Astronomers detect '10th planet'
http://news.bbc.co.uk/2/hi/science/nature/4730061.stm
By Dr David Whitehouse
Science Editor BBC news website
The new planet has a highly-inclined orbit
Astronomers in the United States have announced the discovery of the 10th planet to orbit our Sun.
The largest object found in our Solar System since the discovery of Neptune in 1846, it was first seen in 2003, but only recently confirmed as a planet.
Designated 2003 UB313, it is about 3,000km across, a world of rock and ice and somewhat larger than Pluto.
It is more than twice as far away as Pluto, in a puzzling orbit, at an angle to the orbits of the other planets.
Astronomers think that at some point in its history Neptune likely flung it into its highly-inclined 44 degree orbit.
It is currently 97 Earth-Sun distances away - more than twice Pluto's average distance from the Sun.
Bigger than Pluto
Its discoverers are Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale University.
David Rabinowitz told the BBC News website: "It has been a remarkable day and a remarkable year. 2003 UB313 is probably larger than Pluto. It is fainter than Pluto, but three times farther away.
"Brought to the same distance from the Sun as Pluto, it would be brighter. So today the world knows that Pluto is not unique. There are other Plutos, just farther out in the solar system where they are a little harder to find."
It was picked up using the Samuel Oschin Telescope at Palomar Observatory and the 8-metre Gemini North telescope on Mauna Kea.
Chad Trujillo told the BBC News website: "I feel extremely lucky to be part of a discovery as exciting as this. It's not every day that you find something Pluto-sized or larger!"
"The spectra that we took at the Gemini Observatory are particularly interesting because it shows that the surface of 2003 UB313 is very similar to that of Pluto."
Slow mover
It was first seen 21 October 2003, but didn't see it move in the sky until looking at the same area 15 months later on 8 January 2005.
The researchers say they tried looking for it with the Spitzer Space Telescope which is sensitive to heat radiation, and didn't detect it.
This gives them an upper limit of its size of 3,000 km, they say. The lower limit still makes it larger than Pluto.
The discovery of 2003 UB313 comes just after the announcement of the finding of 2003 EL61, which appears to be a little smaller than Pluto.
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