Is Pluto really a planet?
Thursday, August 24. 2006
SSTL’s inveterate enthusiast for all things space, Stuart Eves spoke recently on BBC Southern Counties radio in the UK about Pluto and the body’s right to remain named a planet alongside the “big 8” in our solar system. This is such an interesting topic that we have decided to put finger to keyboard for our international audience - and those people in the South East of England who were not tuned in at 7:20 in the morning.
Stuart Eves
The debate has arisen from a question of scale, here’s the reasoning in plain English.
If the Earth was the size of a golf ball, then the relative size of Jupiter would be that of a beach ball and the Moon would be the size of a marble. Meanwhile Pluto, currently recognised as the most distant planet in our solar system would only be the size of a pea in our domestic analogy.
Following the recent discovery of a tenth planet, UB313 – nicknamed Xena – which is 4.81% larger than Pluto, the cold planet’s 1930 classification is under threat. The classification of Pluto as a planet is even being reconsidered by the International Astronomical Union (IAU).
A computer generated impression of Pluto created by combining lots of different images of the planet which gives some idea of variations in "colour" (or perhaps terrain?) on its surface
The IAU’s planet classification is driven primarily by size, although other planet-like features are considered (eg. shape, does it have moons). If a body has a size comparable to the recognised planets, then it could be in with a chance. Recent contenders for the title “planet” include Sedna (2004) and larger Quaoar (2002), both dismissed because they were smaller than Pluto.
Unlike asteroids, which are typically irregular “potato shaped” objects, a planet has sufficient mass that its own gravity pulls the matter into a sphere. Another reason for Pluto’s uncertain future is that it is now known to be just one of a large number of objects of roughly comparable size to be found in the Edgeworth Kuiper belt in which it orbits.
An early HST image showing Pluto and Charon as distinct objects
The IAU has announced that they will and vote today (now) on a new classification for the planet, with one possibility being the re-classification of Pluto as a Pluton, which means “a body like Pluto”. This new class of objects would also include Xena, along with Pluto’s largest moon Charon, on the grounds that Charon is so large relative to Pluto that astronomers now consider the Pluto-Charon system to be a “double planet” (or “double Pluton”), rather than a planet and a moon.
Small satellites have certainly come a long way since the academic research conducted by pioneers such as Sir Martin Sweeting, 
