The Man Who Made Stars and Planets

Alan Boss has spent a career predicting how stars and planets form—and has often been right.

By Corey S. Powell|Monday, January 12, 2009
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boss1
Photography by Stephen Voss

Thirty-five years ago, astrophysicist Alan Boss set off on a seemingly quixotic project: He wanted to figure out how planets form around stars. At the time astronomers knew little about the early history of our solar system and had no idea at all whether other stars even had planets. In essence, he wanted to build a model without knowing what he was modeling. Undeterred, Boss spent the next few decades working with the only example available—our own sun and planets—and making his best guesses.

Today those guesses have turned out to be incredibly prescient. Since 1995 astronomers have found more than 300 exoplanets (planets circling other stars), turning up one surprise after another. The first planets found were as massive as Jupiter but huddled much closer to their parent stars than Mercury is to the sun. Nobody had ever anticipated the existence of such “hot Jupiters,” but Boss’s models quickly suggested how these and other gas giants might have formed. Subsequent curveballs included planets in wildly elongated orbits and giant, rocky “super-Earths”—objects that are clearly alien yet also uncannily reminiscent of our own home.

Each discovery has shown just how creative nature can be and has brought Boss a little closer to understanding the intricate process by which a swirling mass of gas and dust turns into a vibrant system of planets. And with each step he gets a little closer to answering the key question behind his work: Is Earth a fluke, or are Earth-like worlds a common result of the way planets arise? In his forthcoming book, The Crowded Universe, he argues that we will soon find many other Earths, and many will teem with life. DISCOVER executive editor Corey S. Powell caught up with Boss in his office at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington in Washington, D.C.

You got into stellar and planet modeling at a time when observing exoplanets wasn’t even possible. How did that happen?
I went to UC Santa Barbara for graduate school, and I was planning to go into high-energy physics. But I took a graduate course from a fellow by the name of Stan Peale, a celestial mechanician, and unlike my classmates I did very well, so he asked me to work with him. He had me look at a book by Viktor Safronov called The Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. It was sort of the seminal work for organizing thought on the formation of solar systems and planetary systems. Somewhere in chapter 2 or 3, Safronov essentially says, “Consider a newly formed star with a disk around it. And let’s make planets.” I said, “How did we get to that point?” I figured I’d just take a few steps back and worry about how stars formed, and then it would lead me, naturally, to the process of planet formation. Coming up on 35 years later, I’m still busily doing the same thing.

But it wasn’t until the early 1990s that we actually saw good evidence of planets around other stars.
There were several decades there when we only had one solar system to worry about. And that made it pretty easy because you could just take any basic astronomy textbook and look in the back and see exactly the masses and the orbits of the planets. You knew what the answer was. Theorists love that—to solve problems where they know what the answer is. By 1995 or so, theorists pretty much knew how to make terrestrial planets, even gas giants. Build up a core first and then pull gas out of it to make the gas giants.

We thought we knew how everything went. Then came the discovery of the planet around the star 51 Pegasi. The planet [a hot Jupiter] was too close to its sun to have formed there, and this implied something had to have migrated inward. Migration had been in the back of our minds, but we considered it a dark, dirty little secret. When 51 Peg came along, we were forced to admit that migration was going to be a big part of the planet formation process. Therefore the whole process had to be rethought.

Nobody expected the planet of 51 Pegasi to be anything like it was. Where did that surprise lead you?
It was more like a eureka moment. It wasn’t a negative thing, though I had published a paper in Science saying the first planets found would be at the distance Jupiter is from our sun. I was spectacularly wrong. And the planet, 51 Peg b, had this perplexing short-period orbit. One night I was lying in bed at 2 a.m. staring at the ceiling and it just sort of came to me. The damn thing must have migrated in and gotten left behind. There’s just no way to understand its forming there. It would have formed somewhere else and migrated in.

