www.nature.com/news/2005/050725/full/050725-13.html
Mark Peplow Competing teams spy objects on fringe of Solar System. The new planet, nicknamed 'Santa' (centre), and its tiny moon (below). M Brown et al / Caltech / Keck Two sets of astronomers have spotted a new planetoid in the outskirts of our Solar System. It is the brightest object in the region after Pluto, and it has its own small moon. It isn't too uncommon to find such objects lurking in the icy Kuiper belt , the region of space beyond Neptune that is filled with rubble left ove r from the formation of our planetary neighbourhood some 45 billion yea rs ago. In recent years astronomers have spotted several Kuiper-belt planetoids, including ones named Quaoar and Varuna; Philosophical debates continue about how large such objects have to be before we call them 'planets' rather than simple lumps of rock. Seeing double Provisionally named 2003 EL61, it was first seen in 2003 by Jose-Luis Ort iz, an astronomer from the Institute of Astrophysics in Andalusia, Spain , and his colleagues. They used the Sierra Nevada Observatory in Granada , Spain, and observations this month confirmed its existence. The object had also been spotted by a group at the California Institute o f Technology in Pasadena, led by astronomer Mike Brown. They first saw i t on 28 December 2004, hence its seasonal sobriquet. "There's no question that the Spanish group is rightly credited with disc overy," writes Brown on his website. "We could have announced the object earlier, but we took a chance that no one else would while we were awai ting our observations from the Spitzer Space Telescope. Ortiz describes it as a "very bright, slowly moving object", which is at least 1,500 kilometres across. This makes 2003 EL61 bigger than Pluto's moon Charon, as well as other known Kuiper-belt planetoids. It is so bri ght that Brown estimates it might be visible using an amateur telescope. Follow-up observations with Spitzer made on 22 July should deliver a prec ise size for the planetoid soon, says Brown. Tiny partner Brown's group also discovered that Santa has a small moon, after more obs ervations in January 2005 from the Keck Observatory in Hawaii. The moon orbits its planetoid once every 49 days at a distance of 50,000 kilometr es. This information allowed Brown to calculate that it makes up just 1% of t he entire mass of the pair, making it much, much smaller than Charon.
The mass of the planetoid itself is around 4 exatonnes (4*10^18 tonnes): a third the weight of Pluto and about the same mass as all the water on Earth. The object spends about half of its time outside Pluto's orbit, a nd half its time closer to the Sun.
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