They’ll decide on the final resolution tomorrow, but astronomers at the International Astronomical Union’s General Assembly have been debating the proposed definition of a planet for the past week, including a meeting that the NY Tmes reports “was described by participants as tumultuous”[subs req], also available here and Washington Post report. Some of the points discussed are detailed in Monday’s edition of Dissertatio cum Nuncio Sidereo III, the official newpaper of the General Assembly, including those of Mark Bailey director of the Armagh Observatory. The result is a new proposal and according to Owen Gingerich, chairman of the Planet Definition Committee of the IAU, under that new proposed definition, “Pluto is not a planet.” Updated with Pluto system image
Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
In short, there are 3 categories proposed for bodies in the Solar System
The new definition offered yesterday would set up a three-tiered classification scheme with eight “planets”; a group of “dwarf planets” that would include Pluto, Ceres, Xena and many other icy balls in the outer solar system; and thousands of “smaller solar system bodies,” like comets and asteroids.
The bottom line, said the Harvard astronomer Owen Gingerich, chairman of the Planet Definition Committee of the union, is that in the new definition, “Pluto is not a planet.”
“There’s not happiness all around, believe me,” he added.
The main difference from the original proposal is that an additional factor is to be considered, oribtal dominance
The committee’s original prime criterion was roundness, meaning that a planet had to be big enough so that gravity would overcome internal forces and squash it into a roughly spherical shape. But a large contingent of astronomers, led by Julio Fernández of the University of the Republic in Montevideo, Uruguay, has argued that a planet must also be massive enough to clear other objects out of its orbital zone. Dr. Gingerich admitted, “They are in control of things.”
So the newest resolution includes the requirement for orbital dominance as a condition for full-fledged planethood, Dr. Gingerich said. That knocks out Pluto, which crosses the orbit of Neptune; Xena, which orbits among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.
Oh, and pluton is out.. for several reasons. Not least the fact that geologists already have a definition of a pluton,
Dr. Gingerich cautioned that there were still many things to be sorted out. For example, the I.A.U. may or may not create a special name for Pluto and other dwarf planets, like Xena and others yet to be discovered, that dwell beyond Neptune. If so, he said that “plutonians” seemed a likelier choice than the previous suggestion “plutons.” That term was protested by geologists, who pointed out that it was already used in earth science for nuggets of molten rock that have solidified and reached the surface.
That’s the current state of play.. as far as I know.. and the vote will be taken tomorrow, it might be available live online, but if you’re interested you can watch the IAU General Assembly heatedly discuss the definition of a planet, from 22nd August, online here[RealPlayer video].