What are the reigning ideas of how planets and solar systems form?
Core accretion is the conventional way to build planets that most people believe in. It basically builds on known physics that we pretty much understand. You have solids in orbit in a disk, and the solids run into each other and stick and build progressively larger and larger bodies. In the absence of much gas, you’ll end up with just a rock, like Earth, or an ice giant, like Neptune, perhaps. If there’s gas around and the bodies get large enough, perhaps something on the order of 10 Earth masses or so, then you can start pulling some gas in on top of your rocky core and make something that looks like a gas giant planet, like Jupiter. You just have to build that core fast enough to be able to pull in gas while the gas is still there. If you can, then Bob’s your uncle.

With the idea of disk instability, you propose a very different mechanism, in which clumps in a star-orbiting disk eventually form a gas giant
Disk instability answers the question “Is there a shortcut that nature can take to make gas giants while the gas is still there?” Can the gas itself undergo an instability where it starts clumping together and in a very short period of time make a self-gravitating clump within which a core can form? In the case of disk instability, ice giants form when the star system is located in the region of high-mass star formation. The ultraviolet light of nearby massive stars evaporates the disk and then reveals protoplanets [planet embryos]. And the protoplanets lose their envelopes as well, so you’re left with an ice giant. Some important work has been done on disk instability recently by the Israeli scientist Ravit Helled and her adviser, Morris Podolak, from Tel Aviv. They wanted to see what would happen if you had a gas giant protoplanet formed by disk instability. What happens to solids in the envelope? Can they settle down and make a core? And she found that, yes, they can. In addition, you can actually trap a lot more solids by having that large, fluffy protoplanet basically vacuum up all the other comets that are in the disk. Disk instability can form planets that look like Saturn and Jupiter just as well as core accretion can. So from that point of view, we now have two ways of making gas giant planets. And I think we need both of them. One or the other is not going to do it because there’s such a wide range of systems.

Given all the chaos, all these things smashing into each other, do you think that in many cases both processes are at work?
Oh, yeah. All these processes are going on at the same time.

How did the planets of our solar system get where they are now?
In our solar system, we think Jupiter and Saturn and pretty much everybody else have stayed where they formed. They haven’t migrated more than maybe one or two astronomical units from where they formed, except for the outer planets, Uranus and Neptune, which perhaps underwent some larger-scale migrations.

Could there have been planets before the ones that we know—planets that got wiped out by a hot Jupiter migrating in toward the sun?
That’s been talked about. That one sounds a little too wild to me. But on the other hand, there’s evidence that the stars have been eating planets, including gas giants. I think that some stars do eat planets.

The pace of discovery has just blown me away. Planets are everywhere. We couldn’t see them because it’s so damn hard to see, but nature is quite robust in making planets.

What has surprised you most about the new observations?
The pace of discovery has just blown me away. When I’ve been interviewed over the last 10 years, I would say, “Oh, well, maybe 20 or 30 years from now we’ll know something really good.” But it’s already happened. Planets are everywhere. We couldn’t see them because it’s so damn hard to see, but nature is quite robust in making planets. One of my colleagues here, John Debes, has been looking around white dwarfs for planets. People are looking around giant stars. They have planets too, it turns out, as long as they’re far enough out that the atmosphere of the giant star hasn’t eaten them up. Pulsars [neutron stars] have them. Maybe F [hot, yellow-white] stars—there are probably some planets around them, too, but I’m not sure if you want to live there. Or even M dwarfs, the slow-burning, cooler stars, sometimes known as red dwarfs. That’s one of the big surprises, that the M dwarfs are actually quite hospitable for habitable worlds. There was a lot of focus about M dwarfs’ being bad places to be because early on they give off an awful lot of noxious radiation, almost as bad as pulsars. But after 2 billion years on the main sequence, they kind of calm down, and they’re not so bad to be living around. The SETI folks are starting to put some M dwarfs on their radio-listening programs and tune in to them.

