2012 Kavli Prize in Astrophysics: A Discussion with Michael Brown and David Jewitt
Two of the 2012 Kavli Prize Laureates in Astrophysics reflect on the discovery of the Kuiper Belt at the outer edges of the solar system and the excitement of exploring a frontier long thought empty.
The Researchers
With their discovery of 1992 QB1, hundreds more objects followed. Later, Michael Brown made startling discoveries of increasingly larger Kuiper Belt Objects, including one he called Eris that is more massive than Pluto, and another beyond the Kuiper Belt he called Sedna. These and other discoveries forced astronomers and the larger public to re-think not only the overall architecture of the solar system but the very status of Pluto, which had been discovered as the solar system’s ninth planet in 1930. After much debate and controversy, the International Astronomical Union voted in 2006 to reclassify Pluto as a “dwarf planet” – and suddenly the solar system had only eight planets.
For their groundbreaking work, Jewitt, Luu and Brown were awarded the 2012 Kavli Prize in Astrophysics. Their work has transformed the way we view the solar system and its constituent planets. And their discoveries have taught us that the outer reaches of our own cosmic neighborhood, once thought to be empty, are not so desolate after all.
In a recent discussion with The Kavli Foundation, Drs. Brown and Jewitt talked about their discoveries of the Kuiper Belt and its larger bodies, the nature of the outer solar system, the episode over the reclassification of Pluto, and where their current research is taking them. (NOTE: Dr. Luu could not join the dialogue because of scheduling difficulties.)
THE KAVLI FOUNDATION (TKF): Way out where we find the Kuiper Belt, it must be very cold and very dark. But can you give a more descriptive sense of what it’s like?
DAVID JEWITT: It's at least 1,000 times darker than it is here. Temperatures are 40 Kelvin – about -230 Celsius – very, very cold. So lots of materials that would be liquid or gas on the Earth are frozen solid there.
MICHAEL BROWN: At the very outer edge of the solar system where you find objects like Sedna and Eris, if you were standing on the surface of one of them and took a straight pin and held it at arms length, you could cover the sun with the head of that pin. We think of the sun as dominating the sky, and out there it's still the brightest star around; but during daytime, you would be able to see other stars in the sky.
JEWITT: The other thing is that it’s incredibly different from what you would guess based on movies like Star Wars. The average distance between objects in the Kuiper Belt is huge, and if you stood on one you would not see another one with your naked eye, unless you were in a binary (that is, on one Kuiper Belt Object in close orbit around another). You would just be out there and you wouldn't see any of the rest of the Kuiper Belt.
TKF: So you wouldn’t be flying through a chaotic jumble of floating rocks.
JEWITT: It's the opposite problem. A spacecraft going out to Pluto and beyond would struggle to find things that it could reach. Even the asteroid belt between Mars and Jupiter, which is dense with objects, is still very empty. We’re sending spacecraft out through the asteroid belt all the time and they never hit anything. They never see any asteroids, unless they are going to one. Space is far more empty than you would ever guess.
TKF: Only a handful of objects have been identified in the Kuiper Belt that are about as large as Pluto, but what is your latest thinking about how large these objects can be?
BROWN: At this point we've done a pretty good survey of the largest things in the Kuiper Belt. There might be a few more things here and there, but it looks like there aren't any other Kuiper Belt Objects that are significantly bigger than Pluto and Eris. An interesting question for astronomers over the next decade will be: “What is beyond the Kuiper Belt, and how big are those objects?” We really don't have a good handle on these icy bodies that got flung out to this outer fringe of the solar system. It wouldn't surprise me if we found things that are the size of Mercury or Mars.
TKF: Are we talking about the Oort Cloud – the sphere of even more distant objects that surrounds the solar system, where comets are thought to come from?
BROWN: In the Oort Cloud there could actually be more of these things. But I'm actually talking about this sort of no man's land between the Kuiper Belt and the Oort Cloud, where Sedna resides. The fact that we found Sedna and it's moderately large is a pretty good clue that there must be a lot of objects there. And if there are a lot of objects the size of Sedna, the next question is how many.
JEWITT: The bottom line is the farther away we go, the less we know. If you double the distance, an object of a given size gets fainter not by a factor of 2 but by 2 to the power of 4, or 16. That means our ability to detect anything at large distances is very, very, very poor. In principle there is room for almost anything you want to dream up out there. But there are no strong predictive theories for what might be there. You can say we had a chaotic phase in the solar system and objects were thrown around and ejected to large distances, but there's enough slop in those models that you can't say anything definitively. We have to find them to make progress.
