The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (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.
This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. 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.
The full report of the conference is here.
3 comments:
I'm glad, and this has been a long time coming. But it seems that their new definition falls short of the merits of a definition, according to this article.
::shrug:: I must admit I don't have very strong feelings either way. I like the idea of a precise definition, but almost certainly this one will need altering. Nearly everything does. That article, I felt, didn't express the objection as clearly as it might, beyond what people "like."
I think it's rather cool to have a new class of planets. :-)
Becca found this resolution regarding Pluto from the California state legislature. It's really funny! http://www.leginfo.ca.gov/pub/bill/asm/ab_0001-0050/hr_36_bill_20060824_introduced.pdf
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