Planetary Radio • Aug 19, 2020

Why Aren't There More Earth-Like Planets in our Solar System?

Please accept marketing-cookies to listen to this podcast.

Download MP3

On This Episode

Stephen kane thumbnail

Stephen Kane

Planetary Research Lab principal investigator at the University of California, Riverside

Bruce betts portrait hq library

Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

Kaplan mat headshot 0114a print

Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

Newly-published research led by Stephen Kane finds room for as many as 6 habitable zone planets around some stars. Why then is Earth on its own? The UC Riverside planetary scientist and astrobiologist will explain. Host Mat Kaplan is not prepared in the least for the latest space trivia contest question posed by Planetary Society chief scientist Bruce Betts. Do you read The Downlink? The Planetary Society’s great newsletter includes space headlines we review.

TRAPPIST-1 Habitable Zone Compared to Our Solar System
TRAPPIST-1 Habitable Zone Compared to Our Solar System The red dwarf star TRAPPIST-1 has 7 known Earth-sized planets closer to their star than Mercury is to our Sun. Our Sun is too hot for liquid water to exist on the surface at that distance. But TRAPPIST-1 is a cooler red dwarf star, placing exoplanets e, f, and g in the habitable zone—the not-too-hot, not-too-cold region where surface water can exist. The habitable zones of both solar systems are shown in green.Image: NASA / JPL-Caltech
Jupiter's Southern Hemisphere
Jupiter's Southern Hemisphere NASA’s Juno mission captured this look at the southern hemisphere of Jupiter on Feb. 17, 2020. This high-resolution view is a composite of four images captured by the JunoCam imager and assembled by citizen scientist Kevin M. Gill.Image: NASA/JPL-Caltech/SwRI/MSSS

Related Links

Trivia Contest

This week's prizes:

The Backyard Astronomer’s Field Guide: How to find the best objects the night sky has to offer

This week's question:

As of August, 2020, what is our Mat Kaplan’s single credit in the Internet Movie Database (IMDB)?

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, August 26th at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

What is the wavelength of the SuperCam laser you’ve been hearing about?


The winner will be revealed next week. 

Question from the 5 August space trivia contest:

What was the only completely unintended splashdown of a spacecraft carrying humans?


Soyuz 23 performed the only completely unintended splashdown of a spacecraft carrying humans, and it wasn’t fun.


Mat Kaplan: Howdy neighbor? A Crowd of Worlds in the Habitable Zone this week on Planetary Radio. Welcome, I'm Mat Kaplan of the Planetary Society with more of the human adventure across our solar system and beyond. Why is earth the only planet in our solar system that is smack dab in the middle of the habitable zone? Okay, maybe Mars is on the edge, but really for life as we know it, the red planet isn't in the same league. Sorry, Martians. This is the question that Stephen Kane wanted to answer, and he does so in a new paper about the conditions that put planets where liquid surface water is likely.

Mat Kaplan: Stephen even tells us where to look in our great conversation. Later, Bruce Betts, will catch me utterly off guard with this week's space trivia contest question. Speaking of Mars, a beauty shot of its south pole tops the August 14 edition of the Downlink, and here are space headlines you'll find at Remember the Dawn spacecraft and its discovery of those crazy bright spots on dwarf planet, Ceres? Scientists now believe we're seeing evidence of a liquid briny water reservoir about 40 kilometers below the surface. I've invited Dawn Mission director, Mark Raymond, to visit us again soon.

Mat Kaplan: NASA's OSIRIS REx successfully completed a final practice session for its descent to asteroid Bennu in October. The spacecraft team believes they are now ready to collect a precious sample for return to earth. Then there's the bad news about the giant Arecibo observatory in Puerto Rico, which has certainly seen more than its share of challenges in recent years. A snapped cable damaged a significant portion of the great dish and the dome that is suspended high above. No one was injured, thank goodness. We hope the great radio telescope will be working again before long.

Mat Kaplan: Here's a calendar alert for you. Virtual events here, virtual events there, but some do stand out. One of these is the Humans to Mars Summit from our friends at Explore Mars. I'd usually be in Washington DC for this terrific annual gathering, this time I'll be moderating a virtual session about planetary protection that will include Alan Stern and NASA planetary protection officer Lisa Pratt. They may put me to work on other stuff as well. H to M runs from Monday, August 31st through Thursday, August 3rd. You can see the entire excellent lineup of presenters and [email protected].

Mat Kaplan: I'm proud that the Planetary Society is once again a cosponsor. Astrobiologist and planetary scientist Stephen Kane is a faculty member in the University of California, Riverside department of earth and planetary sciences and the department of physics and astronomy. One more evidence of his multidisciplinary approach, you'll find it in the planetary research laboratory that he leads at UCR and in the paper that he and several colleagues have just published. That caught my eye because it offered the tantalizing possibility of solar systems with two, three, four, or five, even six worlds in the star's habitable zone; which is kind of astounding, so I asked Stephen to join us.

Mat Kaplan: Stephen, welcome to Planetary Radio and congratulations on the recent publication of this work in the very prestigious Astronomical Journal. You and your colleagues titled that Dynamical Packing in the Habitable Zone: The Case of Beta CVn. And I want to come back to that star, Beta CVn, a little bit later but tell us what you may have learned about the likelihood of worlds like our own in their stars habitable zone.

