June 14, 2007 — Pluto, newly dethroned from its planetary status, isn't even the largest of the dwarf planets, a category created to handle the plethora of discoveries of Pluto-like worlds in the far, frozen reaches of the solar system.
Neighbor Eris, whose discovery in 2003 prompted the review that kicked Pluto out of the planet club, is about 27 percent more massive than its better-known sister.
"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," said California Institute of Technology astronomer Michael Brown, who published his findings in this week's Science.
The discovery of Eris and other large objects in the Kuiper Belt region beyond Neptune prompted Brown and colleagues to petition the International Astronomical Union to reconsider its definition of a planet.
The controversial ruling in August 2006 created a new class of solar system objects, called dwarf planets, and crowned Pluto as its king.
That honor now falls to Eris, which appears to be made mainly of ice and rock, like Pluto.
