Studying Dwarf Planets

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. 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 part four of this five-part lecture given at NASA’s Jet Propulsion Laboratory, Brown talks about Eris, the dwarf planet he discovered, and discusses some other intriguing findings in the farthest reaches of our solar system.

“Our newly discovered dwarf planet is named after Eris, the Greek goddess of discord and strife. She caused the Trojan War by starting quarrels amongst the goddesses. Her favorite method of creating discord and strife is to start quarrels, to cause people to think they are right and everybody else is wrong and to fight about it. I think the name fits very nicely. Somebody said, “Everybody will forget about this fight about defining planets in a couple of years and then you’re stuck with this name.” I remind them that Pluto is the God of the Dead! What was up with Clyde Tombaugh?

Although I hope the showdown over Pluto is now laid to rest, I think it’s an important part of the history of astronomy. I think the debate is a fantastic way to teach people about the solar system and these populations within the solar system.

Eris has a satellite. This doesn’t mean it’s a moon. Mercury and Venus have satellites, also. Of the thousands of Kuiper Belt objects out there, about 10 percent or more have satellites. So having satellites is not uncommon. Scientifically, though, it’s fantastic, because that allows you to figure out how much Eris weighs. You watch how the satellite goes around the object. If it goes around fast, it’s because this thing is really heavy and pulling it around quickly. If it’s more sluggish then it’s a light object.

We know how much it weighs, we know how big it is, so that means we can figure out how dense it is. That will tell us what it’s made out of. Is it made out of uranium, is it a snowball, or is it something in-between? The answer is probably going to be some combination of ices and rocks, but by figuring out the orbit of this little satellite, we’ll be able to figure out exactly or at least get some idea of what this thing is made out of.

Artist’s conception of eight dwarf planets. No clear and detailed photographs have yet been made of these objects. Click image for larger view.

The satellite’s name is Dysnomia. You can shorten it to Dy if you prefer that. We tend to call them Eris and Dy. Dysnomia is the daughter of Eris and the demon spirit of lawlessness. Anybody who thinks a little bit might figure out why we chose that name.

If you take the Earth and the moon and shrink them down by a factor of five, the distance is very close to the system of Eris and Dy. It’s about the same general scale, just a little smaller. That sounds like a coincidence, but we’re convinced there’s a scientific reason behind that, and that Eris and Dy formed in the same way the Earth and moon formed. Most people believe the moon formed when a Mars-sized body smashed into the Earth and spewed off some debris which went in orbit around the Earth, and that debris eventually coalesced to form the moon. We think that same exact process happened with Eris and Dy, just on a much smaller scale.

In talking about Eris and Dysnomia, I’ve neglected some of the other objects out there, and some of them are really cool. We’ve found about 80 objects, and there are two that I think are worthy of study in their own right.

One of them is an elongated, rapidly rotating football-shaped thing. This thing spins every four hours; it’s the fastest rotating object in the outer solar system we’ve ever found. It’s elongated by a factor of about two to one. It has two satellites going around it, and we think that it too was the product of one of these big impacts. Some object smashed into it, made it spin, ripped off all of these ices, some of which became satellites. We’ve also found four Kuiper Belt objects that we’re pretty convinced are chunks of this big impact. Just as we initially nicknamed UB 313 “Xena,” we found this one at Christmas so we called it Santa. But it’s going to get a real name soon, and I’m sure there will be an interesting story behind that.

Artistic representation of 2003 EL61, also known as “Santa,” and satellites. Click image for larger view.
Image Credit: NASA/ESA/HST/A. Field (STScI)

The other interesting object is just a little bit smaller than Pluto, but it looks very much like Pluto. Its official name is now Sedna, and it is located way outside of the Kuiper Belt. Just like with Eris, we really wanted to know its orbit. We didn’t know how big it was – it was bright, so we thought there was a good chance it was bigger than Pluto. We were hoping it was going to be a planet; we were hoping it would have a circular orbit instead of a cometary orbit where it comes in close and goes far out.

It turns out it has a totally crazy 12,000-year orbit around the sun. We just happened to catch it at its closest point. Nobody expected anything like this in the outer solar system. In my mind, this is the single most important scientific find in this whole survey we’ve done. By studying this object and finding more like it, I think we’re going to learn an incredible amount about the birth of the sun and what kind of environment it was in.

But the interesting this is, if it had been a little further away, we wouldn’t have seen it. There was only a 200-year period during which we could have found it. So the odds were one in sixty. Either that means we’re pretty lucky, and we hit that one-in-sixty lottery, or, since there’s a one in sixty chance of finding it, that means there are 60 of them. Maybe there are 30, maybe 90, but there’s a pretty good chance that we didn’t get lucky and find the one that happened to be here. So we think there are probably 60 objects the size of Sedna.

Sedna is not small. It’s about three-quarters the size of Pluto. If there are 60 about that size, we don’t know what the biggest one is but you can make some educated guesses. An educated guess would tell you that the largest object in this region is going to be the size of Mars. Maybe it’s the size of Mercury, maybe it’s the size of the Earth. So it could be that there will be objects out there that are bigger than what we call planets.

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

Scientifically this is going to be an incredible boon. Culturally, I can’t wait. It’s going to be fun to find the first dwarf planet that’s bigger than a real planet. We don’t do this just to make things difficult for the International Astronomical Union, that’s just a side bonus.

We just started this search for these very distant objects in the last few months, and we hope that in the next couple of years we will have found some of the really big ones. So the next time I’m talking about the largest known dwarf planet, it won’t be the view from the now close-by Kuiper Belt, it will be a view from much further out.

It used to be that Pluto was the only of these large objects out there, and it was the oddball at the edge of the solar system. I was always excited about the New Horizons mission that is going to Pluto, because I wanted to see what Pluto was like, but now I’m even more excited, because now instead of just seeing this little oddball at the edge of the solar system, we’re going to see a representative member of this class of big objects out there, and learn about all of these by studying this one that happens to be closer and easier to get to.”

Read part 1 of this lecture; “Defining Planets
Part 2: “Hunting Distant Dwarfs
Part 3: ““The Discovery of Eris

Part 5: “Questioning Dwarfs

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