Stephen Kane: Thank you, Mat. First of all, it's great to be here so thanks for talking to me about this. To give a little background it was a very exciting piece of work to do and I started working with the second author on the paper, Maggie Turnbull, when we were speculating about how many planets you can have in the habitable zone. We were talking about this because both Maggie and I work on various NASA missions, one of which is a direct imaging mission, and so we think a lot about what we can expect to see when some future missions are going to be looking at stars, especially those that are nearby to the sun.

Stephen Kane: And so we were thinking about what are the differences between our solar system and other solar systems. How come we see some systems that have multiple planets in the habitable zone? I should explain what that means. The habitable zone is the region around a star where a planet could have surface liquid water. That's not to say that if it's in the habitable zone it definitely will. It just means it's a way in which that we can target our searches to optimize the search for life. There have been discoveries of systems like TRAPPIST-1. This is a well known system.

Stephen Kane: That is a star which is very different from our sun. It's a very, very small star. It's so small that if it were any smaller it wouldn't actually burn fuel. It would not be a star. It would be something else. And because of that the habitable zone is very close to it. But it has three planets in the habitable zone. When we started working on this, one of the things that I do is I work on dynamical simulations and that means I can create virtual planetary systems, planets of a particular size and mass and distance from their stars, and then in the simulation I start the clock running and see what happens, see how these planets interact with each other.

Stephen Kane: And what I wanted to do was figure out for different kinds of stars, not just TRAPPIST and not just our sun but all kinds of stars in between, what is the maximum amount of planets that you can have in a planetary system. And the key there is that they have to be able to remain in stable orbits for long periods of time. That's really what's setting the limit. You have two planets that pass too close to each other, then they can perturb each other's orbits and then disastrous things can happen. They could collide or one of them could throw the other planet out of the system. We were really looking for those kinds of regions of stability to really put this to the test.

Mat Kaplan: When worlds collide, it's in a book I think I read in middle school. Nice round orbits are advised. I take it that the worlds revolving around TRAPPIST-1, which as you might imagine has surfaced several times on our show, is that what you see in that system that's about 40 light years from us, that these are in pretty circular orbits? They don't get in each other's way?

Stephen Kane: Yeah. Well, as far as we know the thing about the TRAPPIST star, as I mentioned, the TRAPPIST star is very small which also means it's very faint. And because it's so faint, as you may know, the way in which we detected those planets is because it just so happens that they pass between us and the star and they block out some of the light. We call this the transit method. The transit method doesn't tell us a lot about the shape of the orbits, but we know enough about the orbits just from the transit work that's been done to know that the orbits are pretty circular.

Stephen Kane: Usually what we would like to do is we would like to measure the orbit at different places around the star. See, the thing about the transit method is that it only tells you information about the planet and its orbit at that point when it passes between you and the star. There's the whole rest of the orbit when it's invisible to us and we don't actually learn anything about the orbit. But we learned this from data from the Capital space spacecraft which was operating between about 2009 and 2014, something like that. And I did a study back then actually about transiting systems, compact systems like TRAPPIST, and whether they have circular orbits or not.

Stephen Kane: And what we found at that time was that systems where planets are close together they do tend to be circular orbits. We do have some evidence that these kinds of systems, where you have dynamical packing, that they are usually circular by necessity. Because, as I said, otherwise you could imagine there's a selection effect there. Well, they have to be secular otherwise they wouldn't be compact systems. Otherwise, they would dynamically fall apart.

Mat Kaplan: As part of this, you also address the importance or the influence of having a giant world like Jupiter in your neighborhood. I guess that that is, maybe, one of the limiting factors, may be why we don't have habitable next door neighbors here on earth.

Stephen Kane: Yeah. That's one of the big things about this study, Mat. As you may know there's been a lot that's been written over many years about the role of Jupiter in the overall structure and certainly the habitability of a system.

Mat Kaplan: Yeah. And when this has come up in the past here it's been we should thank that big bully out there because it clears a lot of the debris that might have put a stop to life down here.

Stephen Kane: It does. And I've actually read two different sides of that very topic, because some would argue that Jupiter is like the vacuum cleaner of the solar system which is hovering up all of the leftover material which would otherwise come and strike the earth. But the flip side of that particular coin is that in the same way it's absorbing a lot of these potential impacts it's also stirring up. It's changing the orbits of our smaller objects like asteroids and things like that, and sending them towards us. There's two sides to that that I keep reading about, and people have done a lot of work thinking about this.

Stephen Kane: And one very important point there is that if you argue, well, let's just say that Jupiter causes or has a role in causing an increase in impacts on earth is that necessarily bad? Because there are still a lot of ideas that state that a lot of the water which is present on earth is a result of the impacts. And that could be attributed to Jupiter, or at least in some part. All of this is to say that there's a lot of discussion all the time about what role Jupiter plays in impacts and things like that. But in this particular study we looked at the role of Jupiter in changing orbits in our system; because, of course, one of the overarching questions in exoplanetary science and astrobiology is, is our solar system common?

Stephen Kane: I mean, the architecture of our system, is that normal or are most other systems different from us? One of the pieces of our solar system is of course that we have a giant planet at five astronomical units away from the sun, five times the earth some distance, and that's one of the defining features. What role does that play? And so for our simulations, as I mentioned, to get the most terrestrial planets in the habitable zone it's preferred that they be in a circular orbits as possible and then you can pack more planets in there. But if you have a giant planet like Jupiter, then, that can start to mess things up.

