Published online 29 July 2005 | Nature | doi:10.1038/news050725-13

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Santa and little helper seen beyond Pluto

Competing teams spy objects on fringe of Solar System.

The new planet, nicknamed 'Santa' (centre), and its tiny moon (below).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 over from the formation of our planetary neighbourhood some 4.5 billion years ago.

In recent years astronomers have spotted several Kuiper-belt planetoids, including ones named Quaoar and Varuna; the latest has been nicknamed Santa. 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 Ortiz, 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 of Technology in Pasadena, led by astronomer Mike Brown. They first saw it on 28 December 2004, hence its seasonal sobriquet.

"There's no question that the Spanish group is rightly credited with discovery," 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 awaiting our observations from the Spitzer Space Telescope. We were wrong!"

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 bright that Brown estimates it might be visible using an amateur telescope.

Follow-up observations with Spitzer made on 22 July should deliver a precise size for the planetoid soon, says Brown.

Tiny partner

Brown's group also discovered that Santa has a small moon, after more observations in January 2005 from the Keck Observatory in Hawaii. The moon orbits its planetoid once every 49 days at a distance of 50,000 kilometres.

This information allowed Brown to calculate that it makes up just 1% of the entire mass of the pair, making it much, much smaller than Charon.

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The mass of the planetoid itself is around 4 exatonnes (4 *1018 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, and half its time closer to the Sun.

Brown will present further details1 of the planetoid and its moon at the American Astronomical Society's planetary sciences meeting in Cambridge, UK, on 8 September.