Interview with Renu Malhotra

September 21, 2005


CI:      What’s your personal history—how did you get into your field?

RM:    I wanted to study physics all my life; through college I was completely passionate about physics. In graduate school I discovered thermodynamics, and through that planetary dynamics, which was an epiphany. It was a match.

CI:      So you knew you’d found your niche as a grad student. How did you discover physics early in your life?

RM:    I was born in Delhi, and grew up near there. My father only had a high school education, but he eventually worked for the Indian Air Force as a technician. He left and joined the airline when I was six or seven years old, ascended through the ranks through self-study, and eventually became an engineer for aircraft maintenance. He was very patient and open, eager to answer my questions about nature and how things work.

I went to a very good all-girls private school as a charity student; my parents couldn’t afford it. The teachers were excellent—they encouraged my interest. Labs were open, free-form; you could do whatever experiments you chose. I spent a lot of time doing optics experiments, looking at how light bends, lensing, simple high school stuff.

CI:      You were an undergrad in India also?

RM:    Yes, I went to the Indian Institute of Technology in Delhi, where I studied physics. I was one of five or six girls in a class of two hundred and fifty.

CI:      After being in an all-girls school—that’s quite a transition!

RM:    It was. I was overwhelmed—I did some risky things that I probably wouldn’t have done if I’d had a more balanced exposure to the male sex.

CI:      But you felt a meritocracy in the education itself? You felt you could go as far and as fast as you wanted?

RM:    Yes. People ask me, ”How did you overcome the pressures and expectations of that society?” I was oblivious to them, that’s how I sailed through—I had blinders on. My family’s unusual; my parents had no issues, they never raised any concerns about me wanting to do something different from the normal path.

CI:      That’s the ideal of parenting, an unquestioning assumption that your kids can do whatever they want.

RM:    As a child I didn’t recognize how unusual that is in parents. I see it now—they allowed me risks that I wouldn’t allow my own children.

CI:      Well, the world may be a slightly more dangerous place, too.

RM:    Perhaps.

CI:      You went to Cornell. Was there culture shock in coming to the States?

RM:    I came to graduate school thinking I was going to do high-energy physics or theoretical particle physics, or maybe astrophysics. My first summer I did a project on nonlinear dynamics. That’s when I began to realize how narrow my ideas of possible research areas were.

It also helped that I didn’t do well in my classes that first year. [Laughs] That steered me towards other things. High-energy theory and particle physics theory were clearly at the top of the hierarchy in graduate school. We had this team of “three wise men” who were assigned to each incoming graduate student. They discouraged me from high-energy physics or theoretical particle physics. Part of the culture shock was not doing terribly well that first year. I got A’s in some classes but I bombed one. That dampened my self-esteem. But then I discovered nonlinear dynamics, this fascinating subject nobody had ever mentioned and I’d never run across before. I put myself into that wholeheartedly.

CI:      And now it’s a huge theoretical subject. How did you decide that you wanted to follow the astrophysical applications?

RM:    That was serendipitious. One of the “three wise men” was Ed Salpeter; he was the most influential. He put me in touch with people doing planetary dynamics in the astronomy department. At Cornell, the astronomy and physics departments were physically separated. They were in different buildings, so you wouldn’t automatically run into astronomers.

CI:      What was your thesis topic?

RM:    It was on the dynamics of Uranian satellites. Voyager 2 had just had a fly-by of Uranus in 1986, and I got started working on the Uranian satellites around that time. There were some fresh problems that the planetary community had to deal with—we’d discovered some things about Miranda, which is one of the small moons of Uranus. The anomalous precession of these classical Uranian satellites was a long-standing problem.

Miranda and the other satellites weren’t doing what they was supposed to. It was recognized that neither their periods nor the other motions were adequate to what the satellites were actually doing. These satellites have orbital periods of days, and their precession periods are on the orders of a few years; there was adequate astrometry to know that they were doing something not in accord with the periods. The first project I did was very simple; it effectively translates into showing that the mean of cosine-theta is not zero if theta is not omega-T.

CI:      [Laughs] Okay.

