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WASHINGTON — 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 Schaller of California Institute of Technology, co-author of a report in the June 14 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 Brown and graduate student Schaller found otherwise while studying Dysnomia, the moon of Eris, another dwarf planet.

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.

Pluto was discovered by the late Clyde Tombaugh of Las Cruces, N.M., who was 24 when he discovered it while working at Lowell Observatory in Flagstaff, Ariz. Tombaugh moved to New Mexico State University in 1955 and founded the school's research astronomy department.

His friends and colleagues gathered on NMSU's campus shortly after Pluto was downgraded to protest the decision to strip it of its status as a planet. About 50 students and staff members turned out for the good-natured challenge, some wearing T-shirts and carrying signs that read "Protest for Pluto" and "Size Doesn't Matter."

Tombaugh died in 1997 at the age of 90.