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 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.