RM:    Theta is a non-linear function of time; it’s only slightly nonlinear in this case. Omega-T here is a small frequency. There are pairs of Uranian satellites that are close to orbital resonances. There’s no phase locking between orbital phases, but they’re close, so there’s a smaller frequency there. Traditionally celestial mathematicians had assumed that frequency was some constant frequency; that it’s a constant small difference. Cosine of the omega-T averages out over time, so it doesn’t contribute to any long-term precession rates.

This was not known in celestial mechanics at that time. So I pointed out that this is really a cosine of a small term that actually has a cosine inside it, so it does not average to zero. That provides that extra precession rate that was missing in the theory. That was my first project in graduate school.

CI:      Let me ask about the techniques you use. The average person who knows a little bit about gravity is probably aware that things get complicated when there are three or more objects. When you’re working in pure theory as opposed to using simulations and computers, what are the techniques you bring to bear on these complex systems?

RM:    One of the most powerful things I use is Hamilton-Jacobi theory. It proves very powerful in ferreting out these small forces or small determining effects, which can build up over long periods of time and then dominate the long-term behavior of the dynamical system.

CI:      Has the emergence of powerful computers changed the complexion of the field?

RM:    It’s changed it a lot. There are few people trained to work with analytical tools. For the recent generations of graduate students there’s so much to learn, so most of these classical analytical tools don’t show up in their courses.

CI:      Do you think it’s a detriment to the training of intuition?

RM:    Absolutely, it’s a huge detriment. Analytical tools are what allows you to work creatively—in my field and in a lot of astrophysics, in terms of theoretical work, not having command of those tools puts you at a serious disadvantage. If all you’re doing is simulations—which is what graduate students are more comfortable with now—you’ll be limited in what you can predict for the system. You put in what you know, essentially, so your expectations are limited. With analytical tools, the range of things you can project into possible phenomena, things that might interact with each other, is vastly bigger. I’m certainly at an advantage that way, compared to people who are doing large-scale simulations. On the other hand, simulations have their place too. There are equations that we simply cannot solve; simulations can guide you toward the answers.

CI:      Are there areas that haven’t been tackled because the tools aren’t available; phenomena that haven’t been explained satisfactorily even in the Solar System?

RM:    There are plenty of phenomena that haven’t been explained; I haven’t thought about that, why haven’t certain phenomena been explained to this day? I think it has to do more with imagination. The problems I’m thinking of have to do with the early history of the Solar System, which is hard to tackle observationally. You’re doing forensic work—you’re in the present day looking for clues that are four and a half billion years old, and then you’re trying to imagine all the phenomena that might have gone into producing the evidence that survives today. It’s not so much about the analytical and numerical tools, it’s more about imagining the environment of the planetary system.

CI:      When you start with detailed knowledge of the way things are now, what are the limitations of doing the back-projection, of understanding or even constraining initial conditions?

RM:    We can do fairly well. There’s a wealth of new observations and discoveries in the Solar System; we’re beginning to probe farther, so we have this huge new population of small bodies in the Kuiper Belt that we didn’t know existed ten years ago. Observations are crucial; technology is providing us with windows to the early history of the Solar System that we would not have imagined without observations. We could have speculated, and there were plenty of speculations prior to the discovery of the Kuiper belt. Is there a limit to how much we may be able to learn about the earliest history? Yes, but we’re nowhere near that limit yet.

CI:      What about the interplay between the systematic properties that lead to patterns of terrestrial planets and giant planets, and the serendipitious impacts and things that seem more chaotic or stochastic? When you’re trying to tell the whole story of the Solar System, how do you judge the importance of unusual events that may be unique to our history?

RM:    There are systematic effects on our Solar System that are very important, and there are stochastic effects that are equally important. It depends upon the questions you ask. We seem to be aware of the stochastic things—like large impacts—and their relevance to the history of life on Earth. How important are they in telling the big-picture story of the Solar System? Not especially; they don’t throw a monkey wrench into our big picture, because even though we have these stochastic events, we can still use statisical methods over a long period of time.

CI:      We are naturally interested in the history of the Earth, yet we face the fact that it has a rather unusual moon, and the interplay between the Earth and the Moon has had particular effects on the Earth that might play into how life evolved. How can we draw conclusions about what we might find elsewhere? Is the Earth “rare” in some way?

