Judgment day for Pluto

Pluto with its satellite Charon

From today’s Guardian. Nearly 400 years after Galileo first peered at the heavens through a rudimentary telescope, the world's top astronomers have called an urgent meeting to decide once and for all the meaning of the word "planet".

A vote today at the International Astronomical Union meeting in Prague today, will end what has become an embarrassing crisis for scientists. It will also change the nature of the solar system forever: where we now have nine planets endlessly looping around the sun, we may soon have only eight. Or 23, or 39, or more.

At the root of the crisis is a 76-year-old celestial fudge that has until now been papered over: the discovery of Pluto. When scientists at the Lowell Observatory announced they had spotted Pluto in 1930, they claimed it was several times larger than Earth, ensuring its prompt entry into the textbooks as the ninth planet. It later turned out to be a rock substantially smaller than the moon. Until last year, the astronomy community was mostly happy to turn a blind eye to Pluto's apparent crashing of the planetary party. But last year, Mike Brown, an astronomer at California Institute of Technology, discovered a fly in the ointment -another celestial body larger than Pluto, hurtling around with an orbit stretching beyond Neptune.

The Hubble space telescope measured the rocky object at about 1,490 miles in diameter - roughly 70 miles longer than Pluto's. While it is officially known as 2003 UB313, Professor Brown named the rock Xena, after the Warrior Princess TV series, and claimed it as the 10th planet. "The discovery of this object really brought things to a head. It's spherical, it orbits the sun, and it happens to be bigger than Pluto," said Dr Marsden.

Xena with its satellite Gabrielle

The reason the question has become such a headache is that unlike the inner eight planets orbiting the sun, Pluto is a member of a vast band of extremely distant rocks called the Kuiper belt that orbits the sun beyond Neptune. Xena is a Kuiper belt object too, so if Pluto qualifies as a planet, so must Xena. And the headache does not end there. Astronomers know that the Kuiper belt is likely to be full of countless other rocks, so where do they draw the line?

"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."

A popular idea is to count only objects above a certain size. Some scientists want an arbitrary limit - a diameter of around 1,243 miles. Others say the size and rotation of the object should be sufficient to give the rock a strong enough gravitational field to make it spherical.

Whatever the outcome, the IAU's decision could rewrite school textbooks and encyclopaedias and countless science fiction stories. "I don't know about the public, but whatever happens, the astrologers will be upset about it," said Astronomer Patrick Moore.

astronomy
Pluto
Xena