For Pluto, Another Comedown

Alas, poor Pluto, the dissing just keeps on coming.

Last year, the erstwhile ninth planet was demoted because astronomers had discovered a nearby object, now officially named Eris, that was larger in diameter than Pluto. (According to the latest measurements, Eris is 2,400 kilometers across, compared to Pluto’s 2,200.) If Pluto was a planet, then so was Eris—making it the tenth.

But instead of taking that route, which would have opened membership in the club of planets to other large objects such as the asteroid Ceres and Pluto’s own moon Charon, astronomers decided to get pickier about what defines a planet. Pluto would hence be a “dwarf planet,” one that orbits the Sun but that has insufficient mass to “gravitationally dominate” its region of the solar system—that is, enough mass to sweep away other stuff orbiting there. But heck, at least Pluto was the heftiest, most massive dwarf.

Pluto fans who never got over last year’s ouster had better brace themselves. In a paper in tomorrow’s issue of the journal Science, astronomers Michael Brown (who discovered Eris in 2003) and Emily Schaller of the California Institute of Technology report that Eris, having gotten Pluto kicked out of the club of planets, is at it again. Using the Hubble Space Telescope and the telescope at the Keck Observatory, the Caltech astronomers calculate that Eris has a mass 27 percent greater than Pluto’s. That puts its density at 2 grams per cubic centimeter, suggesting a composition of ice and rock very similar to Pluto. “In addition to being the largest, Eris is also the most massive known dwarf planet,” the scientists write. Any hope that Eris is an air head—all diameter and no heft—is now gone.

“This was Pluto’s last chance to be the biggest thing found so far in the Kuiper belt,” said Brown. “There was a possibility that Pluto and Eris were roughly the same size, but these new results show that it’s second place at best for Pluto.”

Pluto could not be reached for comment.

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