Pluto Smaller Than Dwarf Planet Neighbor Eris, Study Finds

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But Brown said that Dysnomia is so small that its mass doesn't throw things off.

"Really, the interesting comparison is Eris to Pluto, rather than the system to the system," he said.

"And since the mass of Dysnomia is insignificant compared to Eris ... we really are just calculating the mass of Eris."

Nail in Pluto's Coffin?

Along with Eris, Brown is also the discoverer of 2003EL61, Sedna, Orcus and Quaoar, and 2005 FY9, which is only a little smaller than Pluto.

All of these objects reside in the far-flung area beyond Neptune's orbit known as the Kuiper Belt, named after 20th-century astronomer Gerard Kuiper.

Brown's work on the region helped spur the International Astronomical Union to make the decision to distinguish between planets and dwarf planets.

With that, Pluto became the ambassador for all of its neighbors in the Kuiper Belt: planetary science's next frontier. But ambassador is a step down from Pluto's planet status and even its more recent conciliatory title, "King of the Kuiper Belt."

Objects in the Kuiper Belt orbit 30 to 50 times farther from the sun than Earth—Pluto is about 3 billion miles (4.8 billion kilometers) from the sun, while Earth orbits 93 million miles (150 million kilometers) away.

Astronomers are intrigued by the region, because it's the source of many comets and it contains frozen evidence from the birth of the solar system. (Related: "Far-Flung Space Crash May Help Solve Mystery of Moon's Formation" [March 15, 2007].)

Meanwhile, Wikipedia has dubbed Brown "Pluto's worst nightmare," and he doesn't seem to mind.

"Pluto sort of had one last chance," he said of his new paper. "Maybe, just maybe, it would be more massive after all. And now we've done it in for good."

Not So Fast

Michael A. Burstein is president of the Society for the Preservation of Pluto as a Planet, which goes by the acronym SP3. The group of astronomy buffs formed in the spring of 2006, when rumors first started circulating that Pluto was in trouble.

Burstein preferred the IAU's initial idea for a planet definition, which was never voted upon at their solar-system-shattering meeting last August. (Related: "Pluto to Get Partners? New Definition of 'Planet' Proposed" [August 16, 2006].)

By that definition—that a planet should directly orbit a star and be massive enough to be round—Pluto would still be a planet, as would dwarf planets Eris and Ceres, a large, round asteroid orbiting near Jupiter.

It's fine if we end up with 50 or even 100 planets as new objects are discovered, Burstein said. We could keep the math easy by calling the old guard, including Pluto, "classical planets," he added.

For now, Burstein's group is laying low to see what the pros do—under the guidance of New Horizons' Alan Stern. Stern is leading the charge of professional astronomers to dismiss the IAU's ruling.

"People just aren't using the IAU definition because it's so substantially flawed," he said. "Even their own members, and I'm one, aren't using the IAU definition."

The debate over a better definition was a hot topic at the April meeting of the European Geophysical Union. And it's already part of the agenda for the February 2008 meeting of the American Association for the Advancement of Science.

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