RickJ
Apr 26, 2012, 07:44 AM
The definition has varied over the years.
In 2006, the International Astronomical Union issued their updated definition of a "planet":
"A planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects around its orbit."
Source (http://www.iau.org/search/?cx=016374242383990648147%3Adstbycjagiu&cof=FORID%3A11&q=definition+of+planet&siteurl=www.iau.org%2F&ref=www.google.com%2Furl%3Fsa%3Dt%26rct%3Dj%26q%3D %26esrc%3Ds%26source%3Dweb%26cd%3D1%26ved%3D0CDMQF jAA%26url%3Dhttp%253A%252F%252Fwww.iau.org%252F%26 ei%3DGF-ZT-OmHIWV0QHRvY3FCg%26usg%3DAFQjCNHqJV9YtaIVG-CBrHh2740EOZxgUg%26sig2%3Dh4oWyrHHwLs4ibnp5dA8XA).