Questioning Dwarfs

In 2005, Michael Brown of the California Institute of Technology and his team discovered a large body in the outer solar system. It was not the first distant object that had been found in the Kuiper Belt — the region is composed of hundreds of icy objects encircling our solar system. But it was the largest known Kuiper Belt object, just beating out Pluto in size, and so their discovery was heralded as “the tenth planet.”

Scientists think eventually we will discover many planet-sized globes in that distant region of space, and that brings to the forefront questions about what defines a planet. Recent discoveries of many unusual extrasolar planets in other solar systems also have put the definition of “planet” up to scrutiny. This planetary debate was put to a vote at a meeting of the International Astronomical Union in 2006, and scientists there chose to reclassify Pluto and other large Kuiper Belt objects as “dwarf planets.”

Artist’s concept of Eris (the object formerly known as Xena) and its satellite, Dysnomia. The sun and planets appear in the distance. Credit: R. Hurt, IPAC

In the final part of this five-part lecture given at NASA’s Jet Propulsion Laboratory, Brown answers questions from the audience. Questions include “Why did the name of Xena get changed to Eris?” and “Could there be another undiscovered planet in our solar system?”

Q:Why did the name change from Xena to Eris?

Michael Brown (MB): Xena was never supposed to be the real name. The naming is done by the International Astronomical Union. They can be very fast, and they had a process set up when we announced what we found, but then it got waylaid by the fact that nobody knew what to call this thing – is it a planet or not? While we were waiting for them to give it an official name, we gave it a nickname to make it easier to talk about, as we always do when we discover these objects. The three of us involved in this, Chad Trujillo out in Hawaii, and David Rabinowitz out of Yale University, and myself, gave it a number, K1021-C, but that’s not nearly as memorable as Xena. The IAU didn’t give it a name for a couple of years, and I think the name Xena first slipped out in an article in the New York Times when they asked me what we called it. So the name got repeated about a hundred thousand times until it became the name that everybody knew. But it was never going to be the real name. There was some argument that modern mythology is equally valid as ancient mythology as a naming convention, and we toyed with the idea of seeing if Xena could still stick, but in the end it’s a tad embarrassing. The next person will come along and name one “Spock” and that’ll be the end of that. I’m a little sad to see Xena go, but her spirit lives on.

Q:On the image of Eris taken by Hubble, there was some irregularity. Can you deduce any of the surface features?

MB: No. All the irregularities were due to irregularities in the mirror of Hubble. We didn’t know how big it was going to be at the time we took the pictures. We were hoping it was maybe twice the size, because then we could’ve maybe eked out some features. But I think there are no features to be seen, because if it’s reflecting 86 percent of the sunlight, it’s mostly white and featureless. A lot of astronomers have spent a time trying to see if it rotates, to see if there’s any kind of variation as it rotates, but so far to my knowledge nobody has seen anything.

Q:I noticed in your map of the Kuiper Belt objects, there was one region that seemed empty. Is that because of your lack of data from the winter sky?

MB: No. When you look at the overview plot of all the Kuiper Belt objects, you see there aren’t any at one region. That’s the region of the Milky Way galaxy where people don’t look. So there are plenty of them there, if you could get all those stars to get out of the way so you could see them. There might even be big ones there, there could be something bigger than Eris hanging out there that will take another 50 years before those things have moved out of the line of the galaxy.

Q:Where’s the Oort cloud?

Illustrations of the Kuiper Belt and Oort Cloud. Click image for larger view.
Credit: NASA/JPL/Donald K. Yeoman

MB: The Oort cloud is at the very edge of the solar system. It’s this spherical group of vaguely-bound bodies that are just on the edge of being ripped out of the solar system by passing stars. Every once in awhile they get a little bit of a kick, and an object comes in and becomes a comet and then goes screaming back out. The Oort cloud is about 10 to 100 times farther away than the farthest that Sedna ever gets. I doubt that we will ever directly see the Oort cloud in my lifetime. Astronomers tend to be clever people, so somebody might come up with a clever way to do it, but it’s going to be tough because it’s really far out there.

Q:Do the orbits of any of these dwarf planets intersect the orbits of the eight planets?

