Is there a set of specific requirements
that is used to classify a planet as a planet versus an asteroid, etc? I
have a student who is doing a research project on the discussion of
whether Pluto is truly a planet.
There is no border line between 'planet' and 'minor planet', although
there have been attempts to clarify the situation.
One suggestion of a line was 1000 kilometres in diameter, and some
astronomers do use this 'de facto' definition, but the International
Astronomical Union (IAU) looked into the matter a few years ago but
has never made a decision.
Even if some forth-coming IAU decision decides that Pluto is a minor
planet [asteroid], history will inevitably keep it a major planet.
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Update: June 2012