NANJING -- Four-year-old Gong Xiaoxin, who has just learned from his father that the solar system has nine planets, may well have to revise his lessons in a couple of days.

Nearly 2,500 astronomers from 75 nations meeting in Prague will vote on a new universal definition of a planet on Thursday.

The issue has aroused interest in China, which has been carrying out astronomical research for a thousand years.

Wang Sichao, an expert with the Zijinshan Astronomical Observatory based in Nanjing, capital city of east China's Jiangsu Province, said, "No matter what the result is, the vote is very important. It signifies that over the past few decades, human understanding of the solar system has made a giant leap, smashing the original framework."

Under a draft resolution presented to the International Astronomical Union (IAU), Pluto would remain a planet and its largest moon, Charon, plus two other heavenly bodies would join Earth's solar system as new planets. Textbooks would be rewritten to say the solar system has 12 planets rather than the nine memorized by generations of schoolchildren.

Wang said scientists have based the draft resolution on scientific factors, but have also taken historical and social factors into consideration. But it is a compromise solution, Wang added, that does not really reflect astronomers' all-round understanding of celestial bodies in the solar system.

The other planet candidates, apart from Charon, are 2003 UB313, the farthest-known object in the solar system otherwise known as Xena, and the asteroid Ceres. Pluto risks being demoted to the status of dwarf planet.

Opponents of Pluto, which was named a planet in 1930, might still spoil for a fight. Earth's moon is larger; so is 2003 UB 313 (Xena), about 112 kilometers wider.

But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 kilometers in diameter that orbits the sun.

Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet Earth's moon would not qualify because the two bodies' common center of gravity lies below the surface of the Earth.