[This is an excerpt from an article by David L. Chandler in the Dec. 8, 2005 issue of New Scientist magazine, called "To Pluto and Beyond", which is available HERE (subscription required for access). It is copyright 2005 by New Scientist]
What is a planet, anyway?
The biggest, most passionate debate raging about Pluto is one that has little to do with science or any of the properties of what has been, for 75 years, the solar system's ninth planet. The burning question is whether Pluto should be considered a planet at all - and if so, what else might also be one. A 19-member committee of the International Astronomical Union (IAU) is going to deliver a verdict - maybe, some day - on what a planet is. How that decision will be received is anyone's guess, because lots of people - astronomers, schoolchildren, museum directors and editorial writers - have already chosen sides and dug in their heels. But it's not likely to get resolved anytime soon. Brian Marsden, a member of the IAU committee, says, "A lot of members haven't shown any sign of compromise." No date has been set for a decision. [Note: now, the decision is set for a vote on August 24] There seem to be five main ideas in circulation, each of which has its passionate advocates and its fierce detractors:
1 Planets are round things
This sets a clear limit, based on physical principles. Any object above a certain size, no matter what it's made of, will be pulled by gravity into a spherical shape. That cut-off is a diameter somewhere around 800 kilometres. Alan Stern is a leading proselytiser for this definition. This would include Pluto and at least four other known Kuiper belt objects - Quaoar, 2003 UB313, 2003 EL61 and Sedna - and also one asteroid, Ceres. It's a simple definition and it fits with people's intuitive ideas about planets: "Say 'draw a picture of a planet,' and all kids will start by drawing a circle," Stern says.
Downside: Inclusion of Ceres and multiple KBOs is a big change from the current nine planets.
[Note: With one crucial change, which adds Pluto's moon Charon to the list, this is the proposal that was presented to the IAU, but it has now been ammended to change it to number 2, below.]
2 Planets march alone
This is the leading argument against Pluto and its Kuiper belt neighbours. The idea is that planets are things that dominate their region of the solar system, orbiting alone in their zones. Thus the swarm of asteroids orbiting mostly between Mars and Jupiter, and the much bigger swarm of KBOs, including Pluto and 2003 UB313 - discovered this year, and bigger than Pluto - do not qualify as planets, but merely as members of a different class of objects. By this criterion, there are only eight planets in the solar system, and that almost certainly will remain so. This is the view supported by Neil Tyson, director of New York's Hayden Planetarium.
Downside: Pluto is excluded, changing what people have been taught for generations.
3 Size matters
Anything over, say, a 1000-kilometre radius (or 1150 kilometres, the radius of Pluto) is automatically a planet. This has the advantage of setting a clear physical limit that also fits with everyone's existing ideas about what is and is not a planet. By this rule, Ceres is too small, as are Sedna and 2003 EL61, but Pluto and 2003 UB313 are planets. So we now have 10 planets, with the exciting prospect of discovering a few more in coming years. Mike Brown, the astronomer who discovered 2003 UB313, favours this plan.
Downside: The size limit is arbitrary, plus it's hard to accurately estimate the size of faraway bodies.
4 History matters
And Pluto is special. This is supported by a subgroup of the eight-planet camp and holds that while the orbits would normally rule out Pluto, it should be "grandfathered in" out of deference to its 75 years in the planetary family. That way, a strict definition can exclude future KBOs, but would still allow schoolchildren everywhere to stick with their favourite variation of "My Very Energetic Mother Just Served Us Nine Pizzas" as a way to remember what and where the planets are.
Downside: Too arbitrary.
5 There are no planets
In an attempt at compromise, some have suggested getting rid of the unadorned word "planet" altogether, instead saddling it with a series of adjectives. Thus, the solar system consists of four types: four terrestrial (rocky) planets, a bunch of minor planets (asteroids), four gas-giant planets, and a whole lot of trans-Neptunian (ice-dwarf) planets.
Downside: Too complicated, and too much of a change from tradition.