TKF: Kuiper Belt Objects are considered relics of the early solar system and they’re largely unchanged. If you could send a spacecraft to the surface of one of these objects, which one would you choose that we've discovered so far and what would you want to find out?
BROWN: Let me deflect the question for a minute. One of the last things that I would do to learn more about the Kuiper Belt is send a spacecraft there. With a spacecraft you get a lot of detailed information about one object. But one of the most interesting things that we’ve found about the Kuiper Belt is how incredibly varied the objects are. So with one spacecraft you’d learn a ton about that object but not much more about the overall Kuiper Belt and the solar system.
JEWITT: So it would be a waste of time?
BROWN: No, it wouldn’t be a waste of time but it's not going to tell us very much about the Kuiper Belt. It's not going to answer the big questions about how the solar system put itself together. For example, when we look at the surface of Pluto with the New Horizons spacecraft in 2015, we’ll learn very little about what it was like at the beginning of the solar system. It’s exciting stuff, and I’m happy that it will happen. But it's not what I would do if I wanted to learn the things that I think are most important about the Kuiper Belt.
But, that said, if I had my choice I would pick a mid-size object. I’d want one that’s small enough that it doesn’t have an atmosphere or a frosty surface that is changing all the time, but also big enough to have had interesting geological processes, interior processes. I’ll be excited when New Horizons gets to Pluto, but I’m actually more excited to see what its moon, Charon, looks like.
TKF: But if you had to choose a specific Kuiper Belt Object?
BROWN: I'd like to explore Orcus, which is an interesting object. It's got an icy surface, and it's got the potential of ammonia anti-freezes on the surface of it. And it has a moon around it, which means you can see two different things in the same place.
TKF: Dave – what would you target?
JEWITT: I would probably find a close binary object, and see what the components are like. Are they made of the same stuff? Do they seem to have the same cratering history? If they are close, can I measure the distortion of their shapes by mutual gravity? Can I look at their rotation and see if they are exactly synchronously locked, so that their spin periods are equal to their rotation periods? Through these careful studies it might be possible to figure out something about the interior structures of these bodies.
I'm not going to give you a particular one to go to, but I would like to visit such an object and try to understand its dynamics. I agree with Mike, however, that sending one spacecraft to one object is kind of a non-starter. What you would really want is a kind of a hopper mission that could go from object to object, so that over time you could build up some perspective.
BROWN: We are only starting to do that sort of thing in the asteroid belt, with the Dawn mission. And asteroid belt objects are so much closer together so it’s easier to go from one place to another – and even there it's taking years and years to just move from one asteroid to the next asteroid. So in the Kuiper Belt the ideal thing to do would be to get a sample of the population. But I'm not going to hold my breath for that one.
JEWITT: The other idea is to develop very small and low-cost “micro” spacecraft, and just shoot out 100 or 1,000.
TKF: What do you think has been one of the biggest impacts of studying the Kuiper Belt – as far as how it’s changed our view of the solar system?
JEWITT: The biggest impact of studying the Kuiper Belt has been the realization that there must've been some outward migration of Neptune, and that once Neptune moved out there it trapped objects in special orbits but also scattered the orbits of others. There doesn’t seem to be any other very good explanation for the orbits of objects that we see in the Kuiper Belt. So, in addition to giving us a huge amount of new real estate in the solar system, the Kuiper Belt has changed that old picture of a boring solar system – where everything moves like clockwork in circles, and it’s been that way for billions of years. It’s a much more dynamic system, where all sorts of interesting things might have happened as the planets migrated from their initial orbits. The name of the game now is to try to figure out what those things were.
TKF: The discovery of the Kuiper Belt and its biggest objects was really a case of finding something where many of your colleagues thought there was nothing.
JEWITT: The outer solar system represented an area of astronomy in which relatively few other people worked. And I’ve always believed that you should work on subjects that are very undeveloped, and where we don't even know the important questions. I think what happened for us was that the outer solar system has been ignored by classical astronomers because they just haven’t cared about the solar system that much. And it’s been ignored by planetary scientists because they are more interested in nearby objects to which they can send spacecraft.
"It’s exciting to think about large objects out there that are so far away that they have basically been unaltered for billions of years. ...And that means they're basically fossil records of a period when the sun was born." – Michael Brown
As a result, the outer solar system – at least in the past – kind of fell into this dead zone where the planet people didn't really pay attention because they were not telescope users and had no way to study this region, and the astronomers who could have studied this region just didn't bother because why would they?
