August 24, 2006
Astronomers strip Pluto of its status as a planet
By Alan Crosby
PRAGUE (Reuters) - Pluto was stripped of its status as a
planet on Thursday when astronomers from around the world
redefined it as a "dwarf planet," leaving just eight major
planets in the solar system.
instantly obsolete, forcing teachers and publishers to scramble
to update textbooks and lessons used in classrooms for decades.
"Pluto is dead," Mike Brown of the California Institute of
Technology bluntly told reporters on a teleconference.
Discovered in 1930 by the American Clyde Tombaugh, the icy
rock of Pluto has traditionally been considered the ninth
planet, farthest from the sun in the solar system.
However, the definition of a planet, approved after a
heated debate among 2,500 scientists from the International
Astronomical Union (IAU) meeting in Prague, drew a clear
distinction between Pluto and the other eight planets.
The need to define what is a planet was driven by
technological advances enabling astronomers to look further
into space and measure more precisely the size of celestial
"This is all about the advancement of science changing our
thinking as we get more information," said Richard Binzel,
professor of Planetary Sciences at The Massachusetts Institute
of Technology and a member of the planet definition committee.
"The significance is that new discoveries and new science
have told us that there is something different about Pluto from
the other eight planets and as science learns more information,
we get new results and new considerations."
Brown added impetus to the decades-old debate on the
definition of a planet when he discovered UB313 in 2003. Xena,
as it is nicknamed, is larger than Pluto, instantly creating a
buzz over whether a new planet had been discovered.
The scientists agreed that, to be called a planet, a
celestial body must be in orbit around a star while not itself
being a star.
It must be large enough in mass for its own gravity to pull
it into a nearly spherical shape and have cleared the
neighborhood around its orbit.
Pluto was disqualified because its oblong orbit overlaps
Neptune's. Xena also does not make the grade of being a planet,
and will also be known as a dwarf planet.
"It's an issue mainly for the public, not really for
scientists. Some people may be upset, but we've long regarded
it (Pluto) as a minor planet," said Richard H. Miller of the
University of Chicago.
Alan Stern of the Southwest Research Institute in Boulder,
Colorado, overseer of science investigations on NASA's New
Horizons mission to Pluto, called the reclassification rash and
illogical. "I think people are going to consider Pluto a planet
regardless," he said.
Officials at the American Museum of Natural History in New
York had been at the vanguard of the movement to demote Pluto
and were feeling vindicated on Thursday.
"We had enormous numbers of telephone calls and I would say
things that verged on hate mail from second-graders -- very
angry children who said, 'What have you done? This is the
cutest, most Disney-esque of the planets. How could you
possibly demote it?"' said Michael Shara, the museum's
While the museum staff was celebrating, Shara said Pluto's
new status was more a victory for the astronomical community
because it now had a "greatly increased understanding of what a
The agreed-upon definition -- the first time the IAU has
tried to define scientifically what a planet is -- comes in
sharp contrast to the draft sent around to delegates at the
General Assembly last week.
That document, which kept Pluto as a planet and would have
added three others, touched off a revolt that grew daily. Some
delegates appeared downright hostile to the notion.
Tombaugh's widow Patricia said the discoverer, like any
good scientist, would have accepted the demotion as inevitable.
"Clyde would have said, 'Science is a progressive thing and
if you're going to be a scientist and put your neck out, you're
apt to have it bitten upon,"' the 94-year-old said from her
home in Las Cruces, New Mexico.
She added that a small amount of her husband's ashes were
now on a spacecraft bound for Pluto.
The new definition creates a second category called "dwarf
planets," as well as a third category for all other objects,
except satellites, known as small solar system bodies.
From now on -- or 'at least for the time being' joked one
delegate -- traditional planets will be restricted to eight:
Mercury, Venus. Earth, Mars, Jupiter, Saturn, Uranus and
(additional reporting by Svea Herbst-Bayliss in Boston,
Bill Trott in Washington and Michelle Nichols in New York)