Pluto, not even the biggest dwarf

BBC reports: [edited]

Pluto has suffered yet another blow to its status. Not only has it been demoted from planet to "dwarf planet", research now shows that it cannot even lay claim to being the biggest of these.

A study has confirmed that the dwarf planet Eris - whose discovery prompted Pluto's relegation from planet to dwarf - outranks it in mass.

The US team, whose work is published in the journal Science, described their finding as "Pluto's last stand". The discovery of Eris, formerly known as 2003 UB313, marked the beginning of the end for Pluto as a planet.

Previous measurements from the Hubble Space Telescope confirmed that Eris was larger in diameter than Pluto, leading the International Astronomical Union (IAU) to rule in 2006 that Pluto could no longer be classed as a planet.

A new category of dwarf planets was adopted, into which Pluto, Eris and another body called Ceres, which is located in the asteroid belt, were placed.

Eris lies some 14.5 billion km from Earth in a region of space known as the Kuiper Belt. It has a highly elongated orbit around the Sun that lasts 560 years. It also has a moon, which is called Dysnomia, and scientists used this satellite, along with the Keck Observatory, Hubble Space Telescope to calculate its mass.

The researchers, led by Eris' discoverer Mike Brown from the California Institute of Technology, discovered that the more distant world has 27% more mass than Pluto. They wrote: "In addition to being the largest, Eris is also the most massive known dwarf planet."
------------