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
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 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
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:
- it must be in
orbit around the Sun
- it must be
large enough that it takes on a nearly round shape
- it has cleared
its orbit of other objects
automatically disqualified because its highly elliptical orbit overlaps with
that of Neptune. It will now join a new category of dwarf planets.
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.