The planet formerly known as Pluto has a volume about 0.5832% that of Earth's ... it's tiny size and odd orbit have led to reclassification as a "dwarf planet" and the new name of 134340 Pluto.
Credit: NASA
PARIS, 14 September 2006: Once the proud outermost planet of the Solar System, Pluto has been consigned to the status of a telephone number after the world's paramount astronomical body tore up its membership of the cosmic A-list.
The enigmatic, icy world spotted in 1930 has been given the official monicker of 134340 Pluto and lumped among 136,562 asteroids and other small bodies by the Minor Planet Centre, part of the Paris-headquartered International Astronomical Union (IAU).
Pluto's satellites, Charon, Nix and Hydra, have been numbered 134340 I, 134340 II and 134340 III respectively under the latest minor planet list, compiled on September 7.
"Every asteroid in the solar system has a number," said Bryan Gaensler, an astronomer at the University of Sydney and member of the Editorial Advisory Board of Cosmos. "It just reflects the fact that Pluto is not a planet, and since it's now been demoted it should have the same classification as all the other objects in that category.
The IAU last month declared Pluto to be a 'dwarf planet' that should not belong among the hallowed ranks of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The organisation is now drawing up a 'dwarf planet' catalogue whose initial members will include Pluto, the large asteroid Ceres and a distant object, 2003 UB313, unofficially named Xena after the lead character in the cult TV series, Xena: Warrior Princess.
Until the IAU meeting last month, 2003 UB313 had laid claim to being the Solar System's 10th planet.
Rebel astronomers are circulating a petition, contesting the 'dwarf planet' definition as unscientific and the decision on Pluto as undemocratic. They intend to hold a conference next year that, in their view, will overhaul the definition of a planet.
"A more open process, involving a broader cross-section of the community engaged in planetary studies of our own Solar System and others should be undertaken," said Mark Sykes, the petition's organiser and director of the Planetary Science Institute in Tucson, Arizona.
Pluto's status had long been contested by astronomers who said its tiny size, odd orbit and orbital plane precluded it from joining the other acknowledged planets.
By the new IAU yardstick, a planet has "cleared the neighbourhood around its orbit" - in other words, it is massive enough to wield a gravity that clears rocks and other debris on its orbital path.
134340 Pluto, however, has not cleared its orbit - it follows an unusual elongated path that crosses the orbit of Neptune. And far from being a solitary wanderer on the outer fringes of the Solar System, the celestial body is only one of a cloud of scattered objects at this distance from the Sun.
"There are numerous other trans-Neptunian objects in that region of space," said Nick Lomb, senior curator at the Sydney Obervatory, who attended the demotion meeting. "So if Pluto is regarded as a planet then we'd have the ridiculous situation of dozens of small icy bodies regarded as planets."
But the pro-Pluto rebels argue that no planet ever fully clears its orbit: Earth has been catastrophically struck several times in its history by large space rocks, and in 1994 even Jupiter - the biggest planet of the Solar System - was wacked by parts of a disintegrating comet, they note.
According to Gaensler, however, Pluto's former classification was just a historical accident. "People wanted a planet in 1930 and then they found something and so immediately it became a planet," he said. "If people hadn't been excited about finding a nice planet at that point, then it would have been just another asteroid ... Pluto really doesn't have much going for it."
with Agençe France-Presse