MB: Yes. A few of the known 200 dwarf planets will cross into Neptune’s orbit, just like Pluto does. This is confusing, because some people say, “Oh, but that means that Neptune’s not a planet, because it hasn’t cleared these objects out.” But Neptune has done better than cleared them out, it has forced all of these things into orbits where they cross just barely, and then it pushes them away.

Q:Is there any chance that there may be another planet in or beyond the Kuiper Belt?

MB: Oh, this is my favorite question! Are there any other planets? What would it take to be classified a “planet” now? If you were out at, let’s say, ten times the distance of the Kuiper Belt, then you would have to get picky about what the definition meant. You would probably have to be bigger than the Earth, maybe not quite as big as Neptune or Uranus, but you’d have to be pretty big. Hard to imagine how something like that could get out there and be in a nice circular orbit and clear its zone.

You would think I would hate this definition. My poor tenth planet got taken away! But in some ways I kind of love it. It’s a challenge to see what else might be out there that you could consider to be a planet. Something the size of Mars would challenge our ideas of what it means to be a planet. Something really big enough to clear its zone probably can’t exist that far out, but that doesn’t mean we’re not looking.

Q:Were people upset about Ceres being demoted as a planet?

MB: They apparently are happy. Because Ceres used to be just a plain old vanilla asteroid, it was going to be a planet for about a week, but now it’s a dwarf planet. There’s a mission going to Ceres…

Q:No, when it was discovered a hundred years ago.

MB: Oh, back then? I don’t know the answer to that. I don’t think so, because Ceres didn’t have a Disney character associated with it.

The orbits of the outer planets in the solar system, with the blue and purple dots representing the location of Kuiper Belt objects. Click image for larger view.
Image Credit: NASA/JPL-Caltech.

Q:I read your comments following the IAU conference, and I want to commend you for taking the high road. Mr. Stern wasn’t so gracious. But I’m still not clear why the new definition of a planet excluded Pluto.

MB: Forget the wording and the definition, because astronomers don’t deal precisely with definitions, they deal in concepts. Pluto is part, and only a small part, of this huge array of objects in the vicinity around it. It’s like picking up a handful of gravel and saying, “This one piece, we’ll call it a planet.” It’s a handful of gravel, and then there are eight boulders. If Pluto had been the only thing out there and the rest of the Kuiper Belt didn’t exist, then the question would never have come up. But it’s part of a population. The IAU vote was, “If you’re a part of this population, then we’re not going to call you a planet.”

Q:What about Pluto’s orbit crossing Neptune…?

MB: That was an unfortunate misstatement in the first AP report that said because Pluto crosses Neptune’s orbit it’s automatically not a planet. It’s because it’s part of the Kuiper Belt, not because it crosses the orbit of Neptune.

Q:And the moon Charon…?

MB: The fact that Pluto has a big moon around it is not important. It’s part of the Kuiper Belt, and if you’re part of the Kuiper Belt, you’re not a planet. If you’re part of the asteroid belt, you’re not a planet. If you’re a part of any belt, you’re not a planet. Black belt, brown belt, it doesn’t matter – no planets.

Q:Is there a possibility that New Horizons could take pictures of Eris?

MB: Unfortunately, it would have to turn around and go in the opposite direction. New Horizons is trying to go to a Kuiper Belt object, the bad news is that from our viewpoint Pluto’s background is the middle of the Milky Way galaxy – the line of sight where Kuiper Belt objects are hard to find. So we currently don’t know of any Kuiper Belt objects that are right on the path. New Horizons doesn’t have a lot of fuel to burn to make a sudden veering, so they have to go straight to Pluto, and then they can go a little bit off. I’m excited that they’re going to go to Pluto, which is a large object in the Kuiper Belt, and then their goal is to go to a very small object. I think you learn much more by going to a large one and a small one then going to two really large ones that are going to be similar in some ways. I can’t wait to see what those pictures of Pluto look like.

The “New Object” in this graphic is Sedna, a dwarf planet with a 12,000-year orbit around the sun. Click image for larger view.

Q:When you find a new object, how are you concluding that its orbit is so vast and elongated, such as the orbit you determined to be 12,000 years?

