The International Astronomical Union (IAU) on Thursday adopted a resolution on planet definition, according to which Pluto had been stripped of the planetary status. (Xinhua Photo)

The International Astronomical Union (IAU) on Thursday adopted a resolution on planet definition, according to which Pluto had been stripped of the planetary status. (Xinhua Photo)

The resolution of a planet definition, the first of its kind in the astronomical history, was adopted here Thursday after days of fierce debate and effectively kicked Pluto out of the planet group in the solar system.

According to the new rules for a planet adopted at an International Astronomical Union meeting in Prague, Pluto doesn't make the grade for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's. Instead, it was defined as a "dwarf planet", but no longer a planet.

Ron Eckers, President of the IAU, which is the world's largest astronomical society, said it may not matter that much how many planets are there in the solar system.

"What really matters is that we have a definition of planet, and with that, we can move on with our studies," he told Xinhua.

The definition, which Ecker described as "carefully refined" with much compromise after considerable discussion, came in line with the contemporary observation that has been changing the understanding of planetary system.

According to the resolution, Pluto, Charon, Ceres and 2003 UB313 are termed "dwarf planets," despite having enough mass and being nearly round.

Still, Ecker, who is strongly in favor of naming the eight planets found before 1900 "classical planets", and including "dwarf planet" into category of "planet", call the result "a pity."

If a larger category of "planet" is created, we can leave open the possibility to discover and define other types of planets, he said.

Fang Cheng, an astronomer with the Chinese Academy of Sciences, also vice chairman of the IAU, told Xinhua that the definition is "clear and compact."

Editor: Yan