Stephen Kane: Jupiter has played an important role in the architecture of our solar system, especially as Jupiter has changed position. Jupiter is currently, as I mentioned, five times the earth some distance away from the sun, but that wasn't always the case during the early stages of our solar system and planet formation. Jupiter actually moved around a bit and so did Saturn. There was all kinds of musical chairs and shenanigans going on with Jupiter and Saturn moving around until they finally settled on their current orbits, but that played a role as well.

Stephen Kane: And if that kind of having a giant planet, it moving around, that could have a devastating effect on the formation of terrestrial planets. Now one thing I should mention is that very small stars like TRAPPIST-1, one thing we do know from our studies of exoplanets, is that small stars tend to not have giant planets. That's not to say that they can't, but they tend not to. And you can imagine that intuitively from the fact that a small star means less material to form planets, meaning you don't have enough material in general to form a giant planet. And so that means that small stars like TRAPPIST-1 could more easily get away with having a larger number of planets in the habitable zone because they tend not to have giant planets. But this is a big part of the study The Role of Giant Planets in all of this.

Mat Kaplan: Stephen, as you create these dynamical simulations, these models in your lab, have you created models where you subtract Jupiter or Jupiter's masses? I don't know, spread out among more worlds? And what happens in our own solar system when you don't have Jupiter? Do you see an effect on the number of at least potential habitable planets?

Stephen Kane: Yeah, that was an important part of our work. As I mentioned, Jupiter has played a very important role in the architecture of our solar system, and one of the ways it does that is through something called orbital resonances. Orbital resonances are when two objects have integer multiples of their overall period, which is how long they take to orbit around the star. If you have an inner planet which moves faster than the outer planet, and let's just say that their overall periods are a factor of two different, then that means that very regularly they line up because, well, one of them keeps catching up with each other at the same point in its orbit.

Stephen Kane: Because of that if the outer planet is something like Jupiter, a large object with a large gravitational influence, then that means it is able to regularly tug on the orbit of that planet consistently at the same place in its orbit through a long period of time. These resonances can result in instabilities in orbits. One of the big examples of these in our solar system are what we call the Kirkwood gaps. The Kirkwood gaps are those locations in the asteroid belt between Mars and Jupiter where we see gaps in the distributions of asteroids. And these Kirkwood gaps are locations where if there were an object there it would have an integer multiple of Jupiter's orbital period.

Stephen Kane: What that means is that Jupiter is essentially clearing out those locations because of these overall resonances. We see that in our solar system. In our simulations what we did was if we have a giant planet like Jupiter and we put it at different locations, then these orbital resonances they move through these locations where you would normally have, say, six terrestrial planets in the habitable zone or five terrestrial planets in the habitable zone. And it can severely disrupt the orbits because over the course of millions of years which is what our simulations, the timescale they run over, you start to see the orbits of the planets change.

Stephen Kane: And as one of the orbits change, it starts to change the other orbits. It has a cascading effect because they move away from these circular orbits we spoke about earlier. Often I refer to planetary interactions as the planets being able to see each other. That means that they can feel each other's presence. That sounds very star Wars-ish, [inaudible 00:17:22].

Mat Kaplan: It's a planetary sixth sense, or the force.

Stephen Kane: They can feel the gravitational presence of the other planet. And as soon as planets start to see each other, then, things can become unstable. That's what happens when you have a giant planet that starts to perturb the orbits. It starts small, but like the well known butterfly effect it can cascade into something far more significant.

Mat Kaplan: Fascinating. If these worlds had been allowed to, or possible worlds, had formed in these gaps where Jupiter prevented them or has prevented them from forming, I guess in our solar system though they'd still be out at the distance of the asteroid belt. Which I assume since that'd be beyond Mars they would not be in the habitable zone.

Stephen Kane: Right. Yeah. In the example I was just talking about the Kirkwood gaps beyond the habitable zone, because Mars is outside the outer edge of the habitable zone and so the Kirkwood gaps lie beyond that, Jupiter gets blamed for a lot of things. Like I said, it's had a major role in the architecture of our solar system. But in particular in this case, if it wasn't for Jupiter we could have had more terrestrial planets beyond Mars. But its presence, the presence of Jupiter, is felt interior to Mars as well. Particularly as I mentioned, Jupiter has moved during its lifetime over the four and a half billion years. It's had all kinds of effects in shaping the modern solar system that we see now that we're still struggling to understand.

Mat Kaplan: Stephen Kane of UC Riverside. He'll be right back with more worlds and great science after this break. We have a new sponsor, and I'm selfishly thrilled. It's Masterclass, the service that enables you to learn from the world's best minds anytime, anywhere, and at your own pace. With over 85 classes from a range of world class instructors, that thing you've always wanted to do or learn, well, it's closer now than you think. Why is this selfish? Because Masterclass has opened up its library to me. I've started with the great Chris Hadfield's class about space exploration, naturally.

Mat Kaplan: It's really a guide to becoming and working as an astronaut. It is superbly produced, and Chris is great of course. There are 29 lessons, all of which are very manageably sized. I think I'll go from here to Neil Tyson's course titled Scientific Thinking and Communication. Then again, I'm a huge fan of magicians Penn and Teller, hi guys, so their class might be my next stop. I highly recommend you check it out. Get unlimited access to every Masterclass, and as a Planetary Radio listener you get 15% off an annual membership. Go to That's for 15% off Masterclass.

