Pluto no longer a planet, say astronomers
Thursday, August 24, 2006; Posted: 10:27
a.m. EDT (14:27 GMT)
PRAGUE, Czech Republic (AP) -- 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.
It was unclear how Pluto's demotion might affect
the mission of NASA's New Horizons spacecraft, which earlier this year began a
91/2-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. ( Watch why some think planet size doesn't matter --
3:39 )
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 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 was pleased by the decision. He had argued
that Pluto and similar bodies didn't deserve planet status, saying that would
"take the magic out of the solar system."
"UB313 is the largest dwarf planet. That's kind of
cool," he said.
Hey! Xena is bigger than Pluto. Cool! 