RM:    I’m on the fence. Regarding the Moon: it provides this large angular momentum within the system that keeps the spin axis of the Earth more stable than it might be otherwise. That translates into a stable climate over hundreds of millions of years, perhaps billions of years. Without the Moon, the Earth simply might not be habitable—that seems like a strong, attractive argument. But I don’t know enough about feedback cycles and climate to say that such a climate would necessarily be completely hostile to life, that climate variations would quench life if the Earth’s spin axis were wobbling a bit more than it does now.

CI:      There’s always that danger in analyzing the habitable conditions on a particular planet. As you say, there are many feedback cycles, and Mars has had a different set of constraints.

RM:    Yes, and to say that Mars doesn’t have advanced life on it because it doesn’t have a large moon would be a stretch. Mars is still in the so-called habitable zone. It’s at the right distance from the Sun, it’s the right size, it could support a liquid water environment near its surface. We can’t say that Mars has not developed biologically the same way the Earth has because of peculiarities such as the Earth having a large moon, Jupiter being where it is, and so forth. We can’t attribute the divergent biological histories of Earth and Mars to those specific conditions.

CI:      There seems to be a danger of cherry-picking the phenomena to fit the idea. Along with the stabilization of the Earth’s climate by the Moon, we also have the existence of plate tectonics and snowball Earth episodes. That goes in the other direction—it’s an extreme variation in climate, yet life has persisted and done quite well. The Earth doesn’t seem to be a Goldilocks environment in any sense.

RM:    That’s right, there are other phenomena going on, pushing Earth to not be habitable. It’s fascinating that Earth should have plate tectonics, and no other planet that we know of.

CI:      Let me ask a question about the future rather than the past. We have a pretty good story of how the Solar System got its present architecture. If you wind the clock forward billions of years, what is the dynamical fate of the Solar System?

RM:    In the Solar System, the major planets’ architecture seems to have plateaued long ago. We’re not likely to lose the major planets through the forces we recognize.

CI:      Does that judgment include the passage of a nearby star?

RM:    Yes. The probabilites of having a deep encounter with a nearby star are so low that, over the next five billion years of the Solar System, that’s not likely to have any significant effect. The architecture of the major planets is fixed, but the small bodies are still evolving. The Kuiper belt is losing mass, the asteroid belt is losing mass, it’s eroding. The mass of the asteroid belt is so little that it’s hardly there.

CI:      What is it, four percent of the Earth?

RM:    Smaller, maybe two percent. It’s so little, but we think there should be a lot more mass, something like a hundred times what it is today. We also infer that it was not lost gradually; it was lost in some major event within the first few hundred million years of the history of the Solar System. The asteroid belt was decimated—decimated squared—reduced in mass by a factor of about a hundred in one dynamical event. There’s just enough left there to tease us. We’re learning something similar about the Kuiper belt—that what remains today is a small fraction of what used to be there. Something happened to remove it.

CI:      And it was the influence of Jupiter that stopped the asteroid belt from becoming a planet in the first place?

RM:    That’s the theory, yes.

CI:      So something else very early on had to remove a lot of the mass.

RM:    That’s right, because Jupiter wouldn’t have been enough. If you were to increase the mass of the asteriod belt by a factor of ten now, it would take a long time to waste away.

CI:      So it’s eroding slowly.

RM:    Very slowly. We need an external mechanism that we don’t see today to get rid of most of the mass. We think the giant planets moved around early in the history of the Solar System, which would do enough damage. Having the planets where they are today is not good enough, but if you move them even a little bit, you can reduce the asteroid belt to a very small fraction of its original mass. That phenomenon probably accounts for a lot of the mass loss in the small body populations.

CI:      Let’s go from that to the debris environment of a terrestrial planet, which presumably affects the evolution of life. With what we know about our Solar System and dynamics of other solar systems, do we expect to see widely varying debris environments for terrestrial planets? Are all solar systems going to have a comet cloud, for example?

RM:    Not necessarily.

CI:      So the architecture and layout of the small bodies could be different?

