Planets and Plutons

Planets

(From: World Wide Words - Issue 501)

If a resolution at the International Astronomical Union's meeting in Prague is passed next week, the word "planet" will formally alter its meaning, requiring the reference books to be rewritten.

Pluto

Pluto is not a dog...

The reason lies with the development of observational astronomy. The discovery in 1930 by Clyde Tombaugh of Pluto, orbiting beyond the then known limits of the Solar System, was a sensation at the time. Because the initial estimates of its size were way too big, it was immediately included in the list of planets. But in recent years many similar objects have been found even further out, in a distant part of the Solar System called the Kuiper Belt, at least one of them larger than Pluto. So is Pluto a planet or a minor body? And if it is a planet, are these other new bodies also planets?

It will seem an arcane and irrelevant argument for many people, but for astronomers a definitive answer will end a controversy that has been running for years, with many in the field wanting to downgrade Pluto's status. The problem is that if they continue to consider Pluto a fully fledged planet, they will also have to include many of these newly discovered bodies.

It would not be the first time "planet" has changed its meaning. It comes from Greek "planetes", a wanderer, and was applied in ancient times to any celestial object that moved against the background of the fixed stars. This included not only all the bodies visible to the naked eye that we still call planets - Mercury, Venus, Mars, Jupiter, and Saturn - but the Sun and Moon as well. With greater understanding of astronomical realities - that the Earth revolved around the Sun, that the Moon was a satellite of Earth, and that the Sun was very different to the others - the word changed its meaning to the modern one of a large body that travels around the Sun in a roughly circular orbit. There are currently nine major planets - including Pluto - plus thousands of minor planets.

The proposed solution is rather neat, though a lot of astronomers don't like it. A planet will be defined as any celestial object not a star that orbits a star and whose mass is large enough for it to have been pulled into a spherical shape by its own gravity. Pluto, by that definition, remains a planet; Charon, its former satellite, becomes the other half of a double planet; Ceres, the largest of the asteroids orbiting between Mars and Jupiter, joins the big- kids' club; the body officially known as 2003 UB313 but informally as Xena becomes the twelfth planet.

But don't be in a hurry to rewrite those reference books: there are dozens of other candidates in the outer darkness that are likely to be added to the list, including those with unofficial names Varuna, Quaoar and Sedna. Three other asteroids, Pallas, Vesta and Hygeia, may become planets, too, if detailed astronomical observations prove them to be spherical. The definition would make the planetary Solar System a great deal more complicated.

Plutons

The same resolution at the International Astronomical Union (IAU) would officially create this new word as a way of distinguishing among several classes of bodies orbiting the Sun. The classical planets are the most massive bodies: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. There are also large numbers of small rocky bodies called collectively the minor planets.

In between would be this new class. They are massive enough to have been formed into a spherical shape by their own gravity, they orbit the sun so far out that one revolution of the Sun takes at least 200 years, and they have elliptical orbits that are inclined to those of the classical planets, the implication being that plutons have a different origin.

The word has been created from the name of Pluto, the ninth planet, which is from Greek mythology, in which it was a euphemism for the god of the underworld, Hades. Literally, "pluto" meant "rich one", in reference to the wealth that came from the Earth. The planet was famously named as a result of a suggestion by the 11-year-old Miss Venetia Burney, of Oxford. Walt Disney's dog, by the way, was named after the planet, not the other way round; popular culture didn't have the influence it does now, when names like Xena - the Greek warrior princess of the US television series - can be seriously considered.

This week's news reports often implied that the committee which put forward the word had invented it. But there are earlier examples: the astronomer Tom Burns used it in the same sense in an article in the Columbus Dispatch in June 1997, as did Frederik Pohl in his SF novel Mining the Oort of 1992; Robert Heinlein created an Earth currency of that name in his novelettesGulf (1949) and Tunnel in the Sky (1955), though that was based on plutonium. "Pluton" is also an established geological term, for a large body of intrusive igneous rock beneath the Earth's surface; that was created in the 1930s as a back-formation from the adjective "plutonic", itself taken from the Greek name, that referred to the action of intense heat at great depths upon rocks forming the Earth's crust.

Daily Telegraph, 16 Aug. 2006: Little Pluto, which had been in peril of losing its place among the planets, keeps its status, but only in a new category of "plutons," distant oddballs wandering outside Neptune in weirdly shaped orbits.

The Seattle Times, 16 Aug. 2006: Dozens more plutons could be added after the objects are more thoroughly reviewed by the IAU.

World Wide Words is copyright (c) Michael Quinion 2006. All rights reserved. The Words Web site is at http://www.worldwidewords.org.