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.
