited the sun but it's orbital separation was just a little bit longer. And so it just sat out there in cold space as the earth drifted away and... [chuckle]

0:32:10.5 JJ: And what was great about Kepler is that it was single-minded in its science goals. It just [0:32:17.4] ____ like no movable or interchangeable parts on that thing. It was like it popped off the desk cover from the front mirror... From the mirror, and it was just that... It just stared at space. [chuckle] 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 its original mission.

0:32:36.4 SC: So it didn't scan the sky. It was just really focusing on...

0:32:38.5 JJ: It didn't scan. It just stared at 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 opportunity transits. And it turned out, transits by the thousands.

0:32:58.1 SC: So I'm betting that there are people asking, Well, why didn't it scan? There are even more stars elsewhere. 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.

0:33:12.1 JJ: That's right. Yeah, yeah. If you take a whole box of hot Jupiters and you dump 'em out into the Milky Way and they all tumble out of the box, only one-tenth of all of those hot Jupiter systems would have the alignment necessary for a transit.

0:33:29.7 SC: Okay.

0:33:30.9 JJ: So the random orientation of all these stars and planetary systems in space is similar to dumping 'em 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 goes... It decreases one over the [0:33:48.6] ____ star, so by the time you get out to the earth's orbit it's only like [0:33:48.7] ____ further away.

0:33:49.1 SC: So just in case that went by...

0:33:50.5 JJ: So you need to look at lots and lots of stars to get this chance alignment to work out for you.

0:33:53.4 SC: 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.

0:34:01.6 JJ: That's right.

0:34:02.6 SC: If the star orbits perpendicular to our line of sight, it's invisible, as far as this technique is concerned.

0:34:08.0 JJ: That's right. Yeah. So the larger the sample that you're looking at, the greater... You'll just find more planets. But you have to get above a certain threshold.

0:34:17.2 SC: And could you see something as tiny as the earth against something as big as the sun with this kind of technique?

0:34:23.5 JJ: In principle, Kepler... I mean, Kepler was designed to be able to detect the earth around the sun as viewed from far away.

0:34:31.1 SC: Okay. Okay. So it could do it.

0:34:34.5 JJ: There are a number of systems that bore a lot of resemblance to the earth's sun in different ways. Some were... Orbit a closer end sometimes the planet's a little bit larger. But 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're more likely to...

0:34:52.2 SC: Sure. And also, how long did Kepler look? You said about a decade?

0:35:00.2 JJ: 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 it got renewed for three years, and then it got renewed for three years. And it just was... It just kept on chugging. It wasn't supposed to have. It broke down. It just actually broke down, 'cause it has these three little reaction mills that keep it pointed at that same place, those little gyroscopes that keep it stable in its angular momentum. And one of 'em... It 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 starting to wobble.

0:35:39.5 SC: Wobble. [chuckle]

0:35:41.1 JJ: And it wasn't... So what they figured out that... The NASA engineers figured out is that they could... The solar panels on the back of it are like the roof on a house. They come up to a point. 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 the photons breaking over the solar panels like it was the bow of a boat, stabilized it in that third dimension.

0:36:05.8 SC: Wow. Wow.

0:36:06.5 JJ: And then that was the K2 mission. And that just went on. Yeah, so I think, all told, it must be about a decade.

0:36:12.6 SC: They are clever, those NASA engineers, sometimes. [chuckle]

0:36:16.9 JJ: Yup.

0:36:18.0 SC: But I'm glad you brought that up because it's a reminder, again, to 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?

0:36:29.2 JJ: Yeah, yeah.

0:36:30.2 SC: Like you said, one wheel broke instantly, and if two had broke instantly, we'd have been in trouble.

0:36:34.4 JJ: That's right. Yeah. [chuckle] That's why space missions are so expensive, in part, just because the... There's just no room for error. Not to mention that you have to put it on this gigantic exploding tower and send it off into earth's orbit, but... [chuckle]

0:36:51.8 SC: And there was a... There was the TESS Mission. Is that a follow-up?

0:36:54.4 JJ: Yeah. So the TESS Mission was the follow-up. And you asked, "Why don't you scan?" And TESS answered that question and said, Yeah, why not? So let's scan. And so what TESS does is it trades off the duration of the survey. So Kepler went years and years. TESS, what it does is it scans a big stripe of the sky 30 days at a time.

