Tuesday, January 17, 2006

Pluto A Planet At All?


That is the question on the minds of NASA scientists launching the New Horizons mission over the next few weeks.

While Pluto is large for a Kuiper-belt object—it has a diameter of 2,300km—its mass is less than 1% of Earth's. This and the presence of other similar-sized or larger things in the belt are fuelling a debate about what should count as a planet. Dr Stern argues that Pluto is a genuine planet, because it is rounded by the force of its own gravity. As he puts it, “it's a dwarf planet by the same measure a Chihuahua is still a dog.”

In that case, though, 2003 UB313 should also be called a planet. But nobody can agree on this, so last year the International Astronomical Union (IAU) convened a group of senior astronomers to suggest a new definition of what it is to be a planet.

Another possibility the IAU group discussed was whether a planet should be defined as something that is “dominant in its own neighbourhood”. Yet according to this definition, Pluto would not count as a planet, because it is only one of many similar objects in the region.
Such a definition would lead to several problems. For one thing, astronomers working on Pluto do not want a demotion. For another, the last time the IAU considered reclassifying Pluto as something other than a planet, it caused a public outcry. Then there is the question of what to call Pluto if it were demoted. For example, the term “minor planet” is today applied to asteroids, but, by any definition, most asteroids are not planets.


By a tiny majority, the IAU group voted that the best option was some kind of arbitrary size-distinction. Dr Williams says that a radius of 1,000km seemed sensible because, “100km is silly and 10,000km would not include Earth”.

The debate is not over yet, however. The final decision will be taken at the IAU general assembly in Prague this August, where astronomers will have the job of trying to balance the scientifically useful with the culturally acceptable. If they can agree on the matter, then 2003 UB{-3}{-1}{-3} will finally get a proper moniker. If it is deemed to be just another miserable lump of rock and ice a very long way away, its discoverer, Michael Brown of the California Institute of Technology, will be allowed to give it a name. But if it is classified as a planet, then it will acquire a suitably grand name belonging to one of the classical gods.

NASA seems to be shaking off its Columbia depression. Two successes in one week is not bad, not bad at all.

Pic: Pluto as captured by Charon

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