Tenth solar system planet found

Solar system bodies more distant than Pluto have been found before, but 2003UB313 is the first that is almost certainly bigger, and so can be classified as a planet (insofar as Pluto can). The new planet’s orbit is even more elliptical than Pluto’s — it’s distance to the sun varies between 38-97 AU over it’s 560 year period, and it’s orbital plane is inclined at 45 degrees to the plane of the other planets. A name has been submitted to the IAU and is currently awaiting acceptance.
See also Spaceflight Now’s coverage; Caltech press release; and discoverer Mike Brown’s excellent explanatory page