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Last Updated: Thursday, 24 August 2006, 20:31 GMT 21:31 UK
Q&A: Pluto's planetary demotion
Jocelyn Bell-Burnell and Pluto dog (AP)
IAU member Jocelyn Bell-Burnell oversees voting at the meeting
Pluto has lost its classification as a planet.

The distant world discovered in 1930 by the American Clyde Tombaugh will be given a secondary status - that of "dwarf planet".

Who made this decision?

The International Astronomical Union (IAU) General Assembly. It has been meeting in the Czech capital, Prague. The IAU is the official nomenclature body for astronomy. The IAU is responsible for naming stars, asteroids and other celestial bodies.

Why was it felt necessary to reclassify Pluto?

A sizeable group of astronomers has been unhappy with Pluto's status for many years. These scientists feel the object's small size and its highly elliptical and tilted orbit set it apart from the other eight planets. They have long campaigned for Pluto's relegation and managed to carry a vote to that effect in Prague.

Why has this debate come to a head now?

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
Improved telescope technologies have made new discoveries in the outer reaches of the Solar System. This research suggests there are many objects which orbit far from the Sun that are very similar to Pluto. One such object, known as 2003 UB313, was recently shown by the Hubble telescope to have a diameter that exceeds Pluto's.

And this presented a problem?

If Pluto was classed as a planet then 2003 UB313 had a legitimate claim to be classed as a planet as well. And with more and more discoveries being made in the outer Solar System, astronomy could have faced the prospect of using the term "planet" to describe many tens of objects - not just the nine worlds that have traditionally gone under that description. Part of the problem here is that the IAU has never really had a strict definition for what constitutes a planet. The new science has forced its hand.

But only a week ago there was talk of increasing the tally?

An initial proposal put forward criteria that kept Pluto's status and brought the club up to 12 - adding 2003 UB313, the asteroid Ceres, and Pluto's largest moon, Charon. But this scheme met with considerable opposition from astronomers at the assembly who felt it still opened the way to a rapid increase in planet numbers in the years ahead.

What was finally agreed at the IAU?

Artist impression of New Horizons spacecraft passing by Pluto

To qualify as a planet, the assembly agreed that a celestial body must be in orbit around a star while not itself being a star. It must also be large enough that its gravity pulls it into a roughly spherical shape. In addition, the world must dominate its orbit, clearing away other objects. It's on this last point that Pluto fails: its highly elliptical orbit overlaps with that of the much bigger Neptune.

How many planets are there now?

Before Thursday, we had nine. Now, the number of planets is eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

What happens to Pluto now?

Pluto goes into a new dwarf-planet category. It is joined by 2003 UB313 - once touted as the "10th planet" - and Ceres. More dwarf planets are expected to be announced by the IAU in the coming months and years. Currently, a dozen candidates are listed on the IAU's "dwarf planet watchlist", which keeps changing as new objects are found and the physics of the existing candidates becomes better known. The IAU is also setting up rules to handle borderline cases. Most objects in the Solar System will still sit in their traditional minor categories: the asteroids, the comets, etc.

Is this the end of the matter?

Unlikely. The Principal Investigator on Nasa's $700m New Horizons mission to Pluto has lambasted the decision to demote Pluto. Alan Stern told BBC News the decision was "embarrassing" and the criteria amounted to "sloppy science". He said that if the clear orbit criterion was strictly applied then even Earth should lose its status because its path is crossed by thousands of small rocks.

Dr Stern said like-minded astronomers would try to get Pluto reinstated.

New Solar System - not to scale (BBC)

Pluto loses status as a planet
24 Aug 06 |  Science/Nature
Crunch time for Planet Pluto
20 Jun 06 |  Science/Nature
Pluto probe launches from Florida
20 Jan 06 |  Science/Nature
Distant world tops Pluto for size
01 Feb 06 |  Science/Nature
Mission guide: New Horizons
19 Jan 06 |  Science/Nature
The girl who named a planet
13 Jan 06 |  Science/Nature
Astronomers detect '10th planet'
30 Jul 05 |  Science/Nature

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