Mat Kaplan: Let's talk just for another moment or two about these simulations that you run. I'm guessing that these are simulations which 20 or perhaps 30 years ago planetary scientists might only have dreamed of. I mean, does this kind of work still take super computer power or can it be done on fairly modest computing systems?

Stephen Kane: Yes. One of the extraordinary things I think that as we have delved into this area of having the technological sensitivity to be able to discover planets around other stars computing power has also progressed significantly during that period. It's not necessarily a 1:1 correlation because the sensitivity is mostly instrumental in nature. We have better detectors and we've developed new techniques to be able to find these kinds of planets. But during that same period when I started doing this in the mid ‘90s the computing power that we had back then was fairly limited, but the infrastructure for the simulations people had planned out.

Stephen Kane: But it was very computationally expensive. You did indeed usually have to use super computing power because there's a lot of calculations. Essentially what you're doing in these simulations is you have, say for our solar system if you were to simulate the entire solar system in terms of the major planets, so you had eight planets. You'll have them all at certain locations. And then what you do is increment time by some amounts, say by one day, so you move all the planets by the amount that they would move during that one day. And then you recalculate the gravitational influence they would have on each other and how that would change the orbits.

Stephen Kane: Which is very intensive calculation because you're calculating not just the effect of the neighboring planets on each other but all of the planets on all the other planets. You do that calculation and then you move the planets again by, say, an increment of one day. And then you keep doing this for, as I mentioned earlier, a simulation period of millions of years. This is a lot of calculations, and so it is very computationally expensive. But these days you can run these simulations on a laptop. You can run one of these simulations for a million years. It may take a few days to run, but that's just one simulation.

Stephen Kane: Usually when we do these simulations we run many, many of them, as we did in this paper that I published, because you want to take into account all the different starting initial conditions of the planetary system. And so that usually requires computing clusters. For this work University of California, Riverside we have accessible to us access to high speed computing facilities. And a lot of universities have that these days. There's also NASA high end computing time that you can apply for. It's reasonably accessible, but it does require that.

Mat Kaplan: Yeah. I'm not going to be doing this, at least to the degree that you are, on my Smartphone before too long, maybe on the iPhone 20 or something. Yeah. And you also make me think of that classic of physics, the three body problem. If you haven't heard of that folks, look it up. We're talking about not a three body problem but, I guess, an n-body problem. And it does get pretty complex, doesn't it?

Stephen Kane: Yeah. Yeah. And that's what we refer to these simulations as, n-body simulations, where usually we have quite a large number of bodies involved.

Mat Kaplan: Speaking of simulations, just before we started to record this morning you pointed me toward a certain YouTube video. Because we started talking with each other, I haven't fully viewed yet but I guess it's from this fellow Anton Petrov. We'll put a link up on this week's show page that you can get to from along with a whole bunch of other links, including the abstract to this paper that Stephen and his colleagues have just published and some other good stuff like Stephen's own site. Tell us about this animation and how you learned about it.

Stephen Kane: That was really interesting because I had seen that there were various news outlets who had taken an interest in the results in our paper. But then last night, one of my department colleagues sent me a link to a YouTube video and he said, “Hey, you really need to check this out because this is a popular science YouTuber who uses various space engines to create simulations of new science discoveries that he talks about. And he's talking about your paper.” I had to look at it, and it's amazing. He does a fantastic job of describing the paper.

Stephen Kane: But not just reading from the paper he creates these background simulations, so while he's talking you can actually see in action what he's talking about. And he does a very, very good job of capturing the essence of the paper. I really highly recommend that people look at it.

Mat Kaplan: Good on you, Anton. Again, we'll put a link up to that YouTube video at Tell us about where we go from here, or maybe rather where you and other researchers go from here. And this is probably the time to bring up that star that made it into the title of your paper, the Beta CVn.

Stephen Kane: Yeah. As I mentioned at the very beginning the premise of this work was that I was talking with a colleague of mine, Maggie Turnbull, because both of us have a very deep interest in future imaging surveys. Direct imaging is, of course, the Holy Grail of searches for planets around other stars. Most of the techniques that we use these days are called indirect techniques. That means that we don't see the planet directly, but we see the effect that the planet has on its surroundings. And so we can't measure the planet, but what we can measure is the star. That's what the planet is affecting.

Stephen Kane: And there are multiple techniques which are used to measure very, very carefully the star the planet is orbiting and infer the presence of the planets. And that's, for example, how the TRAPPIST planets were discovered. As I mentioned earlier, they pass in front of the star and block out the light; and so we don't see the planets we see the star getting dimmer occasionally. That's what we see. And from that we infer that there are planets that are causing that. But, as I said, what we really want to do is we want to be able to see these planets.

Stephen Kane: And so Maggie and I think a lot about which are the best targets to search for planets, targets meaning stellar targets that are in the solar neighborhood. Solar neighborhood is something like all the stars within about 100 to 200 light years away from us. And the reason that we want to look at the stars which are close to us is because you can imagine that if a star is far away than the planet is the separation between the star and the planet on the sky it's going to be very, very small and we may not be able to see the planet. But if you start to move that star closer to us, then, the angle between the star and the planet becomes larger and we can actually detect it.

