Sorry Pluto, you're not even biggest dwarf planet

By Will Dunham

WASHINGTON (Reuters) - Poor Pluto has been demoted again.

Calculations published on Thursday show the distant world that astronomers no longer deem a planet is not even the largest of our solar system's so-called dwarf planets -- it is smaller than recently discovered dwarf planet Eris.

Michael Brown and Emily Schaller of the California Institute of Technology used data collected from the Hubble Space Telescope and the Keck Observatory in Hawaii to determine for the first time that Eris had a greater mass than Pluto.

Eris, discovered in 2005 and named for an ancient Greek goddess of strife and discord, is 27 percent more massive than Pluto, they found. Eris is about half the size of Earth's moon, Brown said.

Pluto, named for the ancient Greek god of the underworld, was discovered in 1930.

It was considered our solar system's ninth planet until August 2006, when the International Astronomical Union declared it a dwarf planet, a term referring to lesser, round solar system bodies orbiting the sun, mostly in an outer region called the Kuiper belt.

"I don't think we're picking on Pluto," Brown, a professor of planetary astronomy who helped provoke the demotion of Pluto, said in a telephone interview.

"It's just the truth. It (Eris) just is more massive than Pluto. It's just the way it is," Brown said.

The findings were published in the journal Science.

Scientists previously had figured that Eris's diameter was bigger than Pluto's but did not know about mass.

NO VACATION SPOT

Like Pluto, Eris is unlikely to become a holiday destination. Both inhabit a remote and frigid region of the solar system. The new data indicated Eris likely is composed of ice and rock, very similar to Pluto.

"It is covered in an almost perfectly uniform white frost. So it looks just like a white billiard ball out there," Brown said.

Pluto and Eris both follow elliptical rather than circular orbits as they journey around the Sun.

Eris's orbit is highly elongated and takes 560 years. It ventures anywhere from 3.5 billion miles to 10 billion miles from Earth, Brown said.