Meet The New Planets

By MICHAEL D. LEMONICK

Neil DeGrasse Tyson owes his colleague Michael Brown a big thank-you--and flowers wouldn't be a bad idea either. Back in 2000, Tyson, an astrophysicist and the director of the Hayden Planetarium in New York City, triggered an international furor when he decreed that in his prestigious establishment Pluto would no longer be listed as a planet. Henceforth, it would be considered just another ball of ice in the Kuiper Belt, a swarm of debris orbiting the sun out beyond Neptune. He was on firm scientific ground: many professional astronomers have been leaning that way for years. But people evidently had a soft spot for the runt of the planetary litter. Almost overnight, Tyson became the Grinch Who Stole Pluto.

But in July, Brown, an astronomer at Caltech, made an announcement that took the debate to a whole new plane. Along with his colleague Chad Trujillo, Brown had found something very much like Pluto, only bigger, and last month he declared that the object known officially as 2003 UB313--and temporarily nicknamed Xena--has its own little moon. Suddenly, the question Tyson had raised to make a provocative educational point became something much larger: if Pluto is a planet, then Brown's new object must be one as well.

And it doesn't stop there. What do you call all the other planetlike objects that have lately been discovered orbiting around our sun, tiny worlds with names like Sedna, Quaoar and 2004 DW? Part of the problem is that there is no precise scientific definition of the word planet. The International Astronomical Union (IAU) is trying to hammer one out, but the decision is proving more difficult than anyone thought. An apparent consensus, reached just weeks ago, seems to have fallen apart. "The current state," admits Brian Marsden, director of the IAU's Minor Planet Center at Harvard, "is rather confusing."

No wonder. The solar system most of us studied in school was a deceptively simple place. There were the sun, a few asteroids and comets and, as of 1930, when Clyde Tombaugh spotted Pluto on a telescopic photograph, nine planets. Memorizing those nine names has long been a childhood rite of passage, up there with learning to tie your shoes. Yes, Pluto was always an oddball: not only is it tiny (two-thirds the size of our moon), but it has a weird, elongated orbit that is tilted at a sharp angle to the plane the other planets inhabit. Still, the gap in size between Pluto and the biggest asteroids was comfortably huge.

But when astronomers started thinking about where comets actually came from, they realized that there was an enormous cloud of icy chunks, named the Oort Cloud (after the Dutch astronomer who proposed it), orbiting invisibly tens of trillions of miles from the sun. A second group of comets, according to Gerard Kuiper (a Dutch American), must come from closer in, falling sunward from the disk-shaped cloud of icy chunks just beyond Neptune that bears his name. Sure enough, when astronomers trained their telescopes on the Kuiper Belt 15 years ago, they started finding all sorts of objects. In the past few years, Brown and Trujillo have been turning up some pretty big ones, including Quaoar (about half the size of Pluto) in 2002 and Sedna (probably a bit bigger than that) in 2004.

Tyson's heretical planetarium exhibit was based on the theory that there must be lots of things out there the size of Pluto. Brown and Trujillo found some of them, and for the past year and a half, the pressure has been on to decide once and for all which are planets and which are not. 2003 UB313 just upped the ante. But it is like trying to define continent, says Brown. "Some geographers call Australia a continent," he says, "and some call it a very big island. There is no scientific definition." It is human nature to put things into categories, but nature rarely cooperates. What, precisely, is the dividing line between a hill and a mountain? A rock and a boulder? A stream and a river?

Most people don't worry much about such distinctions. With planets, however, it's different--as Tyson discovered. How do you resolve the problem he created? One idea would be to arbitrarily set the lower limit for a planet at about 2,000 km (1,250 miles) in diameter, which would let Pluto remain a planet and make 2003 UB313 one as well, but keep the rest of the riffraff out. "Pluto," says Alan Boss, an astronomer at the Carnegie Institution of Washington and a member of the IAU working group, "has historically been considered a planet, and so any definition we adopt really must include it." Another proposal would drop the limit to 1,000 km, letting Quaoar and Sedna into the club as well.

Yet another idea, favored by Alan Stern of the Southwest Research Institute in Boulder, Colo., would open the door even wider. By his definition, any object massive enough for gravity to squeeze into a spherical shape is a planet--unless the object orbits a bigger planet, of course. Otherwise, dozens of moons would have to be reclassified as planets. "Defining planets by size is purely arbitrary," agrees Marsden, who likes Stern's idea. "The Pluto-crats want to cut things off there, but it's absurd to say that an object 2,000 km across is a planet and one 1,999 km across isn't."

The roundness rule would add lots of planets to the solar system in one fell swoop: not just Sedna, Quaoar and 2003 UB313 but also two more icy worlds spotted by Brown and Trujillo--2004 DW, a little bigger than Quaoar, and 2003 EL61, probably about seven-tenths the size of Pluto. The latter made headlines when it was formally announced to the world by Spanish astronomers who, according to Brown, knew where to look because they had used the Internet to tap into his telescope logs (the Spaniards deny the charge). At least five or six asteroids would also qualify, says Marsden. There would probably be two dozen newly designated planets in all.

There's no telling when the IAU might make a decision. It could be as early as the end of this month. But it can't wait forever; Brown and Trujillo have even more discoveries waiting in the pipeline (they've put their logs behind a firewall to keep prying competitors away) and they're not done yet. Just about all the new worlds have been found by looking even farther outside the plane of the solar system than Pluto's orbit. "Nobody really expected to see anything way up there," says Brown. "But based on what we've found so far, we expect to find at least two or three more of these."

Tyson, ever the iconoclast, thinks the word planet should be retired entirely, not just stripped from Pluto. "You tell me something's a planet," he says, "then I have to ask you 20 more questions to figure out what it actually is." As an educator as well as a scientist, though, he is thrilled that the question of planethood has been opened for freewheeling public discussion. "The point," says Tyson, who is working on a book about the Pluto debacle, "is that the solar system is a lot more interesting than just a list of nine planets." And thanks to Michael Brown and his associates, that fact is impossible to avoid.