The International Astronomical Union (IAU) yesterday stripped Pluto of its title, and demoted it to the status of 'dwarf' planet.

Pluto demoted after definition of planet

Wang Sichao, planet expert and researcher at the Zijinshan Astronomical Observatory in Nanjing, the capital city of Jiangsu Province in China's east, said the task of defining a planet was very difficult. The original definition put forward by the Planet Definition Committee proposed that Venus, Saturn, Jupiter, Mercury, Earth, Mars, Uranus and Neptune be the "classic" planets and that Pluto be demoted to the status of a dwarf planet. Celestial bodies Ceres, Charon and 2003 UB313, nicknamed Xena, would also be known as dwarf planets.

However, some planet experts objected to this proposal, prompting spirited discussion. Eventually a second proposal for the definition of a planet was put forward by 17 planet experts, who felt that a planet should satisfy three conditions. They argued that a planet must be a spherical celestial body in a particular area; it must be big enough that its own gravity keeps it roughly round, and that nuclear fusion will not occur in its interior. Only Venus, Saturn, Jupiter, Mercury, Earth, Mars, Uranus and Neptune would satisfy these conditions.

The second proposal was more scientific and the seven-person definition committee, which consists of planet experts, historians and culture experts, accepted it to some extent. They finally dismissed the first proposal and proposed a third following much debate. In this third proposal, a planet had to meet three conditions: it must be big enough and have enough gravity to pull itself into a spherical shape; it must not be a satellite of another planet; and it must orbit the sun. This proposal was the one ultimately adopted.

Wang Sichao said in a briefing that the third proposal had only minor differences from the second with the identical consequence of distinguishing Pluto from the major planets. Comparatively speaking, the second proposal was for a wider definition of a planet and included planets that are outside earth's solar system.

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Pluto: demoted to "dwarf planet"

Pluto's right to be called a planet has long been in dispute since it was discovered over 70 years ago. The process of its discovery was based on an incorrect theory and its mass was wrongly estimated, which resulted in an incorrect classification.

Three possible outcomes following the vote at the meeting of the IAU

Wang Sichao, planet expert and researcher at the Zijinshan Astronomical Observatory in Nanjing, the capital city of Jiangsu Province in China's east, said there were three possible outcomes from the vote.

The first possible outcome was that the second proposal, supported by most exports and put forward by 17 planet exports, was adopted.

The second possible outcome was that the third proposal, outlined by the Planet Definition Committee, was adopted.

The third possible outcome was that the decision on a definition would be postponed to the next meeting of the IAU, to be held in Paris in 2009, as according to some astronomical experts, a decision is premature.

By People's Daily Online