Pluto is a planet no more after the world's
leading astronomers redrew the solar system today, ruling that only
eight celestial bodies deserved the prestigious title.
Discovered
in 1930 by Clyde Tombaugh, a 24-year-old American astronomer, Pluto's
status as the ninth, oddly small planet had been in jeopardy since the
1990s, when powerful telescopes revealed objects just as large also
making distant orbits in the darkness beyond Neptune.
For the last week, an assembly of 2,500 delegates at
the International Astronomical Union (IAU) has been debating Pluto's
future, as well as the status of three other bodies: Ceres, until now
regarded as the solar system's largest asteroid; Charon, one of Pluto’s
moons; and a newly discovered object called 2003 UB313.
One proposal was for the solar system to be expanded
to contain 12 planets in two tiers, but today, by a show of hands in
its concluding session, the IAU decided that there could be only eight.
"The eight planets are Mercury, Earth, Venus, Mars,
Jupiter, Saturn, Uranus and Neptune," said the IAU resolution that
confirmed Pluto's relegation to "dwarf planet" status, where it will
not even be the largest object.
Just 1,433 miles (2,306km) across, Pluto is smaller
than 2003 UB313, an icy being unofficially known as Xena whose diameter
of 1,900 miles (3,000km) was only confirmed earlier this year.
Ceres and Charon, the largest of Pluto’s three moons,
will also be added to the group of dwarf planets that astronomers
believe could grow to include dozens of members in the years to come.
Today's meeting also created a new catch-all category
for other significant asteroids and comets: they will be known as
"small solar system bodies".
Today's vote was the culmination of two years of
debate and semantic adjustment by a seven-strong IAU committee that has
worked to define just what makes a planet in an increasingly complex
and visible solar system.
Eventually, that definition — "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" — made Pluto's case
hard to argue.
Pluto fails to meet this
new definition. Its orbit, far from cleared - which means the object
carries the gravitational clout to draw in rocks and other debris that
otherwise clutter its path - overlaps that of Neptune.
Although last week Pluto's planethood appeared to
have been saved by the compromise proposal to expand the solar system,
its eventual relegation comes after years of steady skepticism from
astronomers.
In 1999, the IAU was forced to issue a statement
saying it had no plans for Pluto's demotion. Soon afterwards, it was
excluded from an exhibition about the solar system that the American
Museum of Natural History in New York.
The next piece of good news for Pluto may not now come until 2015, when Nasa's New Horizons spacecraft
arrives at the edge of the solar system to take the first close-up
images of its surface. In the end, it was a planet for just over a
quarter of a Pluto year, a Pluto spring.
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