Stephen Kane: That's why whenever we talk about directly imaging planets we're always talking about looking at those stars which are nearest to us; because those are the only ones which, at least in the near term, it's going to be feasible to do this kind of work. We spoke about a particular star called Beta CVn. It's a bright star, so it actually has a name. Its name is Chara or Kara, depending on how you pronounce that. It's C-H-A-R-A. This is a particularly interesting star because it's very similar to the sun. It's a very similar size, temperature, even a very similar age. It's about 27 light years away, so it's pretty close to us.

Stephen Kane: This is a star which is of great interest for those reasons astrobiologically speaking because it is so similar to the sun that people speculate that it could have formed planets under similar conditions and that it could have planets that are suitable for life. The question is have we found any planets around this star? And the answer is no, we haven't. We certainly haven't seen any transiting planets. Since the transit method relies on the planet passing in front of the star, then it relies on the orbit of those planets being directly adjourned. And the probability of that is extremely low so you don't expect that the stars closest to us will have transiting planets. That would be extremely lucky.

Stephen Kane: But what we would be able to do is, with direct imaging, we would be able to get a top-down view of those are planets and be able to see them potentially with direct imaging. Fortunately that star, Beta CVn, has been monitored using the ground-based Keck telescope at Maunakea in Hawaii for a few decades. It's been monitored using a method called the Doppler Wobble method. And the Doppler Wobble method is another indirect way of looking for planets, except instead of looking for the potential dimming of the star due to a transit what it's doing is it's looking for the gravitational effect of a planet on the star so you see the star wobble.

Stephen Kane: We've been doing this for Beta CVn, as I mentioned, for a few decades. The orbital period of Jupiter is 12 years. Jupiter takes 12 years to orbit the sun. It means that if Chara or Beta CVn had a Jupiter analog then we should have seen the gravitational effect on Beta CVn. And so, one of the things I did in this paper is that I used those data from Keck to show that we can rule out the presence of a giant planet as small as Mars is about Saturn mass at all kinds of orbital distances from Beta CVn. In other words, we demonstrated that Beta CVn almost certainly does not have a giant planet.

Stephen Kane: This was perfect because in the first part of our paper we argued that if you don't have a giant planet you can have around about six terrestrial planets in the habitable zone of a star like our sun. Here, we have a star very similar to our sun that does not have a giant planet. And so what we showed in that second part of the paper, because the paper really is divided into two halves… The first is the dynamical simulations, the second is applying this to Beta CVn. And we showed that Beta CVn would be a very interesting test of this whole hypothesis of if we were to directly image it. Maybe it does have a large number of terrestrial planets in the habitable zone of that star.

Mat Kaplan: Wow. And of course there have been some images made of some exoplanets, except that they tend to be huge and very hot. You mentioned that you, and I think you said Margaret Turnbull, are working on a potential direct imaging mission. Can you say a little bit more about that, and is this a mission that might be capable of imaging a world around Chara?

Stephen Kane: Yeah, I'd love to talk about that. The mission that we had been working on was, up until recently, called WFIRST.

Mat Kaplan: Oh, yes. Of course.

Stephen Kane: This is a NASA mission.

Mat Kaplan: Yeah. And we've talked about it several times on the show and its usefulness in all kinds of astronomy.

Stephen Kane: Yeah. It was ranked very highly in NASA recommendations that came from the community. These recommendations are released every 10 years in what we call decadals. In the 2010 decadal WFIRST was ranked very highly exactly because of the reason you mentioned. It's a multipurpose instrument that will achieve a lot of different science, everything from large scale, infrared surveys, to exoplanets; so a very different ends of the distance scale because it answers a lot of questions from cosmology to searching for planets.

Mat Kaplan: I'll cut you off there. I think you were about to say it was, of course, recently renamed the Nancy Grace Roman space telescope.

Stephen Kane: Yes. This is a tradition for telescopes when they move to, essentially, the final phase of their development before launch, that they officially received their name. That happened for Hubble, that happened for James Webb, it happened for Spitzer and now this has happened for WFIRST which is named after Nancy Grace Roman. I think a lot of people now are just calling it the Roman space telescope or something like that. But one of the purposes of this telescope is that it will be able to directly image planets around nearby stars. It's a pathfinder in this whole technique in many ways because this is something that we as a civilization have not really attempted from space yet.

Stephen Kane: That uses what we call an a [inaudible 00:34:32] or coronagraph which blocks out the light from the star. And the difficulty, of course, in directly imaging planets around other stars is that you have a faint object, which is the planet, very, very close to a very bright object, which is the star. And so ideally what you want to do is remove the effect of the star so that you can see the planet, otherwise the light from the planet just becomes washed out in the light of the staff. And so that's what the coronagraph does.

Stephen Kane: The name coronagraph actually originates from solar astronomy because the same technique which is used to block out the light from the photosphere of the sun so that we can see the corona of the sun, which otherwise you only get during a total solar eclipse, we just generalize that term to just blocking out the light from a star. The trouble with doing this from the ground, of course, is that we're looking through the Earth's atmosphere. The Earth's atmosphere makes stars move all around on your detector and so it becomes a very difficult to do it's. It's possible to do, and we have done it as you mentioned. We have directly imaged several planets, and those have all been ground-based.

