Pluto's rival is tinier but shinier than thought

Pluto may be the king of the dwarfs after all.

New observations confirm that Eris, the dwarf planet whose discovery got Pluto kicked out of the planet club in 2006, is almost exactly the same size as Pluto and may be a bit smaller.

When Eris was discovered in 2005, images from the Hubble Space Telescope suggested that it was 2400 kilometres wide, 5 per cent wider than Pluto, which is only about 2340 kilometres wide.

Later observations with the infrared Spitzer Space Telescope made Pluto's case even worse, finding Eris's diameter to be around 2600 kilometres but both measurements left room for doubt.

Last November, astronomers got a chance to know for sure which rock ruled the outer solar system, when Eris passed directly in front of a distant star and cast a small shadow on the Earth.

Bruno Sicardy of the Paris Observatory and colleagues compared the shadow's size from two different sites in Chile, and found that Eris's diameter is 2326 kilometres, reported Scientific American's Observations blog. That's hardly different from the best values for Pluto's size.

"It could be smaller, it could be larger; basically, it is a twin," Sicardy said at the meeting, according to the Planetary Society Blog. Sicardy presented the results at the Division of Planetary Sciences meeting in Nantes, France, on 4 October, and they will be published in an upcoming issue of Nature.

Eris is still the dwarf planet heavyweight, though. It is much more massive than Pluto, meaning it is substantially denser.

That suggests Eris is mostly composed of rock, with a relatively thin icy mantle. Models of the solar system's composition at various distances suggest it should have had a thicker layer of ice if it formed where it is. If so, much of its original ice may have been "blasted away" in a catastrophic impact.

The new observations also revealed that the dwarf planet is brighter than fresh snow, and possibly the second brightest object in the solar system, after Saturn's icy moon Enceladus.

That hints at a surface layer of nitrogen or methane frost, the remnants of a collapsed atmosphere which goes through a cycle of freezing and thawing as the small world wheels around the sun.