Page last updated at 08:50 GMT, Thursday, 12 June 2008 09:50 UK

'Non-planet' Pluto gets new class

Pluto and its moon Charon (beyond) SPL
Pluto was stripped of its planet status two years ago

"Plutoid" is the word of the moment for astronomers.

It is the new classification that has been sanctioned for the object that was formerly known as the "ninth planet".

It is nearly two years since the International Astronomical Union (IAU) stripped Pluto of its former status as a "proper" planet.

Now an IAU committee, meeting in Oslo, has suggested that small, nearly spherical objects orbiting beyond Neptune should carry the "plutoid" tag.

As astronomy's official nomenclature organisation, the IAU must approve all new names and classifications.

Its decision at the 2006 General Assembly to demote Pluto from "planet" to "dwarf planet" caused an international furore.

Plutoids or haemorrhoids, whatever they call it. This is irrelevant
Alan Stern

Pluto's relegation was felt necessary because new telescope technologies had begun to reveal far-off objects that rivalled the world in size.

Without a new classification, these discoveries raised the prospect that textbooks could soon be talking about 50 or more "planets" in the Solar System.

That prospect proved too much for IAU members who took the historic decision to redefine the Solar System to have just eight major worlds - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

They relegated Pluto to a grouping that includes Ceres (the largest asteroid), and Eris, an object slightly larger than Pluto that orbits even further out from the Sun in an icy region sometimes referred to as the "scattered disc".

The IAU's Committee on Small Body Nomenclature has now decided that dwarf planets that move beyond Neptune should be placed in a new sub-category, the plutoid.

More plutoids

In a statement released on Tuesday, the IAU further explained the plutoid definition as celestial bodies that "have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared [their orbits of debris].

"The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made."

The plutoids will also need to have a minimum brightness.

Ceres will not be considered a plutoid because of its position in the asteroid belt between Mars and Jupiter.

The classification will not placate those incensed by Pluto's demotion.

Alan Stern, a former Nasa space sciences chief and principal investigator on a mission to Pluto, was scathing in his condemnation of the IAU.

"It's just some people in a smoke-filled room who dreamed it up," he told the Associated Press. "Plutoids or haemorrhoids, whatever they call it. This is irrelevant."

New Solar System - not to scale (BBC)

Planet-hunters set for big bounty
17 Feb 08 |  Science/Nature
Pluto-bound probe's Jupiter vista
10 Oct 07 |  Science/Nature
Pluto status suffers another blow
14 Jun 07 |  Science/Nature
Mission guide: New Horizons
27 Feb 07 |  Science/Nature
Dwarf planet 'becoming a comet'
17 Jan 07 |  Science/Nature
'Plutoed' voted US word of year
08 Jan 07 |  Americas
Astronomers name 'world of chaos'
14 Sep 06 |  Science/Nature
Pluto loses status as a planet
24 Aug 06 |  Science/Nature
Q&A: Pluto's planetary demotion
24 Aug 06 |  Science/Nature

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