[sac-forum] What is a Planet

From http://en.wikipedia.org/wiki/Definition_of_planet:

The IAU published its definition proposal on August 16, 2006. This
definition loosely follows the second option considered by the 19-member
panel in 2005. In its exact wording, it is:

    âA planet is a celestial body that (a) has sufficient mass for its
     self-gravity to overcome rigid body forces so that it assumes a
     hydrostatic equilibrium (nearly round) shape, and (b) is in orbit
     around a star, and is neither a star nor a satellite of a planet.â

The IAU proposal requires that three new worlds immediately become
planets: Ceres, Charon and 2003 UB313.  A further dozen are likely to join
them soon.  Despite what has been claimed in the media, this proposal does
not automatically leave our solar system with 12 planets.  Mike Brown, the
discoverer of Sedna and 2003 UB313, has claimed that this definition
could eventually comprise up to 53 planets in our system alone, with
hundreds, or even thousands, not yet discovered.

Under this proposal, Pluto and Charon are recognised as a double planet
system, though the definition (that the system's barycenter must lie
outside either body) is for now exclusive to Pluto-Charon and excludes
other potential double-planets, such as Earth and the Moon.  The term
"minor planet" is abandoned, replaced by a twofold classification: "dwarf
planet", which includes all planets smaller than the eight "classical
planets" in orbit around the Sun, and "Pluton", a term which specifically
includes those icy dwarf planets in the Kuiper belt and beyond.  Those
objects underneath the "spherical" threshold are termed "minor solar
system bodies."  The IAU has not ruled on what separates a planet from a
brown dwarf.  The final decision is expected by August 24, 2006.

        -Paul

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