There’s something odd floating around in the outer solar system. Actually, there’s lots of odd things floating around in the outer solar system, but 2002 UX25 is one of the most baffling. The mid-sized Kuiper belt object (KBO) measures 650 kilometers (400 miles) across, and yet it has a density less than water (less than 1 gram per cubic centimeter). Yes, if you put it in a huge bathtub, 2002 UX25 would float. ...planetary scientist Mike Brown, of the California Institute of Technology (Caltech) in Pasadena, has taken a measure of 2002 UX25′s density and discovered that it is “the...