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Last Updated: Thursday, 24 August 2006, 13:34 GMT 14:34 UK
Pluto loses status as a planet
Pluto was discovered in 1930 by US astronomer Clyde Tombaugh
Astronomers have voted to strip Pluto of its status as a
planet.
About 2,500 scientists meeting in Prague have adopted
historic new guidelines that see the small, distant world
demoted to a secondary category.
The researchers said Pluto failed to dominate its orbit
around the Sun in the same way as the other planets.
The International Astronomical Union's (IAU) decision means
textbooks will now have to describe a Solar System with just
eight major planetary bodies.
Pluto, which was discovered in 1930 by the American Clyde
Tombaugh, will be referred to as a "dwarf planet".
There is a recognition that the demotion is likely to upset
the public, who have become accustomed to a particular view
of the Solar System.
"I have a slight tear in my eye today, yes; but at the end
of the day we have to describe the Solar System as it really
is, not as we would like it to be," said Professor Iwan
Williams, chair of the IAU panel that has been working over
recent months to define the term "planet".
The meeting had seen some fierce arguments before final
voting
The need for a strict definition was deemed necessary after
new telescope technologies began to reveal far-off objects
that rivalled Pluto in size.
Without a new nomenclature, these discoveries raised the
prospect that
textbooks could soon be talking about 50 or more planets in
the Solar System.
Amid dramatic scenes in the Czech capital which saw
astronomers waving yellow ballot papers in the air, the IAU
voted to block this possibility - and in the process took
the historic decision to relegate Pluto.
The scientists agreed that for a celestial body to qualify
as a planet:
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*
round
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it must be in orbit around the Sun
it must be large enough that it takes on a nearly
shape
it has cleared its orbit of other objects
Pluto was automatically disqualified because its highly
elliptical orbit overlaps with that of Neptune. It will now
join a new category of dwarf planets.
Icy reaches
Pluto's status has been contested for many years. It is
further away and considerably smaller than the eight other
"traditional" planets in our Solar System. At just 2,360km
(1,467 miles) across, Pluto is smaller even than some moons
in the Solar System.
PLUTO - A 'DEMOTED PLANET'
Named after underworld god
Average of 5.9bn km to Sun
Orbits Sun every 248 years
Diameter of 2,360km
Has at least three moons
Rotates every 6.8 days
Gravity about 6% of Earth's
Surface temperature -233C
Nasa probe visits in 2015
Its orbit around the Sun is also highly tilted compared with
the plane of the big planets.
In addition, since the early 1990s, astronomers have found
several objects of comparable size to Pluto in an outer
region of the Solar System called the Kuiper Belt.
Some astronomers have long argued that Pluto would be better
categorised alongside this population of small, icy worlds.
The critical blow for Pluto came with the discovery three
years ago of an object currently designated 2003 UB313.
After being measured with the Hubble Space Telescope, it was
shown to be some 3,000km (1,864 miles) in diameter: it is
bigger than Pluto.
2003 UB313 will now join Pluto in the dwarf category, along
with the biggest asteroid in the Solar System, Ceres.
Named after the god of the underworld in Roman mythology,
Pluto orbits the Sun at an average distance of 5.9 billion
kilometres (3.7 billion miles) taking 247.9 Earth years to
complete a single circuit of the Sun.
An unmanned US spacecraft, New Horizons, is due to fly by
Pluto and the Kuiper Belt in 2015.