0:37:16.0 SC: Okay.

0:37:16.8 JJ: And then it moves over to the next stripe until it kind of paints out the globe around the night sky around the earth. And the detectors... What's really nice about that, though, is that the detectors overlap near the pole, as it's [0:37:30.5] ____ 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, 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 a shorter duration. So it's a shallower, broader, and Kepler was really narrow and deep.

0:37:51.9 SC: And did it... But it found a bunch of planets?

0:37:54.5 JJ: Yeah. That's one that took it from the couple thousand out of Kepler into the 5000s mostly [0:38:00.9] ____.

0:38:01.9 SC: Okay. And that was also the transit method?

0:38:05.9 JJ: Mm-hmm.

0:38:06.4 SC: They were looking for the transit method, they were looking for a little decrease in the brightness of the star?

0:38:08.8 JJ: Yes, yes. They were using the transit method.

0:38:10.9 SC: Is this team work? Once you find the planet candidate with one of these satellites, do you follow up with telescopes here on earth?

0:38:18.9 JJ: Yeah. It is absolutely a team effort. NASA designs, flies and maintains the mission. And they also do a lot of... 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.

0:38:41.9 SC: Okay.

0:38:43.2 JJ: And then that is just, that's just data, right? So like you can collect a lot of data and you can get almost no information from it. Look at it well enough. If you don't use the right way of looking at it. And so then usually where an astronomy graduate student is figuring out how do I take the data that's handed in these files from NASA and how do I turn and print that into use. 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 sector, you can trust that this means this and that. And they understood the instrument and they... So they allowed us to make, reliable transitions from just raw data information.

0:39:33.1 JJ: But then there was a whole extra step where you wanna make sure that the thing that looks an awful lot, like the signal 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 from its star. Yeah. 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, 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 be like a 10th 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 the actual. And Kepler's precision, its photometric precision just traced out that shape with such clarity that like, it was almost like each Kepler light curve was a textbook version of what a transit should look like.

[laughter]

0:40:40.1 JJ: And so, 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 there. We had so little, such little noises with Kepler, but nonetheless, it was kind of accepted protocol that you have to make sure that you take a different telescope, maybe on the ground, maybe if one of those systems that corrects through 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 [0:41:10.3] ____ And so that follow up that's called ground based follow up. So you have the space based mission. That's producing signals and 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, the early Kepler mission, was doing a lot of the ground based.

0:41:28.1 SC: 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?

0:41:42.1 JJ: 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.

0:41:51.9 SC: Okay.

0:41:52.3 JJ: But I think that the procedures that people go through to they actually publish a planet as a new planet, so still include that legacy of doing all your due diligence to check all around it.

0:42:01.5 SC: Oh, let's find a new planet. It's a big deal.

0:42:02.5 JJ: But I like it. I think that it's good to be careful with yourself.

0:42:04.2 SC: Yeah.

0:42:05.0 JJ: Even when, when you have a lot of confidence.

0:42:07.2 SC: 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. There's other... There's this Doppler mechanism, which is an entirely different one.

0:42:22.5 JJ: Yeah. So there's the method of looking for the movements of the star in response to its planet. And when we think about planetary systems, we often think of a star orbiting its planet and to a close approximation, that's exactly what's happening is the star is stationary and the planet is, 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.

0:42:57.9 SC: Yeah.

0:42:58.4 JJ: But it does move and it moves enough that it, for a part of the planet's orbit, it can look like the star is moving towards us. And then as the planet moves around in it's orbit it tugs the star, the star responds, and now it's being away from us. And that back and forth motion is the signal that we look using the radio [0:43:15.1] ____.

0:43:15.7 SC: So it is the Doppler shift of the light from the star that we're looking at, even though we're detecting the planet, we're actually detecting the motion of the star just to check.

0:43:25.5 JJ: Yeah, that's right. Yeah. Every, technique is dependent on a lot of fundamental understanding of the stars in order to understand your benefits. Every technique requires that, and almost all techniques require measuring what the star is doing in response to the, that we see.

0:43:44.7 SC: 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?

[laughter]

0:43:56.6 JJ: Yeah. So the, 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 pitched. And as it moves away from you, it goes lower pitch. It's like, wee, wee, wee, woo, woo, woo. As it moves away. And so that change of and in tone is, is the stretching of the sound waves, or the compression of it as it comes towards you. And the same thing happens with light because light is a 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, a red, a reddening. And so what we do is we look for these minute shifts in the, the features of the star as a spectrograph, which means, we spread out all the light.