MB: The process is you take pictures of it for maybe a month, and then once you have a month’s worth of pictures you can predict where it should have been last year. So you go back to pictures from last year and see if it is where it was supposed to be. You think you find it but you’re not sure, but say “if that’s really it than I should be able to predict where it was two years ago,” and you look and see if you can find it. And so you leap frog back for as many years as you can. For Sedna we went back about 10 years, so when I said 12,000 years it might be 10,000 years, it might be 14,000 years – there’s some uncertainty there. But still, 10 years is a short time period. From your high school physics, if you can bring your mind back to that, all we need to know to know the entire orbit for all time is to know where it is, its position, and its velocity. And if you know any object’s position and velocity and what else is affecting it, which is just the sun, you know everything. So we don’t have to follow it for 12,000 years, we just have to make a good measure of position and velocity. And with 10 years of data we can do that pretty well.

Q:How do you calculate the temperature from the size and density?

MB: When we calculate the density simply by how much it weighs and how big it is, then the question is, what is it made out of? Ice density depends on the temperature, and also depends on how compressed it is. If you took a ball of ice and turned it into a dwarf planet, it would get more dense. So it’s a little bit more complicated then just saying, “Yep, this is the density,” go down the periodic table and it must be made out of bismuth.

Q:Do expect Eris to have any methane or ethane to keep it from being pure ice?

MB: Pluto is covered in ethane, in addition to having nitrogen. A lot of things in the outer solar system have methane. Saturn’s moon Titan is orange because of the methane in the atmosphere. Methane is a carbon atom and four hydrogens, and if you rip off the hydrogens you’ll start to build together these carbons and you get this red gunk, this sludge on Titan. The same thing should be happening on Kuiper Belt objects. It happens on Pluto — we absolutely see red stuff on Pluto. But if you look at Eris, it’s not nearly as red as Pluto. I think it takes some time for this reddening to happen, and if you were to get out your razor blade and scrape off the surface of the atmosphere that’s on the top, I think you would see the reddening underneath.

The large tilt in Pluto’s rotational axis (120 degrees) creates very large seasonal variations. As surface ices melt in the migrating sunlight, their gases are transported and redeposited in dark, cold surface regions as frost. At the same time, in other locations on the planet or as Pluto’s seasons change, some of the gases of the atmosphere freeze on the surface, perhaps covering much of the planet with freshly deposited ices on time scales of thousands of years. Click image for larger view.

Q: How do you know that Eris is round?

MB: We don’t know, but it’s so massive that no reasonable material could withstand the pressures and not come into a round shape unless it was spinning very rapidly. You know, the Earth is not round, the Earth has love handles because it’s spinning a little bit. If something is rotating fast and therefore not round, we’d have to be looking at it almost precisely pole on. If we saw it at a different angle, we’d see it get brighter and darker. In images from Hubble, we tried to see if we could see any elongation, and we didn’t detect any. I think chances are it is not rapidly rotating and in fact it is round. We have other reasons to think that too. We now know the plane of the moon’s orbit, and if it was rapidly rotating you would see it in the moon’s orbit.

Q:Is there is there a good reason to believe there could be an object the size of Mars far away from the sun?

MB: It’s what we in the field call hand waving. There’s enough hand waving there that we could probably have flown across the room. It is a fairly inescapable conclusion that something large is out there. It doesn’t have to be the size of Mars. Maybe it’s the size of Mercury, or half the size, maybe it’s the size of the Earth. But simply by one detection of Sedna, you can follow that chain and say, “One Sedna means 60 other things, maybe 30, maybe 90, but if there are 60 things that are about the size of Pluto, the largest one is going to be about the size of Mars, or maybe Mercury, maybe Earth.” The actual size of the largest one is extremely uncertain, but there are going to be many large objects out in that region that is going to be found over the next 5 or 10 years, so stay tuned.

Read part 1 of this lecture: “Defining Planets.”
Part 2: “Hunting Distant Dwarfs
Part 3: ““The Discovery of Eris
Part 4: “Studying Dwarf Planets

Related Web Pages

Speeding by Jupiter
Questioning Pluto
The Ice Dwarf Cometh
Moons Over the Kuiper Belt
A Glimmer of Pluto on the Horizon
Tenth Planet
To Pluto and Beyond