Surprise from solar system's edge

1.00pm

WASHINGTON - Strange planetoid Sedna, the most distant object in the solar system, was supposed to have a companion moon, but even the Hubble Space Telescope could not find it, scientists said today.

Astronomers who first detected Sedna had expected to find a moon orbiting their discovery, mostly because Sedna was spinning so slowly that they figured a satellite must be slowing its rotation to just one turn every 20 to 50 days.

"We were so convinced that there had to be a satellite, because there is no other good scientific explanation for why something will rotate as slowly as 20 days," Mike Brown, an astronomer at the California Institute of Technology, said at a telephone news conference.

"Much to our surprise, there's no satellite," Brown said.

Sedna, named for the Inuit goddess who created Arctic sea creatures, is the coldest, most distant object ever found in our solar system, three times further away from the sun than Pluto -- at 13 billion kilometres, the sun would appear so small on Sedna's surface that it could be blocked from view by the head of a pin.

Sedna is part of the solar system, but it is not considered a planet. Instead, astronomers consider it a minor planet or planetoid.

Brown and his colleagues announced the find on March 15, and immediately began searching the area with the orbiting Hubble telescope, expecting to see Sedna's moon. Hubble didn't find one, but it could still be there, Brown said.

Sedna's moon might be hiding directly behind it, or the satellite might be fainter than astronomers expected, making it hard for even Hubble to detect.

Other possibilities include the notion that Sedna once had a moon, which slowed the planetoid's rotation, but the moon was destroyed by an impact with another cosmic body or was pulled away by a close encounter with another planetoid.

Moon or no moon, though, Sedna's status as a planetoid remains unchanged, Brown said.

"Whether or not an object has a moon in no way influences what we think of as a planet," he said. "Mercury and Venus have no moons and there's no doubt that they're planets. There are many asteroids that have moons and other objects out beyond Neptune ... which are clearly not planets. so unfortunately this doesn't help to answer the situation."

Brown and other researchers believe Sedna is just one of many such planetoids at the solar system fringe.

For now, though, Sedna is a cosmic oddball: one of the reddest objects in our planetary system after Mars, it takes a highly elliptical path around the sun, reaching 135 billion km at its farthest point. The complete orbit takes 10,500 years to complete.

Even at its closest to the sun, the temperature on Sedna never gets above minus 240degC.

- REUTERS