0:44:45.1 JJ: When you look up 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 a 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 and watch those dark lines move back and forth. The downside is that they don't shift much at all.

[laughter]

0:45:15.6 SC: It sounds very tiny.

0:45:18.7 JJ: Yeah. It's absolutely tiny. So, we're looking at one of those lines moved by 10000th and 1000th of its width.

0:45:27.5 SC: Okay.

0:45:28.8 JJ: So, it... And what it translates into is like on our physical detector we're watching 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.

0:45:48.6 SC: Alright, [chuckle]

0:45:52.7 JJ: The good thing is that the trick that we use is that every one of those thousands of absorption [0:45:58.4] ____ 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 this... And that's where a lot of... If you want the inside review of what astronomy is about, it's not so much the looking up at the night sky at all, and it's a lot of asking like, "Oh my God, how do I do that in software?"

0:46:25.8 SC: [chuckle] In software and hardware, I guess, right?

0:46:30.3 JJ: [chuckle] Yeah, and so you asked us about it being a team effort, is that like... That's where a lot of work behind the scenes goes in, it's like, okay, 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 the telescope operator had got... Oh okay, this is the nitty gritty, this is gonna consume most of my life.

0:46:53.1 SC: And I'm gonna guess that this Doppler technique is also like the transit technique, most sensitive to planets that are in the plane of where we're looking, but maybe not quite as sensitive, if they were a little bit out, perfect alignment, you could still maybe get a Doppler shift?

0:47:09.2 JJ: That's right. Yeah, the down side... Okay, with the transiting planetary system, you know that inclination, because there's only a certain tiny range of inclinations that would give you that signal that you read, 'cause it has to eclipse. So with the transit method, you get that inclination built, you miss like a huge fraction of the actual planet. The Doppler technique is sensitive to almost all of those planets there, but what you don't get is a measure of that inclination. So the first consequence of this is that, if the orbital plane... If you're looking down on the orbital plane, then you're not gonna get much of a signal because the star is moving perpendicular to you. But that 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... I make the star move more, and moving more looks a lot like being edge on, and if I start tilting it, then I start decreasing the signal, which is also analogous to shrinking the mass of the planet. And so those two... The mass of the planet and the inclination of the orbit are not determinable alone. You would get a mixture of those two.

0:48:22.4 SC: 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.

0:48:28.8 JJ: Yeah, yeah, and it's a piece of information that, given that it's missing is, it complicates things in ways that most astronomers made a long period of the planet detection error and it's just the way that the Bayesian probabilities work with inclinations of... But Yeah. So, in the early days of finding planets and actually the first planet around the sun-like star that was detective when 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 inclinations, whereas transit surveys weren't getting 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...

0:49:16.6 SC: 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 perturbing it.

0:49:26.7 JJ: Yeah, yeah. The way that I help my students understand like... They're often... Students are 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 my more musically talented students, one of them maybe like hum an, A, and hold that, and then I have another one do a, B, and then another 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 like signal processing. You can actually separate out those different periods very cleanly and very clearly, it's all using technology that's similar to what our ears have evolved.

0:50:18.4 SC: 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.

0:50:40.3 JJ: Yeah, it's really encouraging. Yeah, each detection technique. One of the those techniques we didn't even talk about was this gravitational lensing technique...

0:50:48.3 SC: Oh right, yeah, I did wanna talk about that. So I'm glad you brought it up.

0:50:51.3 JJ: Yeah, and it takes advantage of the fact that massive body... Einstein gave us a different view of gravity. Einstein gave us the view that massive objects can [0:51:00.6] ____ or bend space and time. And a consequence of a massive body bending space and time is that if you had light traveling towards that object, that maybe would have gone by it and then 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 the 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 lens or a curved piece of glass bends light to our eye, and so this micro-lensing technique.

0:51:38.5 JJ: Take these situations, highly improbable seeming event where you have some background star sitting in the Milky Way on the other side of the Milky, or like towards the centre of the Milky, alright? 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 Earth, 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. And then as it...

0:52:19.2 SC: And the background star becomes brighter?

