August 24, 2006
Astronomers strip Pluto of its planet status
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 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
with 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.
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 with each
day. Some delegates appeared downright hostile to the notion,
saying the committee was going overboard.
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.
"We are just defining a new class of planets and I think
it's very appropriate. We are finding more planets in our solar
system, and some are larger than Pluto," said Philip Diamond, a
professor at the University of Manchester and a delegate
attending the IAU meeting.
"I think what we have done is a good thing, we have
actually expanded the number of planets in our solar system,
but just spread them over two categories."
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