Astronomy Picture of the Day

Explanation: What do you call an outer Solar System object that is larger than Pluto? Nobody is yet sure. The question arose recently when 2003 UB313, an object currently twice as far out as Pluto and not in the plane with the rest of the planets, was verified recently to be 30 percent wider than Pluto. UB313's size was measured by a noting its distance from the Sun and how much infrared light it emits. Previous size estimates were based only on visible light and greatly affected by how reflective the object is. Whether 2003 UB313 is officially declared a planet will be answered shortly by the International Astronomical Union. In the above picture, a scientific artist has imagined UB313 in its distant orbit around the Sun coupled with a hypothetical moon.