Pluto Smaller Than Dwarf Planet Neighbor Eris, Study Finds

Anne Minard
for National Geographic News
June 14, 2007
Poor Pluto just keeps getting downgraded.

The beleaguered former planet has been dwarfed again by a new study that finds its neighbor Eris is significantly bigger.

That makes Eris—formerly known as 2003 UB313 and then Xena—the largest so-called dwarf planet in the solar system.

Eris was the catalyst for Pluto's demotion last summer, when the International Astronomical Union redefined the term "planet" and created the category of dwarf planet for objects such as Eris and Pluto.

Using the Keck Observatory and the Hubble Space Telescope, Michael E. Brown and Emily Schaller at the California Institute of Technology in Pasadena have now put the mass of Eris at a third more than Pluto's.

The paper appears in this week's issue of the journal Science.

Modest Moon

The key to finding Eris' mass was its tiny moon, Dysnomia. Brown and Schaller used Keck and Hubble to capture images of the moon's position over time.

The data suggest that Dysnomia is Eris' only moon and orbits every 15 days.

The researchers also calculated the total mass of the pair—about 1.27 times that of Pluto—and revealed that the objects are made of around 70 percent rock, making them as dense as Pluto and Triton, a moon of Neptune.

Pluto itself has been described as a ball of rock and ice just bigger than Texas.

Alan Stern is principal investigator of NASA's New Horizons mission to Pluto. When he first read the paper, he said the researchers compared "apples to oranges" by putting Pluto in the ring with two objects—Eris and its moon.

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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