2003 UB313, otherwise known as Xena, is now a 'dwarf' planet of our Solar System. Pluto shares this status.
Credit: NASA
PRAGUE, 25 August 2006: Pluto has lost its seven-decade status as the ninth and outermost planet of the Solar System, the world's top astrononomical body has decided, in a ruling that will rewrite text books and reshape notions about space topography.
"The eight planets are Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune," the International Astronomical Union (IAU) declared in a resolution approved by raised hands after what, by the quiet traditions of the skygazing community, was a stormy debate.
Pluto's status had been contested for many years by astronomers who said its tiny size and highly eccentric orbit precluded it from joining the other acknowledged planets.
The anti-Pluto movement gained ground after the discovery of a distant object beyond Pluto's orbit called 2003 UB313, also known unofficially as Xena after the lead character in the cult TV series, Xena: Warrior Princess. Its discoverer said 2003 UB313 was as big as Pluto and thus could lay claim to being a planet.
After spelling out the eight names in the Solar System's A-list of planets, the 2,500 IAU delegates assigned Pluto to a new category - that of 'dwarf planet'.
The following is the official text approved by the IAU at the assembly in Prague on Thursday that defines a planet and other Solar System objects.
"Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding.
"This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
"The IAU therefore resolves that 'planets' and other bodies in our Solar System be defined into three distinct categories in the following way:
"(1) A 'planet'1 is a celestial body that (a) is 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.
"(2) A 'dwarf planet' is a celestial body that (a) is 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
"(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as 'Small Solar-System Bodies'. (...)
"Pluto is a 'dwarf planet' by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects."
The long and sometimes heated debate underlined a dilemma that arose several years ago with the emergence of more powerful telescopes and computer-assisted scanning of the heavens.
These potent tools began to suggest that Pluto, far from being a solitary wanderer on the outer fringes of the Solar System, could be only one of a cloud of scattered objects at this distance from the Sun.
This realisation in turn prompted questions as to whether Pluto and other largish objects could be considered planets or - simply and sadly - rocks.
The IAU defined the core difference between a planet and a dwarf planet as whether the celestial object has "cleared the neighbourhood around its orbit"; in other words, whether the object is massive enough to wield a gravity that draws in rocks and other debris that may clutter its orbital path.
While casting Pluto into, literally, the outer darkness, the IAU also tried to sugar the pill somewhat, saying Pluto was "an important prototype of a new class of trans-Neptunian objects."
Around a dozen other objects, such as 2003 UB313 and the large asteroid Ceres, are already candidates for the 'dwarf' definition.
British representative Michael Rowan-Robinson acknowledged that the rearrangement of the planetary hierarchy might cause deep confusion among the public. And, he admitted, astronomers might be taken for "total idiots" because of what seemed to be an arcane debate. But, he said, it was vital to move in line with knowledge.
"The glass [of knowledge] is filling," he said. "In a few years, there may be 40 of those new dwarves. The fact that Pluto has been demoted is not so important."
Pluto was discovered on 18 February 1930 by a 24-year-old American astronomer, Clyde Tombaugh. His announcement had an enormous impact. It smashed the perceived boundaries of the Solar System, established 84 years earlier with the discovery of Neptune.
Named after the god of the underworld in classical mythology, Pluto orbits the Sun at an average distance of 5,906,380,000 kilometers, taking 247.9 Earth years to complete a single circuit.
Its orbital plane is a whole 17 degrees off the horizontal plane taken by the eight other planets. In addition, its path around the Sun is so egg-shaped that, for 20 years of its agonisingly long orbit, Pluto tracks inside the orbit of Neptune.
Pluto has three moons: Charon, discovered in 1978, and the tiny Nix and Hydra, spotted in 2005 by U.S. astronomers using the orbiting Hubble Telescope.
Charon has a diameter of around 1,212 km, making it about half the size of Pluto's 2,300 km. Hydra and Nix measure between 48 km and 165 km across. There is not enough data yet to gauge their size exactly.
On January 19 this year, the United States launched an unmanned spacecraft, New Horizons, which is due to fly by Pluto and the Kuiper Belt in 2015. New Horizons carries an instrument named after an 87-year-old Briton, Venetia Phair, who as a child aged 11 thought up the name for Tombaugh's discovery.
And it also carries some of the ashes of Tombaugh himself, who died in January 1997, convinced he had achieved eternal recognition as the discoverer of the Solar System's ninth planet.
See the feature "Distant worlds" by astronomer Fred Watson published in Cosmos, Issue 9.
Astronomer Bryan Gaensler will shed no tears for Pluto. Read why.
Footnotes to the IAU text
1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
