Nine Planets - or Eight?

Sir Patrick Moore ... 15 October 2006

One very new development comes under the heading of terminology rather than true astronomy. We are used to talking about nine planets, from Mercury to Pluto, but Pluto has always been an enigma; it is small, and has an orbit which is both inclined and eccentric. We have now found that
it is not a solitary wanderer in this remote part of the Solar System; there are hundreds, no doubt many thousands, of others, making up the Kuiper Belt, and Pluto is not even the largest member of the swarm. For example Eris, discovered in 2003, is decidedly bigger. At its latest general assembly, held in Prague, the International Astronomical Union decided to re-classify Pluto as a 'dwarf planet', along with Eris and the largest main-belt asteroid, Ceres.

As soon as the IAU announcement was made, strong objections were voiced about the 'demotion' of Pluto, but science is not sympathetic, and the facts are clear. It might have been wiser to class the asteroids, Kuiper Belt objects and other trans-Neptunians together as planetoids, but at least we are now back to eight planets, not nine. If a new giant planet is found beyond the Kuiper Belt, which is unlikely but not absolutely out of the question, we will have to think again!

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