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Pluto gets demoted
24/08/2006 15:47 - (SA)
Prague - Leading astronomers on Thursday approved historic new guidelines under which distant Pluto is no longer defined as a planet.
After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. It is the first time that scientists have had a formal definition of what is - and is not - a planet.
Thursday's decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.
For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Much-maligned Pluto doesn't make the grade under the new rules for a planet: A celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit.
Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's. 'Dwarf planet'
Instead, it will be reclassed in a new category of "dwarf planets", similar to what have long been termed "minor planets". <>The definition also lays out a third class of lesser objects that orbit the sun - "small solar system bodies", a term that will apply to numerous asteroids, comets and other natural satellites.
Although astronomers applauded after the vote, Jocelyn Bell Burnell - a specialist in neutron stars from Northern Ireland who oversaw the proceedings - urged those who might be "quite disappointed" to look on the bright side.
"It could be argued that we are creating an umbrella called 'planet' under which the dwarf planets exist," she said, drawing laughter by waving a stuffed Pluto of Walt Disney fame beneath a real umbrella.
The decision at a conference of 2 500 astronomers from 75 countries was a dramatic shift from just a week ago, when the group's leaders floated a proposal that would have reaffirmed Pluto's planetary status and made planets of its largest moon and two other objects. Plan 'highly unpopular'
That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto's undoing.
Now, two of the objects that at one point were cruising toward possible full-fledged planethood will join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto nicknamed Xena by its discoverer, Michael Brown of the California Institute of Technology.
Charon, the largest of Pluto's three moons, is no longer under consideration for any special designation.
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