0:52:22.0 JJ: The background star appears to get brighter because the foreground star whose light you don't even see, [chuckle] 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 on the backgrounds, now you have two massive objects, the first massive object causes its characteristic up and down, and then superimposed on top of that up and down signal of light is little planets blip, [chuckle] 'cause the planet's gravity got in on the act.

0:52:54.3 SC: Yeah.

0:52:54.5 JJ: And so if you see that characteristic double or actually sometimes multi-peak signal and you're just staring at a bunch of stars night after night, after night, after night, the micro-lensing folks have been able to measure an estimate for how many planets should be out at about three astronomical, three times the earth width.

0:53:16.3 SC: Okay.

0:53:17.4 JJ: Just out... Like for the asteroid belt, and that region nicely intersects the tail of what the transmissions consume and also where the radio velocities and that the numbers are coming up and starting to match in that region, this is wonderful, this is like those science moments where you're like, [chuckle] "Oh, this is how we learn to... This is how we start to believe something."

0:53:40.1 SC: Yeah.

0:53:40.5 JJ: 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 with the lengthening baselines and the increasing technology that allow us to see transit signals and radio velocity signals and direct imaging signals, all of these are gonna start overlapping with the micro-lensing, we're gonna start getting a very clear picture of planet.

0:54:00.8 SC: Wow. Okay.

0:54:01.3 JJ: And I think that's one of the more exciting things that to come out of the whole enterprise of finding planets, is like it's really cool that they're there, and sometimes these the systems are very wacky and they have retrograde orbitals or whatever, but the ensemble of all of those planets starts telling you... Starts showing you a picture, start showing a picture like the truth in the universe that you couldn't get at any other place.

0:54:21.8 SC: Yeah, that's a very, very beautiful story that you have 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 re-emphasising how the collection of solar systems is a little bit more heterogeneous and vary than we would have expected before we actually looked.

0:54:55.7 JJ: I think in one way that you look at the solar system is that it has the sun in the centre, and that the sun is this ball of hydrogen that is its mass, we I always reference all the other stars and into solar masses so it is just as this one solar mass object that's got yellow, it spectrum peaks in the yellow part of the spectrum, so it appears yellow to our eyes, but there's all these other kinds of stars, there's less massive stars called the Red Dwarfs, and as you survey the entire galaxy and look at the distribution of star masses that are out there, what you find is there's far more of the least massive stars than there are the most massives. It's very skewed towards the small, low mass star, so that in the immediate solar neighbourhood, there is... You tell to about 100 stars, about 80 of them are gonna be Red Dwarfs, with masses less than about half of the solar mass.

0:55:56.1 SC: Okay.

0:55:56.8 JJ: And so what we found is that Red Dwarfs have planets in abundance, there's lots and lots of planets, and that statistic of like at least five planets per star comes from an analysis of planet occurrence, and so it looks like Red Dwarfs by having planets, at roughly the same rate as their higher mass star, they now just automatically move into what is typical in the galaxy. The most typical star is a then Red Dwarf. So if you look at it through that lens, then our sun looks very unusual in the solar system. Because It's very unusual.

0:56:33.0 SC: And the Red Dwarfs last longer also, right? They have longer lifetimes?

0:56:36.6 JJ: Yes they are, much longer. So, the sun should live to be about 10 billion years old. It's a lot of years.

0:56:47.2 SC: That's a lot. [chuckle]

0:56:47.5 JJ: Think of it as like 10 billion. Well, the Red Dwarfs are gonna live anywhere from like a 100 billion up to like indeterminate, unmeasurable, beyond the life of the universe, type of world.

0:57:03.2 SC: Okay. [chuckle]

0:57:05.0 JJ: So, the very last star that will be shining in the Milky Way will be...

0:57:06.8 SC: Yeah, I mean, those lifetimes are much longer.

0:57:08.6 JJ: And so if there's any planetary system around them, those would be the oldest planets and the last planets left.

0:57:15.8 SC: So, that's interesting to put it in perspective, we're still in the kind of young, vibrant, and then in the solar system's case, short-lived phase of the universe's evolution.

0:57:25.9 JJ: Yeah, we're in the very interesting, very bright phase, and as the universe gets older [chuckle], everything is gonna start reaching its lowest energy state and it's just gonna 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.

[laughter]

0:57:46.4 SC: 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.

