What does a planet have to have or need to be
classified as a planet?
In my opinion, a planet needs to be large enough to have condensed
as a sphere, and that it cannot be a moon of another planet. If
that definition is adopted, it would include Pluto! The IAU
definition, which adds that a planet must have cleared out its
neighborhood, means the Jupiter and Earth are no longer planets either!
Astronomers develops the definition of what constitutes a planet. The International
Astronomical Union (IAU) meets to determine these definitions as they apply to
astronomy. Recently (2006) the body voted to change the definition of a planet based
upon information that research in astronomy has provided. Basically, a planet must;
be an object which independently orbits the Sun,
have enough mass so that gravity pulls it into a roughly speroidal shape, and
be much larger than anything else which crosses its orbit.
Science is fluid and can change the definition again
if the research warants an addition change. That is how science works.
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Update: June 2012