Aug. 24, 2006 — Leading astronomers declared Thursday that Pluto is no longer a planet under historic new guidelines that downsize the solar system from nine planets to eight.
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. The new definition of what is — and isn't — a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.
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 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.
Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have 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.
