By Dr David Whitehouse
BBC News Online science editor
Sedna, the Solar System's farthest known object, does not have a moon, puzzled astronomers have revealed.
Artist's impression of noontime on Sedna
Its slow spin was thought to be due to the gravity of a small, companion body.
Researchers have now discounted this but say the unexpected finding may offer clues to the origin and evolution of objects on the Solar System's edge.
Sedna's discovery announced on 15 March led to huge excitement and an argument among scientists over whether the small world could be classified as a planet.
Co-discoverer Mike Brown, of the California Institute of Technology (Caltech), US, was so convinced at first that Sedna had a satellite that the artist's concept of the object commissioned to accompany the announcement included a hypothetical moon.
Brown's belief was based on the fact that Sedna appears to have a very slow rotation, which could best be explained by the gravitational pull of a companion body.
Almost all other small, solitary objects in the Solar System - such as asteroids and comets - complete a spin in a matter of hours.
The revelation that Sedna has no satellite in orbit has now raised new questions for the Caltech researcher.
"I'm completely baffled at the absence of a moon," he said. "This is outside the realm of expectation and makes Sedna even more interesting. But I simply don't know what it means."
Immediately following the announcement of the discovery of Sedna, astronomers turned the Hubble Space Telescope (HST) towards the new planetoid to search for the expected companion moon.
The space telescope was required because the turbulence of the Earth's atmosphere makes Sedna's image unstable to ground-based telescopes.
The Hubble picture, taken on 16 March with its Advanced Camera for Surveys, shows only the single object, Sedna, along with a faint, very distant background star in the same field of view.
Hubble gets a sharper view than ground-based telescopes
"Despite HST's crisp view (equivalent to trying to see a football 1,500km away), it still cannot resolve the disc of mysterious Sedna," Brown said.
But Hubble's view does place an upper limit on Sedna's size of approximately three-quarters the diameter of Pluto - about 1,600km.
Brown had expected the predicted satellite to be visible as a small dot in Hubble's image.
The object is not there - although there is a very small chance it was behind Sedna or transiting in front of it, so it could not be seen separately.
The rotation of Sedna had been calculated from earlier observations of periodic changes in light reflecting from the object.
The light curve gives a long rotation period exceeding 20 days (but not greater than 50 days).
If true, this would make Sedna the slowest-rotating object in the Solar System after Mercury and Venus, whose slow rotation rates are due to the tidal influence of the Sun.
One easy way out of this dilemma is the possibility that the rotation period is not as slow as the astronomers think.
But Brown's team is convinced the calculations are correct. He said: "I'm completely lost for an explanation as to why the object rotates so slowly."
Pluto's rotation has been slowed to a relatively leisurely six-day period because it is tidally locked to the revolution period of its satellite, Charon.