RM:    Very different. We know of a few examples of planetary systems around other stars but what we see are only the multiple giant-planet systems. In these systems—in particular systems like 47 Ursa Major or Upsilon-Andromeda—there’s no room for terrestrial planets dynamically, and there’s no room for debris. Debris in our Solar System is only in the inner part of the system; the only way to support debris in the system is to have smaller bodies, or to have debris at large distances, beyond the giant planets; that gets into the regime of about ten astronomical units and beyond.

Some of these stars have higher luminosity than the Sun—Upsilon-Andromeda is thirty percent more luminous than the Sun. You have to presume their solar systems would be somewhat bigger than our Solar System, but there’s no place for a terrestrial planet in what you would nominally call the habitable zone. I don’t think there’s any room for something like an Oort comet cloud, either.

The reason for that is this fine-tuning of where a giant planet is located. The source of Oort cloud comets was close in, somewhere in the range of ten to thirty AU; that’s where the giant planets can control these objects and put them into wide orbits. Then they’re circularized and cannot return to the system. To do that, you have to tune the mass of the giant planet to its distance from the star. If Jupiter were the only large planet in our Solar System, we would not have an Oort cloud, because Jupiter would eject small bodies. You need something slightly smaller than that. We have Saturn and Uranus and Neptune, and they did the bulk of the work in creating and placing objects into distant orbits. Having only giant planets in this region where you potentially have a source of comets is a way not to have an Oort Cloud.

CI:      Let’s come back to the debris and the impact environment of our terrestrial planet, and maybe others. Since these impacts play so heavily into the idea of contingency and evolution of life, we’re faced with these Copernican questions of how unusual our one example is. We can look at a terrestrial planet and the details of its twists and turns in the road to evolving a biosphere. Do we know enough, either from theoretical expectations or using inference on the extrasolar planets discovered so far, to say what the impact environment of terrestrial planets might be in general?

RM:    In the examples we have, there’s no room for terrestrial planets. Our observations haven’t yet accessed systems that have room for terrestrial planets, and if they were to have room, they almost certainly would have room for debris.

How finely tuned is our system to have Earth’s impact history, and how typical do we think that might be? Over the years, I’ve been more and more impressed by how finely tuned the Solar System is in terms of having orbital stability over time scales of a billion years. Stability by itself for a billion years would not be unusual; in the formation process and the evolution process, unstable planetary systems should be weeded out. There are plenty of orbital configurations you could expect to be stable on billion-year time scales. Our Solar System seems to be unusually stable. It doesn’t have to have the fairly circular orbits of the giant planets in order to be stable for five billion years. There’s room for greater orbital variations.

CI:      What induction would follow from that?

RM:    That there’s some kind of self-regulation, perhaps, or some mechanism within the formation and dynamical evolution of planetary systems that drives you to be highly stable. How important is it for life on Earth? Let’s say Jupiter and Saturn and Uranus and Neptune had higher orbital eccentricities, for example, or that they were not as closely aligned in the same plane as they are. Their orbits would vary a lot more than they do on long time scales. That variation would be important to orbital variations for the inner planets. It might destabilize some planets. As I said, one can support the inner planets with much larger orbital variations of the other planets. How would that transmit to the evolution of life on Earth? I don’t know, it’s a question that needs to be studied.

CI:      That’s good, it means we haven’t tapped out the inference we can draw from this solar system.

RM:    Yes, I think we’re nowhere close to the limit. By inferring the history of the Solar System, and inferring more than just the history of this one example, we can generalize from this one sample for the potential for other neighborhoods in deep space.

CI:      Let’s get to your recent work. I have this picture in my head of huge bombardments that diminish with time in the first half-billion years of the Solar System as the debris is mopped up or ejected. And we think life started somewhere toward the end of that first half-billion years. But you think it was more of a pulse than a steady decline, right?

RM:    The record of large impacts in the inner Solar System is basically the crater record on the geologically dead bodies—Mercury, the Moon, and Mars. We can’t infer it from the Earth because the Earth is geologically active and erases all early craters; the typical age of the rock on the surface of the Earth is a couple of hundred million years, whereas the typical age of a rock on the Moon is four and a half billion years. Thanks to the Apollo Program, we’ve known for the last thirty years that the ages of the large impacts on the Moon, the ones that made these big basins, all cluster at about 3.9 billion years old.

