New Hubble images of the most distant object ever spotted in the Solar System are perplexing astronomers. They show that the planetoid Sedna does not seem to have a moon as they expected.

"I'm completely baffled at the absence of a moon," says Mike Brown, a planetary scientist at the California Institute of Technology in Pasadena. "This is outside the realm of expectation and makes Sedna even more interesting."

Brown and his colleagues discovered Sedna in November 2003, using a telescope at Caltech's Palomar Observatory near San Diego, and revealed the find in March. Follow-up observations showed that the reddish coloured planetoid takes about 10,500 years to orbit, drifting up to 900 times farther from the Sun than the Earth.

Astronomers also noticed subtle changes in the amount of sunlight reflecting from Sedna's mottled surface, and the pattern of these changes suggest Sedna is rotating very slowly, completing a revolution just once every 20 days or so. Most lone objects in the Solar System rotate much faster than that.

Sedna's slow rotation could easily be explained if it has a moonlet, which would brake the planetoid's rotation by exerting tidal forces on it. To see if they could spot a companion moon, Brown's team observed Sedna using the Hubble Space Telescope on 16 March.

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