Astronomers have discovered the coldest and most distant object ever found in the solar system, a dark and frigid world a bit smaller than Pluto and three times farther away.  

The new "planetoid," named Sedna after an Inuit goddess who created Arctic sea creatures, is more than 13 billion kilometers from the Sun and never gets above minus 240 C, astronomers said on Monday.

"The Sun appears so small from that distance that you could completely block it out with the head of a pin," said Mike Brown, an astronomer at the California Institute of Technology, who led the research team.

Sedna is one of the reddest objects in the solar system, after Mars, and takes 10,500 years to travel its highly elliptical path around the sun.

Brown and the other astronomers detected Sedna on November 14 during a survey of the outer solar system. As they peered into space, they saw stationary stars and other cosmic bodies, and a very slowly moving object that turned out to be Sedna.

"Anything that moves very slowly across the sky, we know it's something in the solar system: a satellite, a planet, an asteroid,'' Brown said at a telephone news conference. "But this is the most slowly moving object we've ever seen moving across the sky, and we knew it must be something very far away."

Sedna revolves once every 40 days, a slow revolution that suggests it might have a moon slowing its twirl, Brown said.

To check this, he and his team plan to use the Hubble Space Telescope, which is peerless at looking at distant objects that appear close together or even fused and determining whether they are separate.

Sedna is part of the solar system, but that does not mean it is a planet, according to Brian Marsden, director of the Minor Planet Centre of the International Astronomical Union.

"I think it would be misleading to call it the 10th planet," Marsden said in a telephone interview. "Just as I think it's misleading to call Pluto the ninth planet."

To be called planets, astronomical objects must be a certain size, and Pluto is at the lower limit of planetary dimensions, Marsden said. They also must "participate" in the events of the solar system, and there again, he feels Pluto does not qualify -- its orbit is neither circular nor in the same plane as the other planets.

(China Daily March 17, 2004)