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.