The moon that isn't there.

Contrary to predictions, the most distant object known in the solar system doesn't appear to have a moon. According to Hubble Space Telescope images released last week, the remote body dubbed Sedna roams the solar system's edge without a partner, making its origin and evolution especially puzzling.

When Michael E. Brown of the California Institute of Technology in Pasadena and his colleagues announced the discovery of Sedna on March 15 (SN: 3/20/04,p. 179), they were convinced that the so-called planetoid, some 13 billion kilometers from the sun, must have an unseen moon. The researchers based their assumption on observations suggesting that Sedna rotates unusually slowly--just once every 20 days. Most bodies in the solar system rotate every 10 hours or so.

The leisurely rotation can best be accounted for by the gravitational tug of a companion, Brown says. "There is no other good scientific explanation for why something would rotate as slowly as 20 day's" he argued during a press briefing on April 14.

However, he announced that 35 images that his team took with Hubble's Advanced Camera for Surveys show no evidence of a moon.

It's possible, although not likely, that a moon might have been hiding behind Sedna or was passing directly in front of it, so that it wouldn't have been apparent during the Hubble observations, says study collaborator David Rabinowitz of Yale University in New Haven, Conn. Alternatively, a moon might be too faint for Hubble to have detected it.

Although the Hubble images were taken in blue light to record the sharpest possible pictures, Sedna still appears blurred. But the new images do reveal that Sedna's diameter is no bigger than three-quarters of that of Pluto. Brown says that now he'd like to schedule Hubble to take images at redder wavelengths, where a moon might appear brighter.

Scott Tremaine of Princeton University says that one of the most likely explanations is that a moon that's since vanished slowed Sedna. Another planetoid passing close by might have yanked the moon from Sedna's gravitational grasp. Planetary scientist Alan W. Harris of the Space Science Institute in La Canada, Calif., had previously suggested a similar scenario for slowly rotating members of the asteroid belt, a reservoir of rocky bodies between the orbits of Mars and Jupiter.

Another possibility is that Sedna's moon was smashed to bits by a collision with another object at the solar system's fringes, Brown adds.

A more mundane explanation for the lack of a moon is that Sedna rotates more quickly than astronomers have considered. The team observed Sedna about once every 24 hours, so a rotation of slightly more than once around in that time would appear the same as the small amount of rotation reported. However Rabinowitz says, "It would require the unusual coincidence that Sedna's rotation period would have to be unusually close to the Earth's rotation period."

Sedna is now too close to the sun for a good view from Earth, but next fall Brown's team plans to monitor the body for a full night to pin down its rotation speed.