Is Pluto A Planet?

It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.

In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:

• Resolution 1 for GA-XXVI: "Precession Theory and Definition of the Ecliptic"
• Resolution 2 for GA-XXVI: "Supplement to the IAU 2000 Resolutions on reference systems"
• Resolution 3 for GA-XXVI: "Re-definition of Barycentric Dynamical Time, TDB"
• Resolution 4 for GA-XXVI: "Endorsement of the Washington Charter for Communicating Astronomy with the Public"
• Resolution 5A: "Definition of 'planet' "
• Resolution 6A: "Definition of Pluto-class objects"

• (a) is in orbit around the Sun
• (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
• (c) has cleared the neighbourhood around its orbit.

So, is Pluto A Planet?

The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

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