Distant 'planetoid' seen in our solar system

1.00pm

WASHINGTON - 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 km from the sun and never gets above minus 240 degrees, 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 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."

As distant and cold as Sedna is now, its orbit around the sun takes it more than 10 times further, to a distance of 135 billion km.

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 doesn't mean it's a planet, according to Brian Marsden, director of the Minor Planet Center 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.

Since Sedna is smaller and far more eccentric in its path than Pluto, Marsden questioned its potential planetary status. Brown echoed this assessment, and even seconded Marsden's opinion that Pluto is no planet.

Marsden questioned the planetoid's suggested name, given by its discoverers and not yet approved by the astronomical union. The formal designation for Sedna is 2003VB12.

First detected with the Samuel Oschin Telescope near San Diego, California, Sedna was observed within days on telescopes from Chile to Spain, Arizona and Hawaii.

Nasa's new orbiting Spitzer Space Telescope, which looks at the universe with infrared detectors that peer through cosmic dust, was also trained on the distant object.

The Spitzer scope found that Sedna probably has about three-fourths the diameter of Pluto, which would make it the biggest object found in the solar system since Pluto's discovery in 1930.

- REUTERS