CI:      The Moon formed a crust within what, a hundred million years?

RM:    Within fifty to a hundred million years. We know that the Moon formed 4.53 billion years ago. The oldest meteorites in our hands also go back to roughly that period of time. Stellar evolution models for the Sun also date the Sun in that neighborhood. All those ages hang together, that the planets formed at about 4.5 billion years. But there are no samples of rocks from the Moon that we can identify with large craters dating to anything prior to about 4.05 billion years. Since then, and up to the present, the impact frequency hasn’t changed much. So all the large basins on the Moon formed within a period of about a hundred million years.

CI:      So what happened between 4.5 billion years and 3.9 billion years?

RM:    That’s been a subject of controversy. One of the possibilities is that there was debris hanging around that was slowly swept up, and as it was decaying away, the impact rate was decaying and finally came to an end at 3.9 billion years. But that explanation is not in accord with having all the large basins form right at the end.

Another argument is that these large basins wiped out any record prior; we only retain the record of the last few large basins. But that doesn’t fit the crater record. There’s plenty of surface area on the Moon. The big basins are not overlapping. You have to clutch at straws to say that all the large basins simply came at the end, and it was a steady decay.

There’s also chemical evidence. If you look at the lunar meteorites from all over the surface of the Moon, they have impact melt ages, or shock ages. We have a technique to measure when the rock was last metamorphosed by shock waves, which only happen in large impacts. Those have a wider spread of ages, but they peak at 3.9 billion years.

Bob Strom, Professor Emeritus at the University of Arizona, has been studying craters his whole career. He had suspected that there were two different size distributions in the crater record. When we look at the Moon, we see the dark areas and the bright areas; the lunar highlands are the bright areas and the dark areas are the seas or mare. The mare were created by volcanism as recently as 3.2 billion years ago, so they’re younger, and all we see on them are impacts from the last 3.5 billion years, because the volcanism wiped out the older crater record. The crater size distribution on the highlands is clearly different from the crater size distribution on the mare. The size distribution of the craters on the mare is very simple. The lunar highlands, on the other hand, have a complex size distribution, with bumps and valleys. We find the same thing on Mars.

Bob’s team compared the projectiles that would have made these two different size distributions. We can infer the sizes of projectiles that would have made a crater. It turns out that the young craters are made by projectiles that have the same size distribution as the near-Earth asteroids. So we compared those asteroid populations. We can only do that now, we couldn’t have done it three or five years ago, because we didn’t have asteroid sizes down to the sizes needed to compare with the crater record.

This new data gives us statistics for the size distribution of asteroids. It turns out that the main belt asteroids have a different size distribution from the near-Earth asteroid population. Asteroids that leak out of the main belt and become potential impacters for the inner Solar System do so by slowly spiraling in under radiation forces that slowly make them lose or gain orbital energy. Then they get nudged into resonances with Jupiter in the asteroid belt; and once they hit a resonant orbit, they get kicked into crossing orbits on time scales of a few million years. It’s like a filter—the small asteroids are easier to move with these forces, so the ones leaking out are preferentially the smaller ones. That means the near-Earth population is going to be different from the main belt size distribution, more heavily biased towards the small guys. And that’s in the right direction to explain the difference between the near-Earth asteroid size distribution and the main belt size distribution.

The amazing correspondence was that the craters on the young surfaces on Mercury, the Moon, and Mars, correspond exactly to the size distribution for near-Earth population, whereas the craters on the lunar highlands, Mars highlands—the heavily cratered ones, the ones that go back four billion years—have exactly the same size distribution as the main belt asteroids. That raises a question: we’re looking at the main belt today, four and a half billion years after those craters were made, and it has the same size distribution—so what happened? We know that the asteroids that hit us now come through this filter, so that we have more small guys hitting us, but four and a half billion years ago everybody from the asteroid belt was hitting us, without that filter. That tells us that the process by which asteroids were launched into the inner Solar System was a dynamical process with no regard for size. So how and why was that filter imposed?

CI:      And what was so special about 3.9 billion years ago?