0:58:02.9 JJ: The planetary system around it?

0:58:04.1 SC: Yeah.

0:58:04.2 JJ: Yeah. In some ways, it... I mean, yeah. Again, it all depends on how you wanna look at it. So, if you think what's typical, like what... How do we wanna characterise the solar system? If we characterise it by coplanarity, all of the planets orbiting in the same plane, then actually, I think Proxima planetary system starts looking a lot more typical, a lot more like the castles, but the scale of it is just completely different. It's like in response to shrinking down the star, the system of planets around it overreacted and shrunk even faster, but...

[chuckle]

0:58:40.5 JJ: And that's what we're finding around Red Dwarfs is that in that population of stars, what you find are solar systems that are extremely compact. The whole system goes from, there's a four-day planet and there's an eight-day planet, and 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, that's just...

0:59:04.3 SC: These are the orbital periods, these are the years for these planets.

0:59:06.9 JJ: Yeah, these are the years, it is the orbital period. A year on these planet... Around the... 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. It's not... [chuckle] Everything just gets much more compact.

0:59:27.3 SC: Right. And Proxima is also part of a triple star system, right? Alpha Centauri would be bright in the sky as a double star.

0:59:34.1 JJ: It is... Yes. So there's Alpha Cen A and Alpha Cen B which are stars that are much closer in mass and brightness and color to our sun, and then away away from that pair of Proxima A and B is little... I'm sorry, not Proxima, Alpha Cen A and B, way, way far away from that pair is Proxima Cen C, which is this little Red Dwarf, that is associated gravitationally but kind of on the out with the other two.

1:00:05.2 SC: Yeah, something like half a million year orbital period around Alpha Centauri, something like that.

1:00:09.2 JJ: Very, very long. Yeah.

1:00:10.3 SC: Yeah. But that's just, the craziness...

1:00:11.3 JJ: They're barely, barely down together.

1:00:13.9 SC: I love it. I think that is a good reminder of how different the universes is or going to be.

1:00:20.7 JJ: 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. Most stars have other stars that they share an orbit together, and so that we don't have one also might make us... We do know that there are planets that orbit in binary star systems, triple star systems like Proxima, 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 circumbinary planets. And those are very unusual, very wild, and we kind of caught those by luck, but we're still starting to get a handle on how common that system might be, that kind of arrangement.

1:01:11.7 SC: So, you're saying Star Wars was a documentary and Tatooine could be real? [chuckle]

1:01:14.3 JJ: Yes, Tatooine... Yeah, the dual sunset of Tatooine 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.

1:01:25.4 SC: You mentioned the habitable zone. Tell us what that means.

1:01:29.1 JJ: So the habitable zone is more of an idea that we can attach, like physical values to. 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 what we expected to find based on looking at a 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 are 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 distance in central star and it forms like a band around the star. And so if a planet is in that band, then we would consider it potentially habitable.

1:02:47.5 SC: Yeah. So just to be super-duper clear, that does not mean it is inhabited?

[laughter]

1:02:51.8 JJ: That's right, those are two different... Yeah. Habitable speaks to potential, not actuality.

1:02:57.1 SC: And an obvious question to ask is, are there ways of gathering data about all of these 5000 planets we found that would indicate whether any of them had life on them?

1:03:07.1 JJ: Oh. That is tricky. In principle, you could. And we're gonna start maybe testing that notion, that's in principle and now we might start practicing it soon with the Webb Space Telescope, JWST, which recently launched, and it has the capability of looking closely enough at the star, the light from the central star of a system that has a planet that is known to eclipse its star, and it can look during the eclipse and it can see the star spectrum so clearly, it 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 planet. And if everything lines up just right and we find just the right planetary system and all things work on JW as 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 biosignature.

[laughter]

1:04:20.3 SC: Okay. So we're anticipating...

1:04:21.2 JJ: And I think that those types of arguments will just rage on in the astronomy community from some point in the near future for some amount of time, and then eventually we might have a large enough sample to draw a statistical conclusion that there gotta be life there somewhere. But these are very, very tenuous signals based on 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 a guess at what spectroscopic signature we're gonna see, and we're really crossing our fingers that we understand chemistry and biology.

1:04:58.5 SC: Yeah that's what I'm saying.

1:05:00.3 JJ: Well enough that we can predict that that signature would be there, that it was detected, and that we're not fooling ourselves somehow.