Stephen Kane: To do this in a more sensitive way though, we need to go to space. And so WFIRST and now the Roman space telescope will be able to do this but mostly for giant planets. Roman space telescope won't quite get us to where we need to be in order to directly image terrestrial planets in the habitable zone. That will come later, because there is a timeline for NASA in these missions. Roman is part of this timeline but then what's been developed now are subsequent missions are. One of them is called LUVOIR, which you may have heard people talk about. LUVOIR is essentially a much larger James Webb space telescope, but it also has a coronagraph so it will be other directly image planets.

Stephen Kane: The ultimate goal is a mission called HABEX, which is the habitable planet explorer. And that mission which may be launched in, say, a decade or two, that mission will be the one which we'll be able to finally directly image planets in the habitable zone around the nearest stars. That is the timeline that we're talking about.

Mat Kaplan: Very exciting stuff. In the more immediate future, you must therefore be very pleased to see that Congress year after year seems to be committed to the Roman telescope, the former WFIRST, because it keeps restoring funds for that telescope that get eliminated by the executive branch.

Stephen Kane: That has been very, very gratifying to see the confidence that Congress has in the science that we're trying to achieve with Roman. And, as you say, year after year there have been attempts to cut the budget to Roman but Congress often not only restores that but increases the funding. There's been a fantastic effort from myself and other colleagues who have petitioned our local representatives. In some cases my colleagues have even gone to the Hill and done this in person. But it's been a fantastic response from Congress to see that they are all behind the science that we're trying to do here.

Mat Kaplan: As is the Planetary Society as you may be aware. Let me bring it back home. We've been talking out about 27 light years to Chara, but back here at UC Riverside you're in both the departments of physics and astronomy and the department of earth and planetary sciences at UCR, which seems like a very appropriate straddling for a planetary scientist like you. But you're also an astrobiologist. All of this stuff seems to be, by definition, multidisciplinary.

Stephen Kane: Yes. It's been an interesting pathway because my interest originally, when I was a teenager and thinking about what I want to do with my life, was in planetary science. But then when I studied that at undergraduate level I majored in astronomy. I did my PhD in astrophysics. When you think about what exoplanetary science is it's essentially astrophysics because, as I mentioned, most of the techniques that we're using, the indirect techniques, are studying the star. I went down this pathway heavily into exoplanet detection, characterization, studying the orbits of planets, but all of that really involves studying the star and inferring what we can about the planet.

Stephen Kane: But then when the Kepler mission launched in 2009 I had a deep realization that we would soon be discovering planets which are terrestrial, rocky planets. Up until then we were mostly finding planets like Jupiter. I started to go back to my planetary science interest I had had a decade earlier realizing that most of the exoplanet community were very ill-equipped to fully understand the terrestrial planets that we would be discovering. And I had a particular, and still do have, a particular interest in the dichotomy between earth and Venus and their evolutionary pathways, how one became habitable then another one is uninhabitable.

Stephen Kane: That was really my divergence from just pure astrophysics and exoplanetary science into planetary science. At that time I was a professor at San Francisco State University then I was invited to come down to a faculty position at University of California, Riverside. Up until then I'd only been in physics and astronomy departments, but this was earth and planetary science. We do have an astrobiology program at UC Riverside and it seemed a perfect fit for me. As you mentioned, it's completely multidisciplinary topic.

Stephen Kane: When we think about astrobiology we think it's this unison of astronomy and biology, but actually that term refers to everything from planetary science, climate science, geophysics, all of this rolled into one. It's all about what we refer to as system science, that is understanding planetary processes. How is the atmosphere of a planet influenced by the geology, by the biology that goes on at the surface? These days I find myself engaging in extremely cross-disciplinary fields, and it's been very, very exciting especially approaching the language barrier that exists between fields and trying to solve that so that we can move forward.

Mat Kaplan: Sounds like a fun place to be. You also lead the planetary research laboratory at UCR. Just say a word or two about your team there and the other work that's underway.

Stephen Kane: Yeah. The planetary research laboratory is something that I started at San Francisco State University. I moved it to UC Riverside and it's grown quite large now. We're a NASA-funded laboratory. We have about five graduate students, about three post-docs, several faculty members involved. The work, based on what I just said, as you might imagine is extremely diverse. We have folks who are looking for giant planet analogs like Jupiter and Saturn around other stars. We're heavily involved in the tests mission which I'm sure you familiar with, the transiting exoplanet survey satellite, which just completed its primary mission.

Stephen Kane: We've led numerous discoveries from tests. As I mentioned, we're involved in direct imaging both in the ground and space with Roman and other facilities. But we're doing a lot of work on planetary processes as well. I mentioned about Venus. A lot of my work is to do with understanding why Venus diverged from earth. One of my students is looking at the other side, which is the difference between earth and Mars and how small can a planet to be in order to be habitable. We also look at the biological implications of different exoplanets. One of my students is a biologist by training and she studies the effect of temperature variations on extremophiles and how this could be applied to terrestrial exoplanets. It's a very diverse group which is by design of course and it leads to a lot of interesting discussions.

Mat Kaplan: All stuff we love to talk about here on Planetary Radio, and across the Planetary Society really. Stephen, I got just one more for you. I'm an LA kid, a Los Angeles kid. I grew up narrowly under all that city light pollution but also when the smog was about 10 times worse. You grew up in the Australian Outback. Should I envy you because of the dark skies you had overhead?