It seems that the prospects for planets keep getting brighter. Nature seems to be breaking in our favor.
And it seems to be more permissive of what we consider plausible. Who would have thought of planets around a pulsar, a rotating neutron star? Who would have thought planets with such low masses would be there? One pulsar planet is just a few Earth masses, and another is closer to a lunar mass. To think that they would be there—and that we could detect them—is truly remarkable. That was a hard one to predict.

Now that we’re seeing the first good evidence of rocky planets, what are the chances of Earth-like planets out there?

I think we’re going to find lots of planetary systems that in a vague sense resemble our system. You’re not going to find something exactly like ours because the process is chaotic. If you went back and let a butterfly flap its wings on Venus, at some point Venus and Earth might have been interchanged. You’re not going to find something exactly like ours.

What about the possibility of Earth-like planets with liquid water?
The hardest ones to make, in some sense, are the gas giants, and we see them all over the place. The Earths and ice giants are probably easier to make. If we’re talking about Earth-like planets, I would guess somewhere between 1 and 10 percent [of G-type stars, like our sun] may have Earth-like planets. Which is huge. Kepler [NASA’s Kepler photometer mission] is going to be launched early next year. Its main mission is 3.5 years. Along the way we will start getting more hot super-Earths and warm super-Earths and cool super-Earths. I think the census is going to start turning up planets really quickly.

Do you have any guess on the odds of finding life out there?
My basic feeling is it’s unlikely that we’re going to find [a planet] nearby that has intelligent life on it. But I think it’s almost impossible for that planet to avoid having some sort of life. If you take a planet that has water and organic material, which you can’t really avoid having in some sense, and you let that thing evolve for a couple of billion years, how are you going to stop it from forming life? There are all sorts of reasons why we’re out of phase with the development of life. It took hundreds of millions of years on Earth for life to evolve from single-celled animals up to multicellular animals to intelligent beings. We could very well be out of phase with the nearby planets, which could be either ahead of us or behind us in some sense. They might have already had their life. So maybe you’re going to be pessimistic and say 10 percent of the nearby stars formed Earth-like planets, and let’s say only 10 percent of them have water and organics. I think that’s very conservative. Now you’re down to 1 percent. How are you going to stop that 1 percent from forming life of some sort? I think we’re going to find that microbic life is quite commonplace in the universe. And wouldn’t we love to see what these worlds look like?

It’s amazing, given how long people have wondered about this.
Absolutely. Human beings have been staring at the stars for hundreds of thousands of years. And pretty soon we’re probably going to know that essentially all those points of light have planets around them. It’s often talked about being the golden age of astronomy, but “golden” doesn’t quite do it.

What is the next big step?
Well, I think the first step is finding some nearby Earths. Because we want them to be nearby if we want to be able to characterize them later on. We can get the mass and its orbital period, and next the characterization, meaning you actually want to get some light from the planet. Spitzer [NASA’s Spitzer Space Telescope] and Hubble [NASA’s Hubble Space Telescope] are not going to do that. Spitzer is going to run out of cryogen pretty soon, for one thing, and Hubble just doesn’t have the sharpness of eyesight to see it. So you are forced to go back to a specialized space telescope, and that puts you back into the terrestrial planet finder category. The basic concept is either looking in the optical [wavelength] with a single dish chronograph or looking in the infrared with a multiple interferometer.

What has the discovery of hot Jupiters and cold super-Earths revealed about the origins of our own solar system?
For the last 30 years all of us have thought about single stars sitting all by themselves—no binaries, not parts of a cluster. We’ve thought our solar system formed in an isolated region like Taurus. But stars form in different environments. Most stars form in regions like the Orion nebula [a stellar nursery], where there are hundreds of [hot bluish] O stars being formed and tens of thousands of lower-mass stars forming and little protoplanetary disks forming. That is actually the most common environment for young stars to form. Understanding how our solar system formed—that’s my wild dream. I mean, to my mind it’s taken 30 years, but I’m starting to think I understand what happened. I expected at some point I’d get old and not care anymore. But I care more and more. It’s just amazing. I think I’ll keep my day job.

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