1:05:05.2 SC: Well, 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, change that much because you've collected all of this data?

1:05:20.9 JJ: I think what it's done is it's... It's open the door wider to the possibility that we'll find those bio-signatures, the evidence of life, the signatures of biology is what it mean by biosignatures. And what I mean is that if two decades of looking for planets left us with the conclusion that earth-size 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've never seen it, no hope of seeing it, not even a statistical sniff of it is not happening. Well, then that would just close the door on the possibility of finding those biosignatures. Because we don't even have the initial conditions right. We haven't found the evidence of the initial conditions in this, 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, it's... If the life exists it's likely that we'll be able to observe it in just the right way to detect it, is leaving the door open for the possibility, but what sits on the other side of that door? Just tons of technical challenges. And, but at least we don't have to foreclose on the possibility.

1:06:37.2 SC: And do you have any favorite thoughts about... Why we haven't found them yet, the Fermi paradox?

[chuckle]

1:06:43.4 JJ: Yeah, I wonder if some of the assumptions that go into the existence of that paradox hold, like the idea that advanced civilizations necessarily have to get to the point of space flight as one of the achievements or are necessary pre-condition for other achievements. 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 is no real big push because there's no such thing as a Cold War.

1:07:19.1 SC: Yeah.

1:07:22.4 JJ: There's like, the... Everybody's just like everything we ever want is right here and the night sky is so beautiful to look at and everything is taken care of to the point where we have the luxury of 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.

1:07:40.9 SC: Right.

1:07:41.9 JJ: They're just right... [chuckle] They... Maybe they just go through their entire existence as sentinel beings without 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 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 sending telescopes up there, and then we started wondering like, Wow, I wonder if somebody out there is 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 and we just happened to live in that realization that had happened.

1:08:24.9 SC: Well, or I suppose you could imagine that humanity could change a little bit along those lines, I mean...

1:08:31.2 JJ: Yeah, yeah. Maybe we're not there yet.

1:08:33.5 SC: Yeah, yeah. I mean do you... What are your feelings about the 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?

1:08:46.3 JJ: I look forward to living in a world where I feel like that's one of the most important things. But there's just too many...

1:08:52.1 SC: We are not there yet.

1:08:53.1 JJ: Needs left unaddressed here on earth for me to really think too much about that beyond hoping for the conditions for that. So I think that space exploration right now is not being conducted among nation states, it's being conducted among wealthy oligarchs, and it's... I don't trust them to make the right, 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. So but I have a different take the most astronomers, so I recognize that, but...

1:09:38.4 SC: No, that's good.

1:09:39.4 JJ: I think it's good to have a variety of those ideas out there.

1:09:42.0 SC: Good to have a variety of takes, but I guess then for the final question, let's... 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... 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?

1:10:12.7 JJ: Well, I think it's not flashy, but I do think it's the most important, and it's the thing that gets me most excited is the next set of major advances in understanding stars, there's a whole sub-field 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.

[laughter]

1:10:44.2 JJ: Exoplanets are actually piggy-backing on a lot of the technology and techniques that were developed to study stars. And there's gonna be a moment where we reach a limit where... The fundamental... And 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 one of the most impressive feats of the science of astronomy that we happen to know anything. Not much less that we know them, to some cases within a percent, it's just awkward it's what we do, nonetheless, a percent is not good enough sometimes. And so I think, which every graduate student does the unfortunately, not very flashy thesis, but the most important thesis of figuring out new ways of incorporating new physics into our understanding of stellar interiors and Spectra, or understanding the ways that stars, these big fluffy balls of hydrogen vibrate and move around and obscure the signals that we are looking for...

1:11:49.1 JJ: Now, once that breakthrough happens, that's where you're gonna really see the floodgates open up, and so that's where a lot of my interest is these days, is just like, how do we advance our understanding of stars and I often joke that I'm an exoplanetary scientist by day, but by night, I'm a stellar astrophysicist. And so, it's... This is another example of that I think that it's really important to evaluate what we prioritize or absolutely like might start undercutting the things that we say that we're prioritizing, and so... I don't know, again, it's not the most standard take, but it is something that gets me...

1:12:29.1 SC: I like it, I like it 'cause 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.

1:12:34.9 JJ: Thank you for having me, it's been fun.

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