Astronomy Picture of the Day

Explanation: Is that a tenth planet? A faint, slowly moving dot discovered by computer shows clear signs of being a deep Solar System object at least as large as Pluto. The object, designated 2003 UB₃₁₃, is currently situated nearly 100 times the Earth-Sun distance -- over twice the average Pluto-Sun distance. That far out, the only way a single round object could be as bright as 2003 UB₃₁₃ would be if it is at least as large as Pluto and completely reflective. Since 2003 UB₃₁₃ is surely not completely reflective, it could be substantially larger. One of the discovery frames is shown above digitally expanded and artificially brightened. 2003 UB₃₁₃ was identified initially on frames taken by the automated 1.2-metre Samuel Oschin Telescope at Palomar Observatory in California, USA.