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Science  22 Jan 2016:
Vol. 351, Issue 6271, pp. 330-333
DOI: 10.1126/science.351.6271.330

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Astronomers have announced evidence for a new ninth planet in the solar system: a giant nearly the size of Neptune that takes 15,000 years to orbit the sun. They have not detected it directly, but rather, have inferred its existence based on the way it gravitationally sculpts the strange orbits of six small objects beyond Pluto. Yet the claim is the strongest yet in the search for a "Planet X," a quest to extend the solar system beyond Neptune that has been plagued by far-fetched claims. The new evidence comes from a pair of respected planetary scientists, Konstantin Batygin and Mike Brown of the California Institute of Technology in Pasadena. Brown first achieved notoriety for the discovery of Eris, a distant object nearly the size of Pluto, which led to the dwarf planet's demotion. But he now has his sights set on confirming the existence of a world that would certainly be considered a planet. He calls it Planet Nine.