There are only eight planets now, Pluto is out because of its orbit

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. “Many more Plutos wait to be discovered,” added Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology.

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 neighbourhood 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. Experts said there could be dozens of dwarf planets catalogued across the solar system in the next few years. It was unclear how Pluto’s demotion might affect the mission of NASA’s New Horizons spacecraft, which earlier this year began a 9 1/2-year journey to the oddball object to unearth more of its secrets.

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.

That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto’s undoing. In the end, only about 300 astronomers cast ballots.

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 whose discoverer, Michael Brown of the California Institute of Technology, has nicknamed Xena. Charon, the largest of Pluto’s three moons, is no longer under consideration for any special designation.

Brown, who watched the proceedings from Cal Tech, took today’s vote in stride — even though his discovery won’t be christened a planet. “UB313 is the largest dwarf planet. That’s kind of cool,” he said.

WILLIAM J KOLE