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.
