Bigger Than Pluto? Then It’s a Planet!

The trans-Neptunian body, UB313, discovered a little more that a year ago was thought to be larger than Pluto based on its observed brightness. But because high brightness alone isn’t a good measure of size, Frank Bertoldi and his colleagues of the University of Bonn in Germany used the IRAM 30-meter telescope in the Sierra Nevada mountains of southern Spain to observe UB313 in the infrared range. Based on observations made over nine nights in August 2005, UB313 appears to have a diameter of between 3,094 and 2,859 kilometers. Even the smallest size in that range would make the object’s diameter more than 500 kilometers larger than Pluto’s. Their research appeared recently in the journal Nature. So, the bottom line: either UB313 is a planet, or Pluto isn’t.