Stephen Kane: Well, in that respect, yes. As you might imagine, Mat, growing up in a small town in Outback, Australia was a double-edged sword; certainly one side of that was the beautiful skies. Myself and many of my classmates just dreamed of getting out of the small town atmosphere, although many of my classmates did stay in that town as it turns out. I did enjoy very much the extremely dark skies. For those of you who are listening who have never been to the Southern hemisphere, especially during the Southern winter which is the middle of the year, when the view towards the galactic center is maximized the amount of stars that you can see in a dark location just is amazing.

Stephen Kane: It's the best view on earth of the night sky. That was part of my whole interest in astronomy. It was really triggered a lot as well by a visit I had to a planetarium when I was about 12 years old. It was part of a school trip. You might wonder what does a planetarium look like in Outback, Australia in the mid ‘80s. Well, it was in a sheep paddock.

Mat Kaplan: How appropriate.

Stephen Kane: It was essentially a large shed, but what the operators of this planetarium had done was rather than any fancy projections which didn't exist back then they had a giant orrery of the solar system which they had built. And they would operate this giant orrery. It was one of the most amazing things I'd ever seen. And they would have this voiceover. You could imagine the kind of Morgan Freeman type of voiceover describing each of the planets in turn. And that was enough for me to really spark that deep interest in planetary science. I remember coming away from that thinking, “I've got to know more about this. I just have to understand this better.”

Stephen Kane: And the other thing is, during the ‘80s, you remembered that was the heyday for the Voyager spacecraft 2 of [inaudible 00:45:54] of the solar system. I remember distinctly in 1986 watching on television the animation produced from images from Voyager 2 as it passed by Uranus. It was one of the most amazing things I'd ever seen. And then in 1989 we saw a similar footage in animation produced by JPL from the encounter with Neptune. Those two really left a strong impression on me. Those kinds of pieces all came together to ensure that this was definitely what I wanted to do.

Mat Kaplan: Well, I'm sure glad they came together and all those great influences because they've resulted in where you are today. And as regulars to this show know, I thank the Griffith observatory for shaping much of my love for all of this and how fortunate I feel when I get to talk to folks like you, Stephen Kane. Thank you so much. Happy hunting Stephen. We'll look forward to all the discoveries to come as we zero in on these planets that might be out there, almost certainly are out there, that could be twins of our own.

Stephen Kane: Thank you very much Mat. It was a real pleasure to talk to you.

Mat Kaplan: Here we go. Time for What's Up on Planetary Radio. Bruce Betts is the chief scientist of the Planetary Society who's back with another comprehensive report for us on the night sky and much more. Welcome.

Bruce Betts: Thank you. And I'm not sure how comprehensive it is but it will, hopefully, at least be accurate.

Mat Kaplan: We'll go for accuracy. Here it is. I have a late submission for your acronym contest. Remember from last week?

Bruce Betts: Yeah.

Mat Kaplan: It's only been a week, of course you remember. [Soda Pong 00:47:36] in New York. He got this and he knows it's late, but I think it's worth mentioning. And of course this is the acronym that you were asking for people to come up with for Mastcam-Z, those cameras that are now headed to Mars on Perseverance. Here it is. Microscopic Asteroid Strike Turned Charming Astrojournalist Mat into Zombie.

Bruce Betts: Oh, no. I've always feared that would happen.

Mat Kaplan: Don't touch it. Yeah, that's it. Thank you Soda Pong. Nice work. We're ready to hear about that sky.

Bruce Betts: Planets, Jupiter really bright over in the Southeast in the early evening with Saturn to its lower left looking yellowish. It'll be visited by the moon. Jupiter will on the 28th. I mean, well, literally but in the sky. And then a really close moon conjunction with Mars, well, within about a degree or so about within a couple moon diameters Mars is coming up a little bit later in the evening, but getting earlier and earlier. Check it out looking red in the East and Mars has done it. It is now brighter than Sirius, the brightest star in the night sky, so can't miss it if you're looking over on the East Southeast in the mid evening. It will continue to get more and more spectacular, brighter and brighter, through October 6th the closest approach.

Bruce Betts: And then in the predawn sky, Venus is about as high as it gets in the sky. You can check it out in the predawn or a couple hours before dawn looking super bright. There you go. Okay, we move on to This Week in Space History. It was a Voyager 2 week. We had the Voyager 2 launch in '77, Saturn flyby in ‘81 and Neptune flyby in ‘89, all by the intrepid Voyager 2.

Mat Kaplan: Just amazing, and it keeps on ticking. Well done.

Bruce Betts: On to Random Space Facts.

Mat Kaplan: That computes.

Bruce Betts: The surface area of mercury is about equal to the combined surface area of Asia and Africa.

Mat Kaplan: This immediately made me think of that line about Mars, that it may only be a third our size. But the total land area is roughly equal to the total land area on earth?

Bruce Betts: Yes. It's one of my favorite random space facts. If I didn't prohibit myself from repeating them, I would give it to you frequently.

Mat Kaplan: Well, I took care of that for you. I think I got that one from you. We're ready to take on this interesting contest.

Bruce Betts: I asked you in the trivia contest, what was the only unintended splashdown of a spacecraft carrying humans? How'd we do, man?

