What is a planet?

2500 astronomers are gathered in Prague this week for the IAU general assembly. One issue is dominating the media coverage – is Pluto a planet?

Frankly, I don’t see the problem. The answer is clearly “no”. Consider this:

  • Pluto was discovered during the search for a hypothetical “Planet X”, which was thought to influence the orbits of Uranus and Neptune. In fact (a) the orbital anomalies were largely due to inaccurate mass estimates and (b) the mass of Pluto plus its moons is too small to have a major influence on other orbits. However, its discoverers were keen to describe Pluto as a planet – partly, I suspect, to bolster the importance of their discovery and partly to reconcile the discovery with long-held assumptions about “Planet X”. If Pluto were discovered today, it would not be called a planet.
  • There is a pattern in the distribution of objects in the solar system, which stems from the density of matter and its proximity to the sun when the system formed. Near the sun we get rocky planets – Mercury, Venus, Earth and Mars. Further out we get gas or ice giants – Jupiter, Saturn, Uranus and Neptune. And then there’s Pluto. A tiny speck of dirty ice, rather like a comet, way out past all the other planets in an unusual orbit. Surely this makes it a Kuiper belt object.

That said, there’s no scientific definition of a planet. One proposal is that planets should have sufficient mass to maintain spherical shape through their own gravity. So planet = spherical? Not very satisfying. Size is also problematic – where do you draw the line?

The IAU are now floating the idea of classical planets – the eight from Mercury to Neptune, plus a new class for Pluto-like objects called “plutons”. Not only is this a ridiculous name that sounds like an alien race dreamed up by Douglas Adams, but it would lead to the inclusion of large asteroid Ceres as a classical planet. Surely it would be simpler if Pluto die-hards just got over their sentimentality, accepted the fact that Pluto’s status is no more than historical accident and we call Mercury -> Neptune planets and everything else something else – asteroid, minor planet, Kuiper belt object or whatever.

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