We believe it's a primordial population, left over from the epoch when the solar system was forming. It's very far away from the sun, so we think it's pretty pristine, because it hasn't been heated a great deal. It kind of evolved over the last four and a half billion years, the age of the solar system, [mostly] due to collisions. In the beginning there were a lot of small things -- these things collided with each other, and made bigger things, and so on and so on.
The [belt's] orbit has certain characteristics. Each object [is in] a very definite group, and we know how they're grouped. Some are like Pluto, and those we call the Plutinos because they're in these locations called "resonances" that give them some special properties, one of which is to be protected from a close encounter with Neptune. Then there are the scattered objects, and these have very elliptical orbits that bring them far away from the sun. So there are all kinds of funky orbits going on in the Kuiper Belt.
We think they collide with each other, and that's why people are interested in their role in solar system formation. [Astronomers] think they see dust around stars where they believe planets are forming, and that is similar to what we see in our own Kuiper Belt.
NASA's New Horizons spacecraft is on its way to the Kuiper Belt, and is scheduled to fly by Pluto in July 2015. Is there anything you're hoping it will find once it gets there?
Anything would be fantastic. I don't know if it's going to do a rendezvous with another Kuiper Belt object or not. I think there's a plan to do it, but I don't know if the orbit is going to allow that to happen. But anything – a picture – anything at all would be great, because besides Pluto and Charon, we really don't know anything about these objects. For the few [inner solar system] asteroids that were visited by spacecraft, all those images were spectacular, they were all so unexpected. The asteroid didn't look anything like what we expected it to be. So I imagine if we got pictures of the Kuiper Belt they would be spectacular.
What did you think about the controversy when Pluto was "demoted" to a dwarf planet?
I remember back when [fellow Kavli Prize winner] Mike Brown was very keen on keeping Pluto's planethood, because he found other things that were comparable in size, so if Pluto remained a planet than those other things that he and his team found would be planets, too. So he was very much against demoting Pluto. Then I guess he realized that you can't buck the trend forever and, scientifically, Pluto's planethood is difficult to defend. So I guess after a while he gave up. Then he wrote a book called How I Killed Pluto [laughs]. You adapt with the times.
Are scientists still finding lots of new Kuiper Belt objects?
There are lots of surveys happening. But the remarkable thing is…I worked on the field until we discovered [the first one] in 1992, and I kept working on it for another 10 years or so. We predicted from our survey an estimate for the population of the Kuiper Belt objects, and that hasn't changed.
What is that estimate?