RM:    There must have been a mechanism to destabilize the asteroid belt. There’s no consensus on that yet. My notion is that Jupiter’s orbit was changing. It was in a stable, non-changing orbit for five hundred million years and then the process of this migration of the planets started. That was fueled by planet formation in the outer Solar System. Making Uranus and Neptune was a problem. Making Uranus at that stage of the Solar System seemed to be difficult through standard planet formation processes—accumulation of planetessimals, making a core, and then collapsing gaseous envelopes. It’s not hard to do for Jupiter and Saturn, because you can make those cores with the higher densities of rocks beyond the so-called snow line. But making Uranus and Neptune is harder because the orbital frequencies are long, so collision frequencies are low. The surface density of solids is falling off quickly as well.

CI:      What was your idea?

RM:    My idea is that it took about a half-billion years to form the cores of Uranus and Neptune. Uranus and Neptune have twenty percent hydrogen, we believe; the question is whether the gas hangs around that long in the solar nebula. The general understanding is that it does not, that the solar nebula does not last for a half-billion years; it’s gone in only ten million years. So how do you create this envelope for Uranus and Neptune? They have very little gas compared to Jupiter and Saturn. They probably have similar-sized cores, but their hydrogen and helium content is about twenty percent of the gas, as opposed to ninety percent for Jupiter. Some new work on hydrogen absorption on ice crystals might give us an explanation. You can absorb lots of hydrogen in ice crystals and then, later on, once you’ve formed your core, evaporate it and produce a hydrogen envelope. We don’t know yet whether you can do enough of that to make this ten-to-twenty percent hydrogen atmosphere.

That was my idea for how long it took Uranus and Neptune to accumulate their cores. Once they got to be ten-fifteen Earth masses, they became very disruptive in the outer Solar System, and they moved the debris in the outer Solar System around. Say a comet from the Kuiper belt would get thrown into an orbit that crosses Jupiter’s orbit, and then Jupiter would eventually throw that out, and thereby lose some orbital energy and move in. I think that process might have started five hundred million years after the formation of the Solar System. Moving the planets is important in destabilizing the asteroid belt; moving Jupiter and Saturn pushes all these disruptive forces across the asteroid belt. It’s extremely efficient; you can lose practically all of the asteroid belt if you move Jupiter and Saturn.

CI:      And as a side effect of that, you end up with a pulse of big impacts on Earth. Thinking of the history of life, it’s an interesting time scale because it’s about the age of the oldest evidence for life. We think the ingredients were available fairly quickly—oceans had formed, there were probably a couple hundred million more years for life to have existed beyond the age of the oldest carbon-type tracer evidence of life. Perhaps things might have been a little balmier and pleasant before? Was it a sterilizing pulse?

RM:    I look at it this way: what’s the mean time between ocean-vaporizing impacts, and what’s the time to re-condense the oceans? My back-of-the-envelope calculations suggest that it was not sterilizing. The oceans would recondense in a short time compared to the mean time between ocean-vaporizing impacts.

CI:      So an ocean-vaporizing impact is not sterilizing?

RM:    Jack Szostak has suggested that it may not be, if life can sequester itself in rocks. But prior to that pulse, there was no heavy bombardment, and that would be a period of almost a half-billion years.

CI:      Quite a lot could have happened in that interval in biological terms. I want to get to extrasolar planets. As the statistics of extrasolar planets accumulate, and they’re mostly super-Jupiters, mostly on close, tight orbits, with high ellipticities, people started to wonder—is this the norm? Are we just looking at the tip of the iceberg? What type of conclusions can we draw about the population?

RM:    We’re still looking at a biased sample. In terms of what’s possible, in terms of mass distribution, in terms of orbital distribution, solar systems like ours are still beyond our observational knowledge. I don’t see why they would be prohibited. Observations at the present time do not rule out solar systems like ours.

CI:      Why are there so many super-Jupiters on tight elliptical orbits?

RM:    We believe these planets form at just a few AUs and migrate inwards, in the process pushing out any terrestrial planets that form. That seems to be the consensus. I am not wholly wedded to that idea. People have tried to argue that you cannot form these planets where they are, and they sound like very robust arguments—but do they really rule that out? How can we test that, how can we test that these planets migrated—is there some clear-cut test that says that the radius of this planet rules out that it formed anywhere but five AUs? I would like to see a test like that.

