Scientists pin down mass of "10th planet"

Pluto can't seem to catch a break. It was ignominiously demoted to 'dwarf planet' status after astronomers discovered an even larger icy world in the outer solar system. Now, new observations have pinned down the mass of that world, called Eris, revealing it outweighs Pluto by a hefty 27%.But Pluto still appears to have the upper hand in one respect – it has three moons, while the new observations suggest Eris has just one.Because its diameter is slightly larger than Pluto's, Eris was dubbed the 'tenth planet' when its discovery was announced in 2005. After re-igniting a debate over the definition of a planet, it became the largest "dwarf planet" – a new category of object that included Pluto – when astronomers officially defined the term in 2006.Since Eris was found to have a small moon about 1/16th its size, astronomers knew they could observe the moon's orbit to calculate Eris's mass, using a simple equation derived by the 16th-century astronomer Johannes Kepler.

Now, Caltech researchers Michael Brown and Emily Schaller have observed the moon, called Dysnomia, over seven nights in 2005 and 2006 using the Keck Observatory in Hawaii, US, and the Hubble Space Telescope. It appears to be on a 15-day circular orbit, suggesting Eris has a mass of 17 million trillion tonnes – making it 27% more massive than Pluto.Since Eris is larger than Pluto in size – spanning 2400 km compared to Pluto's diameter of 2320 km – its larger mass is not especially surprising. Nor is its density – calculated from its mass and physical size – surprising; it appears similar to Pluto's, suggesting Eris is made of rock and ice.