Aug 24, 2006, 18:20 GMT
Prague - The world's astronomers agreed Thursday to create two categories of solar-system planets, demoting the distant world Pluto with the label 'dwarf planet' to distinguish it from eight larger planets stretching from Mercury to Neptune.
But the astronomers, meeting at a International Astronomical Union conference in Prague, also agreed that the word 'planet' generally could be used to describe Pluto as well as the body Ceres, traditionally known as an asteroid, and a recently discovered object called UB313.
The decision effectively increased the number of scientifically recognized planets in the solar system to at least 11 from nine, and cleared the way for more objects beyond Pluto to be added in the future.
'Pluto is a planet, but it is in the dwarf planet category,' declared IAU President Ron Ekers after members approved a carefully worded resolution crafted by a experts over the past two years.
Scientists said the new definitions will help clarify a system of classifying sun-orbiting bodies that's become outdated by recent discoveries, including the 2003 discovery of UB313 beyond Pluto.
In addition to distinguishing 'planet' and 'dwarf planet,' the scientists agreed to apply the phrase 'small solar system bodies' to objects that are not 'nearly round' but orbit the sun.
The scientists said several questions were left unanswered, such as whether Pluto's companion object Charon may qualify as a planet and what to call objects that may be found orbiting other stars.
So far, our solar system is the only known celestial system with planets orbiting a star. But scientists are sure other solar systems and orbiting objects are waiting to be discovered, and will need to be scientifically labelled.
Moreover, an IAU statement noted 'Pluto is recognized as an important prototype of a new class of trans-Neptunian objects' for which the IAU 'will set up a process to name these objects.'
German astronomer and IAU president-elect Catherine Cesarsky said one goal of the wordsmith exercise was to make space science 'more understandable' to the public including schoolchildren.
'There's been great interest in this all around the world,' she said. 'It behooves us to be conscious of that public support.'
Although Pluto has been called a planet since its discovery in 1930, it now falls into the 'dwarf' category but still meets the general criteria for 'planet' set out in the IAU resolution.
The new definition says a planet 'is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit.'
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