Sedna’s Presumed Moon Is Nowhere to Be Seen

Astronomers are convinced that the newly discovered planetoid called Sedna has a moon, but a recent look at the object by the Hubble telescope found no evidence of a satellite, leaving researchers baffled.

Sedna, which is a little less than 1,000 miles in diameter and the most distant known object in the solar system, was discovered last month. It rotates once every 25 days, which astronomers believe indicates that it has a moon. Objects that don’t have a moon generally rotate much faster.

“There is no other explanation for something to rotate so slowly,” astronomer Michael Brown of Caltech said this week in a telephone news conference. “But no satellite whatsoever was visible” when the team trained the Hubble on it.

Brown said there were only a few possible explanations:

* The moon might be directly in front of or behind the planetoid, about a 1 in 100 possibility.

* The moon might be fainter than expected, which would be surprising because Sedna itself is so bright.

* Sedna might have had a satellite in the past that slowed it down, then was lost or destroyed by impact with another object.

* The research team might have misinterpreted the rotation rate, a 5 in 100 possibility.

Sedna, which revolves around the sun in a highly elliptical orbit, is now about 8 billion miles from the sun, about three times as far as Pluto. It is now on the opposite side of the sun from Earth and will not be visible for six months. Researchers hope to resolve some of their questions when it reemerges.