Mat Kaplan: Oddly enough, a very small number of people came up with the Gemini mission that Neil Armstrong saved from disaster because it started to tumble. Most of you know that story. Well, that was always intended to come down in the ocean, so we don't understand that. The vast majority of you came up with, I think, the answer Bruce was looking for including our winner, a first time winner, Christopher Mills in Arlington Virginia who said that that unintended splashdown was Soyuz 23.

Bruce Betts: Indeed it was, in 1976.

Mat Kaplan: Christopher, congratulations. You are going to get your choice of that 40th anniversary T-Shirt from The Planetary Society, the one that shows where a lot of stuff in our solar system was on the day the society got its start; or the vintage society T-Shirt, the one with the original clipper ship logo. I have an original clipper ship logo T-Shirt, and they're very cool. They are selling like hotcakes, I'm told, from Chop Shop. That's where the Planetary Society store is. You can easily get there at I've got some stuff from some listeners, but anything else you want to say about this mission?

Bruce Betts: Yeah, it was a pretty exciting experience. The spacecraft landed in Kazakhstan as it was supposed to, but it landed in the ice-covered Lake Tengiz and crashed through the ice covering; was in the water floating for 10/11 hours, I believe, before it was recovered. It was subfreezing outside. It was foggy. It was quite the experience and we didn't know about it over in the West until after Glasnost and the breakup of the Soviet Union. It was pretty well hidden that this went weird, but they worked it out. It was unpleasant for the cosmonauts, but they got through it.

Mat Kaplan: We heard from a number of you that, yeah, the capsule was found after nine hours but that the recovery crew figured the astronauts had died.

Bruce Betts: Yeah.

Mat Kaplan: Two hours later, 11 hours, they opened the hatch themselves. The inside of the capsule was covered in frost according to Nick Bell in Indiana. Thorsten Zimmer in Germany said, “At least the astronauts got some real quality time.”

Bruce Betts: They were just looking for some nice quiet time together.

Mat Kaplan: Yeah, or at least kept the vodka chilly at least. Laura Dud in California. “Mission was full of failures and mishaps,” she reports, “no wonder these two guys never went back to space.” A great fun fact from O'Connor Cottrell down in Panama, the country of Panama, despite landing in a lake this is actually the saltiest splashdown. Lake… How did you pronounce it? What's the name of the Lake?

Bruce Betts: I said Tengiz, but I can't guarantee that that's a proper pronunciation.

Mat Kaplan: Anyway, that lake has 50 to 200 grams of salt per liter versus the ocean's average of 35 grams per liter, so we still haven't had a fresh water splashdown yet. Thank you, Carter.

Bruce Betts: That's pretty cool.

Mat Kaplan: Finally, this from Dave Fairchild, the poet laureate out in Kansas, “Back in October of ‘76, the Soviet sent up a ship, but they couldn't dock it, their new Soyuz rocket, and so had to call off the trip. And when it came down, it didn't land on the ground but sank in a Lake like a gator. But there was good news, they recovered the crews a total of nine hours later.” Or 11, we won't quote it. We're ready for another one of these.

Bruce Betts: All right. I always like to lead in to things like this by saying Mat does not know I'm about to ask this.

Mat Kaplan: We keep each other in the dark, usually.

Bruce Betts: As of now, so August 2020, what is our Mat Kaplan's one credit on IMDB? Go to contest.

Mat Kaplan: What?

Bruce Betts: Are you reacting to that or the fact that you only have one credit?

Mat Kaplan: Yeah. I mean, what happened to all my Academy Award performances? I haven't the foggiest. I hadn't the slightest idea.

Bruce Betts: You don't know the answer?

Mat Kaplan: Well, this deeply space-related question comes your way from the chief scientist and must be answered. I'm going to look it up. It must be answered by Wednesday, August 26th at 8:00 AM Pacific time. One of you will win, if you find this answer and you're chosen by This is brand new. It's the Backyard Astronomer's Field Guide from David Dickinson who co-wrote the Universe Today's Ultimate Guide to Viewing the Cosmos, which I think we gave away a while back, so it might be kind of a companion. How To Find the Best Objects the Night Sky Has to Offer, it's excellent.

Mat Kaplan: It's very good. And it's in one of these ring binder formats so that you can take it out and the pages will stay open as you stand next to your telescope or hold your binoculars or whatever. It's very cool, I'm paging through it right now. And we'll put a link up to it, of course, from this week's episode page IMDB, huh?

Bruce Betts: Yeah.

Mat Kaplan: My hands are involuntarily reaching for the keyboard so that I can look it up right now, but I'll wait until you close this out.

Bruce Betts: All right. Everybody, go out there and look up the night sky and think about what character you would like Mat play on TV, radio or in the movies. Thank you, and good night.

Mat Kaplan: That's Bruce Betts. He's the chief scientist of the Planetary Society. I play the host of Planetary Radio, and therefore, get to act out these scenes with him every week here in What's Up.

Bruce Betts: Oh, don't give away the answer.

Mat Kaplan: Planetary Radio is produced by the Planetary Society in Pasadena, California and it's made possible by its members who are worlds apart from the ordinary. That sounds like you too. Find out by becoming a member at Mark Hilverda is our associate producer. Josh Doyle composed our theme, which is arranged and performed by Peter Schlosser. Ad astra.