updated 6/14/2007 2:28:13 PM ET 2007-06-14T18:28:13

Pity poor Pluto, the puny former planet is facing yet another indignity. Demoted from planethood a year ago into a new category of dwarf planet, it now turns out that it isn't even the biggest one of those.

"This is sort of Pluto's last stand," joked Emily L. Schaller of California Institute of Technology, co-author of a report in Thursday's issue of the journal Science.

When the International Astronomical Union redefined planets last year, it created the new subcategory dwarf planets, and Pluto was thought to be the largest in that group.

Planetary astronomy professor Michael E. Brown and graduate student Schaller found otherwise while studying Dysnomia, the moon of Eris, another dwarf planet.

Slideshow: Cosmic Sightings: From Earth to stars Using the Keck Observatory and Hubble Space Telescope they were able to calculate the movement of Dysnomia and, with that information, calculate the mass of Eris at 27 percent more than Pluto. But even though Eris tops Pluto, Earth is still 360 times more massive.

"Pluto and Eris are essentially twins — except that Eris is slightly the pudgier of the two," Brown said.

Eris, by the way, is named for the Greek goddess of, among other things, rivalry.

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