Pluto Downgraded Again

| | Eris and its moon, Dysnomia. Courtesy NASA|

First it was demoted to a "dwarf planet," leaving only eight proper planets in our solar system. Then scientists reported that another dwarf planet, Eris, is bigger than Pluto. And now the final blow: Eris is 27 percent more massive than Pluto.

Scientists Michael E. Brown and Emily Schaller at the California Institute of Technology in Pasadena used the ground-based Keck Observatory and the Hubble Space Telescope to determine the mass of Eris: 16.6 billion trillion kilograms. Brown and Schaller calculated the mass by measuring the time it takes for Eris's moon Dysnomia to complete an orbit.

Earlier observations by Hubble had already shown that Eris has a diameter of about 2,400 kilometers, which is slightly larger than Pluto. Both dwarf planets are probably made of rock and ice. Brown says they are "essentially twins—except that Eris is slightly the pudgier of the two."

Like Pluto, Eris resides beyond Neptune in the Kuiper belt. The icy bodies in this belt orbit the sun at distances 30 to 50 times greater than Earth's orbit. Eris is three times farther from the sun than Pluto, and takes twice as long to orbit it.—Dawn Stover