Pluto and Charon as seen from Earth

Credit: NASA/ESA

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PARIS, 2 September 2006: Only a week after Pluto was stripped of its status as a fully-fledged planet of the Solar System, rebel astronomers have launched a campaign to have it restored in pomp and glory.

A petition already signed by more than 300 professional researchers is circulating among the astronomical community, attacking the International Astronomical Union (IAU) decision that expelled Pluto from the Solar System's A-list and declared it a 'dwarf planet'.

"We as planetary scientists and astronomers do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed," says the petition, placed on the Internet.

The petition organiser, Mark Sykes, who is director of the Planetary Science Institute in Tucson, Arizona, said the IAU definition of a planet "does not meet fundamental scientific standards and should be set aside."

"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," Sykes said.

Rebels intend to stage a conference next year that will fix the definition of a planet, and as many as 1,000 astronomers will attend, they hope.

A co-sponsor of the petition is Alan Stern, executive director of the Centre for Space Exploration Policy Research at the U.S. Southwest Research Institute in San Antonio, Texas, USA.

Stern heads NASA's New Horizons mission to Pluto. An unmanned spacecraft - which also bears some of the ashes of U.S. astronomer Clyde Tombaugh, who discovered Pluto in 1930 - blasted off in January 2006, months before the IAU decision that relegated Pluto's status.

Stern said "only about 428" of the IAU's nearly 10,000 members were involved in the vote at the IAU's assembly in Prague last week. That tally is barely more than the number of people who signed the protest petition within the first five days of its being launched, he said.

The IAU declared on August 24 that the Solar System comprised eight planets: Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto's status had been contested for many years by astronomers who said its tiny size, highly eccentric orbit and orbital plane precluded it from joining the other acknowledged planets.

The IAU assigned Pluto and other large objects to a new category - 'dwarf planet'. The chief difference between a planet and a dwarf planet is 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

Two other objects, officially (but temporarily) known as 2003 UB313 - as well as the large asteroid Ceres - are already included in the 'dwarf' definition. The IAU is, among many other things, the official guardian of the names of celestial objects. Its next assembly is in 2009.

Two other objects, officially (but temporarily) known as 2003 UB313 - as well as the large asteroid Ceres - are already included in the 'dwarf' definition. But they could be joined by many large objects believed to be circulating, like 2003 UB313, in the Kuiper Belt which is in Pluto's general vicinity.

The pro-Pluto group says 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.

The planetary club, instead of being reduced to eight, should be enlarged to other planetary-sized objects, they say, an argument that others contest as unwieldy and confusing for the general public.

Named after the god of the underworld in classical mythology, Pluto orbits the Sun at an average distance of 5,9 million km, taking 247.9 Earth years to complete a single circuit. But 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.

The IAU is, among many other things, the official guardian of the names of celestial objects. Its next assembly is in 2009, which means a long battle could be in the offing among astronomers, whose quiet and bookish exterior can mask steely resolve when it comes to disputes.

Scientific reputations, history, the teaching of our Solar System and the definition of planets that orbit other stars - all are at stake in the Pluto debate. "The only thing missing when [the IAU] announced the decision at their press conference was the 'Mission Accomplished' banner," said University of Colorado researcher Jeffrey Bennett. "Yes, Im afraid this matter is about as settled as the Iraq war in 2003."

Read why astronomer Bryan Gaensler will shed no tears for Pluto, and firmly believes it had to go.

See the feature article "Distant worlds" by astronomer Fred Watson published in Cosmos, Issue 9.