spacetoday.net: Eris and Pluto similar in size

Posted: Fri, Oct 28, 2011, 8:12 AM ET (1212 GMT)
Eris illustration (ESO)

Astronomers have found that the dwarf planet Eris is nearly identical in size to Pluto, making it smaller than expected and likely different in makeup to the former ninth planet. In a paper published in Nature this week, astronomers reported on an occultation in November 2010 when Eris passed in front of a distant star as seen from South America. Observations of the occultation allowed astronomers to calculate the diameter of Eris, resulting in a value of 2,326 kilometers. By comparison, Pluto's estimated diameter is between 2,300 and 2,400 kilometers. Astronomers had previously thought Eris was about 25 percent bigger than Pluto based on its brightness and estimated mass, which was based on observations of Eris's moon, Dysnomia. This suggests that Eris contains a greater fraction of rock than Pluto, and that its surface is covered with a bright icy layer, perhaps a frozen-out atmosphere.