Astronomers have found a tenth planet, larger than Pluto and nearly three times farther from the sun as Pluto.

Temporarily designated 2003 UB313, the new planet is the most distant object yet seen in the solar system, 97 times farther from the sun than the Earth. It also is the largest body yet found orbiting in the Kuiper belt, the group of icy bodies including Pluto which orbit beyond Neptune.



Like Pluto, 2003 UB313 is covered by methane ice, and at its present distance is chilled to just 30°C above absolute zero, says Mike Brown, the Caltech astronomer who announced the discovery on Friday. The sleep-deprived father of a three-week-old daughter, Brown said the discovery was “almost as exciting as having a new baby.”



Pluto was the only object known in the Kuiper belt until 1992, but since then astronomers have spotted hundreds more faint, icy bodies with orbits beyond Neptune. Five years ago, Brown’s group began a systematic search for big Kuiper belt objects, which earlier yielded Quaoar – about 800 miles in diameter – and Sedna, previously the most distant object known at 91 times the earth’s distance.



The survey also yielded the new planet and two other objects only slightly smaller than Pluto, which Brown kept quiet as he analysed the survey data and made new observations to learn more about the 2003 UB313.



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