Earth, Pluto, Charon, and Earth's Moon compared
Pluto's moon Charon is over half the size of Pluto itself, leading astronomers to originally classify the Pluto-Charon system as a "double planet." They were also considered "binary planets" because the smaller Charon doesn't actually orbit around Pluto, rather Pluto and Charon orbit a common gravitational center (the "barycenter') located above Pluto's surface.
In 2006 the International Astronomical Union (IAU) demoted Pluto from its status as the Solar System's 9th planet to a dwarf planet, and as the IAU has yet to formalize a definition for binary dwarf planets, Charon is currently regarded as a satellite of Pluto. Next to the recently discovered dwarf planet Eris, Pluto is the second-largest known dwarf planet in the Solar System.
As no space probe has yet visited Pluto*, nobody is sure what this dwarf planet looks like close up. The Hubble Space telescope has revealed that Pluto's surface displays areas of marked contrast second only in intensity to Saturn's satellite Iapetus.
* NASA's robotic space probe New Horizons is currently en route to Pluto. Launched in 2006, the probe is scheduled to fly to within 7,000 miles of Pluto in 2015.
Copyright © Walter B. Myers. All rights reserved.
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