CI:      What about the question of whether they’re stable over the long term?

RM:    We can only ask that question about multiple-planet systems, because two-body systems are stable. The orbits of most of the multiple-planet systems that we’ve found are not sufficiently well-determined to say whether they’re stable or not. Occam’s razor would say you have to assume they’re stable

The highly eccentric orbits of these giant planets are interesting. I did some work on them, and there’s a piece of information in the orbits that people don’t pay attention to—how the long axis is oriented. If you look at multiple systems, there are two or three orbits that have their long axes oriented relative to each other. You can study that parameter and ask what clues that can provide about the ellipticity. This elliptical orbit is oriented in one direction. You have another neighboring planet whose orbit is oriented relative to this guy in some fashion. Is there any correlation there? Turns out there is for some of these multiple-planet systems—there’s information in the phasing of those orbits that suggests that these planets actually formed in circular orbits, and have been disrupted. In other words, something like a passing star knocked one of the planets into an elliptical orbit, which then transmitted that ellipticity to all the other planets in the system.

Here’s another explanation: maybe a planet got thrown out of the system and left a scattered planet, so the surviving planet would then be left in an eccentric orbit, and then through these long-term conditions would transmit that eccentricity to the other planets. The phasing combination there has to do with transmitting the eccentricity to the other planet. That can really help in the periodic oscillation—it creates a periodic exchange between two orbits. At any given time, those other planets might have eccentric orbits, but they would periodically fall back to zero where they started. There are at least two systems where you have evidence of this, and we know their orbits well enough; Upsilon-Andromeda is one of them. We have good enough orbits there to say that in this system, the inner planet is going through this periodic oscillation in eccentricity. You can’t directly observe it; those periods are a few thousand years long. But if you know the initial conditions well enough, you can project.

CI:      Looking forward to when we are finding terrestrial planets in significant numbers, we’ll be able to play the same game. We’ll have dynamical information that will tell us about aspects that must affect their biospheres by analogy to the Earth. Do you think it’s too early to say what the nature of the typical terrestrial planet environment is, because we haven’t sampled the population yet?

RM:    Absolutely. This is a biased sample. But one thing they’re learning from this sample is that planetary ejection is probably more common than if it only occurred during a close stellar encounter. That means we have Jupiter-mass planets floating around. Almost every planetary system that forms can eject a Jupiter-mass planet. So we have free-floating planets throughout the galaxy.

CI:      Can those free-floating planets have moons? Is that plausible or not?

RM:    Would the moon survive that scattering? I don’t know the answer to that.

CI:      What I’m getting at is that if life just needs an energy source, it might be tidal heating or radioactive decay. We’ve got such a strong presupposition that illuminating stars are essential; maybe that’s not necessary, maybe free-floating planets are habitable, or large moons orbiting those planets. We’ve honed our vision on single, solar-type stars for the most part; it’s the reasonable thing to do. What can we say dynamically—given that most stars are not single stars—about the stability and longevity of planetary systems in binaries?

RM:    The general conclusion is that there’s practically no difference for wide binaries in terms of stability of orbits; we’d expect that in close binaries the planets would be on close, tight orbits around those stars. Again, to me the question is: couldn’t life transcend any of these differences of orbital architectures? We don’t understand the ways in which biological systems can compensate for a variable environment. Even with chaotic orbits and not-nice circular orbits, and no large moon to stabilize your spin axis…couldn’t biology compensate for any of those external environmental variables?

CI:      If you look at the chaotic radiation environment of the Earth, the geological changes, and then the planetary dynamics, does our system seem unremarkable in the spectrum of architectures we’re going to find elsewhere?

RM:    Yes, even among short-lived systems. Suppose you make a planetary system that’s stable for only three hundred million years. Primitive life can probably get started in that time. I don’t think we can say it couldn’t get started.

CI:      And if it was in a star cluster, the panspermia probability would be plausible, too. You could kick life off even in short-lived environments in a star cluster, and then eject life-bearing rocks, seed the cluster with life. So the game is definitely wide open.

RM:    Yes.