The discovery of the Kuiper Belt was really a classic example of doing research in a crack between established fields. So this example basically says to me that this is what people should do; this is where the action is. Working in areas where nothing is known is really where science moves forward the fastest.
BROWN: And I would say that it's just fun. It's a lot more interesting to not know what you're going to find than to measure one more decimal place on the property of some object that everybody's been studying for decades.
TKF: Half a decade has passed since Pluto was reclassified as a Kuiper Belt Object. Looking back on that period today, Mike, how has that episode changed the way you think about the scientific process, and the public’s relationship with science?
BROWN: As much as astronomers were scared to finally reclassify Pluto, they had known since at least 2000 that Pluto had been misclassified to begin with and should have been part of the Kuiper Belt. But I think astronomers knew that there would be a huge public outcry, and nobody really had the guts to irritate the public. In some ways I think that was the best possible thing that could've happened. It’s only led to a lot more discussion of the solar system.
TKF: And school kids have gotten new solar system maps.
BROWN: Five or six years later, it's talked about correctly in schools. It only took us that long to take this mistake from the 1930s, an honest mistake, and fix it. I think there's more work to be done, but everybody now knows that Pluto is not a planet and if you ask someone, “Why?” he or she will likely say because it's too small. What I would love to have happen in the future is if I go to some kids and say, “Okay I know you think Pluto is not a planet because it's too small and you were told that, but I would like you to draw the relative scales of all the planets.”
TKF: It’s tough, if not impossible, for people to understand the scale of things in space.
BROWN: Right now if you have people do that, they get it incredibly wrong. Most people think the Earth is about half the size of Jupiter, and Pluto is a tiny bit smaller than that. And it's because that's what you see on lunch boxes and placemats and everything else. I think our next job as educators is to explain the solar system in a way that finally helps people understand the scales of these things – and as Dave discussed earlier, how incredibly empty the solar system really is.
We’ve shown that the public is indeed willing to listen to reason, eventually. There's a lot of contention at first, but when scientists finally say, “Look, we kind of screwed it up early on and we’re just fixing things right now,” I think everyone is finally willing to go along.
TKF: Dave, at the time when Mike discovered Eris and the debate ensued about whether to reclassify Pluto, what did you think about how things developed?
JEWITT: It was interesting, although not scientifically interesting because it wasn’t a scientific issue at all; it was really a sociological and educational subject more than a scientific subject. When I give a talk to the public I always show a plot of Kuiper Belt Objects and ask people to show me which dot is Pluto. And of course nobody can do that, because Pluto is really just one of many in the group. It's one of many objects with similar orbits and orbital properties. We recognized that in 1995, but it took another ten years to diffuse into the public domain.
Why did it cause a big reaction? I suspect because the educational system in the U.S. still emphasizes memorization over understanding. It's as simple as that – that people who learned that there are nine planets could not unlearn that and did not want to unlearn that. There was resentment: “You told me this fact, so how can you tell me it is not a fact?” I think that the educational system encourages this rigid way of viewing the world. We memorize a lot of things and expect them to be true forever.
"...[S]cience is continually evolving. It’s a very self-critical enterprise that basically does not stay the same. Everything that we know at the present time will eventually be shot down or modified in some important way in the future, and that’s a good thing." – David Jewitt
TKF: But that is not how science works.
JEWITT: Right. That is exactly not what you get from science, because science is continually evolving. It’s a very self-critical enterprise that basically does not stay the same. Everything that we know at the present time will eventually be shot down or modified in some important way in the future, and that’s a good thing! Scientists understand that but I think the message hasn't really gotten through to the public yet, at least not in schools.
TKF: If the reclassification of Pluto was kind of this teachable moment, do you think people learned anything?
JEWITT: As time goes by and people are born they’ll definitely grow up with new ways of seeing the world. My daughter is 11 and she is completely happy with the idea that there are eight planets. She doesn't care, and that is exactly the correct attitude. What difference does it make what you call an object? That's not the point. It's more important to understand the object, how it came to be, and what it means for our solar system. That's what’s important.
BROWN: I think we do a bad job of educating. It's not just that people memorize that there are these eight planets or nine planets. They also have pictures of the planets, even from NASA, that show them as the same size. They're all the same size because for people who study planets, the overall architecture of the solar system is generally not as interesting. But if you start presenting people with images of what the solar system is really like, they’ll realize that there's interesting structure out there. That's what I hope eventually happens, but I'm not convinced it's going to happen anytime soon.
