Kavli Meets Kuiper

Two decades later, three scientists are rewarded for discovering a new body of objects in our solar system.

Courtesy Jane Luu

Nearly 20 years ago, a Massachusetts Institute of Technology professor and his graduate student embarked on a project to find new objects in the outer solar system, motivated by the call to arms behind so many scientific discoveries: "If we don't do it, who will?"

Each year, the Kavli Foundation gives awards to scientists making "fundamental contributions" in their field of study. In 2012, David Jewitt, the former MIT professor now at the University of California, his former student Jane Luu, and Michael Brown of the California Institute of Technology were jointly awarded the $1 million Kavli Prize in astrophysics "for discovering and characterizing the Kuiper Belt and its largest members, work that led to a major advance in the understanding of the history of our planetary system."

 Air & Space spoke with Luu, who has moved on from the world of observational astronomy and now works on laser radar systems at MIT's Lincoln Laboratory. She told us about studying the Kuiper Belt, what it means for demoted planet Pluto, and what we still hope to learn.

A&S: Were you surprised to win the Kavli Prize?

Luu: Oh, absolutely. I did this research 20 years ago. So yeah, I'm extremely surprised. I just assumed that everybody forgot about it, and that was that.

Why did you and David Jewitt start looking for the Kuiper Belt, and how did you go about it?

I had just come to MIT as a graduate student and was looking for a project to work on, and he said, "Why don't we look for things in the outer solar system, beyond Neptune?" His idea was that the only object known at that time beyond Neptune was Pluto. He thought that was kind of odd, because there were so many things inside Neptune's orbit, and there was nothing else outside. According to the model of how we believe the solar system originated, it formed from a disk of dust and gas. That disk didn’t have any sharp cut-off around Neptune's orbit. So why is there such a sharp drop-off in the population of the solar system? He said, "Well, maybe it's really empty, maybe something happened and things that used to be out there got sucked away somehow. Or maybe things just seem empty because nobody looked."

The way to answer that question is to do a survey. We can do this with a CCD [charge-coupled device], but a CCD at that time didn't have many pixels; the CCD you have in your camera now has many more pixels than on the ones we used back then. The biggest CCD we had access to was 500 pixels by 500 pixels, so the field of view was very small. To use such a tiny little camera to survey a big part of the sky seemed like a pretty daunting prospect. Up until then, all the surveys of the solar system were done with photographic plates, and those have huge views, several degrees on a side, and that's how people covered big areas of the sky. I remember asking Dave, "Isn't it kind of crazy to do a survey with a CCD?," because nobody had ever done this before. And he said, "Yeah, but if we don't do it, who will?" So that was the beginning.

How long did you think it would take?

We were realistic. We didn't think this could be done in six months. We had a very small field of view, so we knew it would take us a while to cover any area of significance. It wasn't my Ph.D. thesis, because the prospect of finding nothing was very, very real. 

What did you think when you discovered that first object in the belt?

We searched for five years before we found that one, so we were used to failure and we were very cautious. I remember Dave spotted it first; the two of us would go observing, and one person would run the instruments of the telescope and one person would look at the data as they came out.

The way you search for moving things is, you pick a spot in the sky where you're going to take a picture, then you take a picture of that spot and you wait a little while, and you come back and take a picture of the same piece of the sky, at least two or three times. The idea is that all the things like stars and galaxies don't move over time scales of minutes and hours, so anything that moves would be a solar system object. The speed at which these things move tells you how far away they are, so things that are closer would be moving much faster than something out at Pluto's distance.

We would take multiple images of a piece of the sky, separated by an hour or two, and then we could look at them sequentially to see if anything moved. The computer can do that very well. You just load the images into memory and it plays the images sequentially, very rapidly, like blinking. You remember when you were small and you had those little flip-books where, if you flip the pages very rapidly, it looked like things were moving? Same idea.

Dave was looking at two images, and he saw something move and said, "Hey, come take a look at this." It looked plausible, but it could have been an artifact – CCDs are very good at detecting cosmic rays. We would take a minimum of four images, so if you see something moving systematically in the same direction at the same speed in four images, then you can be pretty sure that it’s real. He spotted something after two images and I thought, Oh that's good, that's a good candidate, but… we didn't jump up and down because we'd been doing this for a long time. Then we took a third image and that object was still there, it was moving in the same direction, at the same speed. So then we were pretty excited. And we took a fourth image and it was still there. The next day we went back to the same field and it was still there. By the second night we knew that it was something real.

How many objects did you eventually find?

Many dozen, we lost track of them. Finding something is one thing, but if you don't follow up with more observations, you just lose them. Let's say you take 10 observations in two nights of some object. The only way to find that thing again is to fit an orbit [taking enough position measurements to extrapolate accurately] so that you can predict where it will be six months later, a year later, two years later. If you can't fit a good orbit, you'll never see it again.

We found lots of things, but to be able to fit an orbit so that these things are recoverable, that took time – time that we didn't have. We wanted to characterize the entire population, and we were just hoping that somebody would do the follow-up observations so we didn’t lose them.

What do we know about the Kuiper Belt now?

We now know Pluto is not alone. There is a huge population of objects beyond Neptune, and Pluto is just the largest one. This population is now called the Kuiper Belt. Not because [Dutch-American astronomer Gerard] Kuiper deserved the name. He wasn't the first or the only person to hypothesize this population, but somehow the name stuck.

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?

It depends on the size. For 100 km size objects there's on the order of 100,000. And then if you go to smaller and smaller sizes, it gets bigger and bigger, and by the time you get to say, one meter, or one km, there's 1011 or something. When you talk about Pluto class, there's about on the order of 10. When I was at Harvard in 1995, I gave a seminar titled "How Many Plutos Are There?" The answer was between five and 10. I still believe that number.

What do you think we'll learn about the Kuiper Belt in the near future?

I think the dynamics still holds a lot of surprises, because we still don't understand the origin of these weird orbits. We have ideas, like maybe there were close encounters with passing stars. If the sun formed in a cluster so that there were other stars relatively nearby, they would have had an influence. They could have stripped off the outer part of the Kuiper Belt – the Kuiper Belt population drops off drastically beyond about 50 astronomical units [one AU is the distance between the Earth and the Sun, about 93 million miles]. Some people call that the Kuiper Belt cliff. It's unlikely that the solar nebula, the disc that formed everything in the solar system, just got truncated sharply at 50 AU. I think dynamics is still an extremely rich area that holds lots of surprises.

[Studying their] physical properties, that still goes on. It's difficult, because they're so faint. There's a handful of telescopes that are capable of doing these things: Keck, Gemini, the big telescopes. Those big telescopes are rare and everybody wants to use them, so getting time on them is even more rare. So I think the dynamicists might be faster, they might be more efficient in coming up with new answers than the observers, simply because they don't need the telescopes.  There's a lot of work to be done in both theory and observation, but I think theory is going to yield more, just because these things are so faint. 

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