HUBBLE images of the most distant object ever seen in the solar system have left astronomers puzzled. The planetoid Sedna appears not to have a moon, which would have accounted for its slow rotation.
A team led by Mike Brown of the California Institute of Technology discovered Sedna in November 2003. The planetoid may be as much as 1600 kilometres across, not far short of Pluto鈥檚 2300-kilometre diameter. Its 10,000-year orbit carries it out 900 times as far as Earth is from the sun, putting it in the inner reaches of the Oort cloud of icy bodies that surrounds the solar system.
Astronomers noticed slow changes in the amount of sunlight reflected from Sedna鈥檚 mottled surface, which suggest that it rotates just once every 20 days or so. But most lone objects in the solar system spin much faster.
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Sedna鈥檚 slow rotation could easily be explained if it had a moon, which would brake its rotation by exerting tidal forces, but when Brown鈥檚 team observed Sedna on 16 March there was no sign of one. 鈥淚鈥檓 completely baffled at the absence of a moon,鈥� he says.
There鈥檚 a small chance that the moon was hiding behind Sedna; or its face may be very dark. Or perhaps Sedna once had a moon, but it collided with a chunk of interplanetary ice that knocked it out of its orbit.
Sedna lies behind the sun at the moment, but astronomers will continue their search for a moon when it emerges from the sun鈥檚 glare in around six months.