‘10th Planet’ May Not Be as Large as Estimated

An icy ball discovered last year in the outer solar system is only slightly larger than Pluto, astronomers reported Tuesday. Previous estimates by ground-based telescopes suggested the object officially known as 2003 UB313, but called Xena, was 30% bigger than Pluto.

But the latest measurement by the Hubble Space Telescope has a smaller margin of error, said astronomer Michael Brown of Caltech, who discovered the so-called 10th planet. According to Hubble, UB313’s diameter is 1,490 miles, give or take 60 miles. Pluto is about 1,422 miles across.