Pluto - the tail that wagged the dog

Is Pluto a planet? Astronomers once thought it was, but now they don’t. Indeed it never really was, but it has taken them three quarters of a century to admit it. As Abraham Lincoln once supposedly asked, “If you call a tail a leg, how many legs does a dog have?” Lincoln’s answer: “Four. Calling a tail a leg doesn’t make it a leg.” In planetary terms, Pluto was that tail, and calling it a planet didn’t make it one. By demoting Pluto, astronomers have pinned the tail back on the dog.

Had Pluto been discovered today, it would not have been classified as a planet. Pluto’s now-rescinded status as the ninth planet is a historical accident. When Clyde Tombaugh discovered it in 1930 astronomers had expected that it would be much bigger than it really is.

Every new measurement of its diameter came up with an ever-smaller value, until by the 1970s it became clear that Pluto was smaller even than our own Moon – embarrassingly small for a supposed planet. Around the same time, an icy object called Chiron was discovered in the outer Solar System, shuttling between the orbits of Saturn and Uranus. Although only one-tenth the diameter of Pluto, there was much about Chiron’s eccentric orbit and icy composition that was reminiscent of Pluto.

The situation became worse in the 1990s when astronomers began to discover a whole swarm of icy objects at Pluto’s distance and beyond. This swarm is known as the Kuiper Belt and astronomers began to speak of Pluto as simply the largest Kuiper Belt object (KBO) or Trans-Neptunian Object (TNO).

Matters came to a head in 2005 when a newly discovered TNO, now known as Eris, turned out to be larger than Pluto. If Pluto was a planet, then shouldn’t Eris be one also? And were there even larger objects lurking beyond, yet to be discovered? It was time to decide what was meant by a planet.

Strange though it may seem that astronomers did not have a comprehensive definition of a planet, none had previously been needed. Everyone knew that a planet was an object that orbited a star and didn’t emit light of its own. Simple. The upper limit between a planet and star was reasonably easy to define, but the lower limit proved more difficult. When does a planet become just a lump of rock or ice?

At the 26th assembly of the International Astronomical Union (IAU) in Prague in 2006 August, astronomers were presented with a proposal to define a planet as anything large enough to be pulled into a round shape by its own gravity. That included Pluto, but it would also have allowed into the planetary club various other objects not previously considered as planets such as the asteroid Ceres and any number of TNOs yet to be discovered.

The proposal was overwhelmingly rejected, to general relief. Instead, the assembled astronomers decided that a planet is a body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

This definition retains the originally proposed criterion of roundness, which sets a physical boundary, but the added criterion of “clearing the neighbourhood” excludes objects in populated areas such as the asteroid belt and the Kuiper Belt. Large asteroids and TNOs become dwarf planets under this new definition, while smaller bodies become – well, Small Solar System Bodies. For a while there seemed to be no place in this scheme for the long-established term “minor planet” but eventually the term was retained, to the relief of the IAU’s own Minor Planet Centre.

A vote at an IAU Assembly is not Papal doctrine. It gives a lead, but will succeed only if other astronomers follow. My expectation is that many astronomers will continue to regard Pluto as a large TNO and eschew the “dwarf planet” designation.

Either way, it won’t be possible to throw away three quarters of a century of history just like that. We are used to thinking of Pluto as a planet, and most books still tell us that it is a planet. For now, Pluto should be afforded dual status as both a planet and a TNO, in much the same way that humans can have dual nationality. Eventually, we will cease to talk of it as a planet, except to note the historical curiosity that it was once considered as such. This dual personality is already manifested by Ceres, which manages to be an asteroid as well as a dwarf planet.

Does it matter? When all is said and done, the whole debate comes down to swapping name-tags. Whatever you call it, Pluto remains a fascinating object at the edge of the Solar System which we still know little about. Currently a NASA space probe called New Horizons, launched in 2006 January, is on its way to Pluto. When it was dispatched from Earth its mission was to investigate the only planet not to have been reached by space probe – but when it arrives in 2015 it will be investigating not the ninth planet but a large TNO.

Ian Ridpath

Read my article Pluto: Planet or Impostor? published in the December 1978 issue of Astronomy magazine (5 MB PDF file).