TKF: Dave, you and Jane Luu wrote in 2007 in the journal Daedalus that the Kuiper Belt might have been detected earlier if only astronomers were open to discovering it. What kinds of other objects in the solar system do you think are going unnoticed or at least being not studied enough?
JEWITT: The example we gave in Daedalus was that many of the brighter objects discovered in the Kuiper Belt have also been subsequently found in data gathered as far back as the 1950s. So, in principle, if someone wanted to look at these photographs taken in the 1950s they could have discovered them. We could have known about the Kuiper Belt before the space age even started.
However, nobody thought to look because why would you if you think Pluto is the last planet in the solar system and that's the end of it? It’s a perception problem, and it’s very hard to get over. We can detect things, but yet fail to perceive them – and that’s just the way our brains work. We are very sensitive to things that we can easily interpret or have seen before, and we are very insensitive to things that are completely new. So, how can we say what the next step will be? Presumably there are many things in existing data that are completely new but which are completely beyond our perceptual horizons at the present time. But how to break through and perceive them is a really difficult question. I don't know the answer.
BROWN: We had been doing our large outer solar system survey for five years, and all the large objects that we had found were between 30 and 50 Astronomical Units from the sun. (An Astronomical Unit is equal to the mean distance between the Earth and the sun, or about 93 million miles) It's very difficult to find things that are farther away than that because they're moving very slowly, and when you're trying to distinguish between a very slowly moving object and the star, sometimes the stars look like they jump around a little bit.
So we imposed an artificial cutoff and said we wouldn’t look for anything slower because it's too hard. Only years later did we decide to go back to see if there was anything even farther out that we missed. And it was hard. We had to re-do all the software, figure out how to limit the stars, and everything else. But, it was only from that re-analysis years later that we actually found Eris, which is the most distant object that anyone has ever seen in the solar system. And it is moving more slowly than anything we’ve seen. We had taken the data in 2003 and analyzed it in our typical way. Eris was there in the data, but we didn't see it until almost two years later when we re-looked at it.
TKF: Where is each of you now headed with your research?
BROWN: I'm still searching for these much more distant objects we talked about earlier. The question is, “How large an object could be out there between the Kuiper Belt and the Oort Cloud?” It’s exciting to think about large objects out there that are so far away that they have basically been unaltered for billions of years. They've not been pushed around or tugged by anything in the solar system like the Kuiper Belt Objects have. And that means they're basically fossil records of a period when the sun was born.
It's hard to find these objects, as Dave keeps saying. As you get farther and farther away they are really faint, so the surveys that we are doing now are with the biggest telescopes in the world with the biggest fields of view. Even so, we’re talking about objects that are still hundreds of times more rare than Kuiper Belt Objects and significantly fainter. So they’re hard to find. This is a new exciting realm that nobody knows anything about, but it’s where I think the new excitement is going to be.
JEWITT: About six years ago, Henry Hsieh and I discovered a group of objects that orbit the sun within the asteroid belt. They look dynamically like asteroids, but they have the physical appearance of comets. We first thought that they must all be ice-containing bodies that are being heated by the sun; that their gas evaporates and dust blows out just like in a regular comet. These asteroids are in a region very close to Earth – about 3 Astronomical Units from the sun – and this is the region from which some people think the Earth obtained its oceans and the organic ingredients of life. The biological precursors to life probably come from this region of the asteroid belt. So it's a very interesting region of the solar system that I’m trying to explore with the hope that we can connect this back to the origin of the oceans, and maybe to the origin of life.
TKF: So these newly discovered objects represent a new frontier in solar system exploration.
JEWITT: It’s really another example how our perceptions can limit our thinking. Think about it. The asteroid belt was discovered in 1801, and people have been studying it in every way possible for 200 years. But these new objects that orbit the sun like asteroids yet have properties similar to comets are right under our noses but went completely unnoticed until the last decade or so. How could that happen? True, some of them are faint, but some of them are not faint. They should have been noticed.
I think what has happened is that we've broken through another barrier of perception. Asteroids in people's minds should look like point sources. They should look like inert rocks. So to have an asteroid that is any different is just not something we can easily absorb. To me the whole subject is just awesome because it's like the Kuiper Belt, which sprang out of nowhere. This kind of work leads in directions that are hard to predict, and that is really, really cool. It's really exciting and that's why I love astronomy.
TKF: So Mike, you are casting your gaze a little farther out from the Kuiper Belt, and Dave, you’re casting you’re gaze closer in.
BROWN: Yeah, there's just more to be found everywhere.