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.
鈥淚鈥檓 completely baffled at the absence of a moon,鈥� says Mike Brown, a planetary scientist at the California Institute of Technology in Pasadena. 鈥淭his is outside the realm of expectation and makes Sedna even more interesting.鈥�
Brown and his colleagues discovered Sedna in November 2003, using a telescope at Caltech鈥檚 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.
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Astronomers also noticed subtle changes in the amount of sunlight reflecting from Sedna鈥檚 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鈥檚 slow rotation could easily be explained if it has a moonlet, which would brake the planetoid鈥檚 rotation by exerting tidal forces on it. To see if they could spot a companion moon, Brown鈥檚 team observed Sedna using the Hubble Space Telescope on 16 March.
Dark face
The Hubble observations show that the maximum diameter of Sedna is about 1600 kilometres, or about three-quarters the width of Pluto. But there is no sign of a moonlet, which might have showed up as a little dot nearby.
There is a very small chance that the moon was hiding behind Sedna or was simply too close to the planetoid to be visible, something that future observations can test.
Alternatively the moon鈥檚 face may just be very dark. And another possibility is that Sedna once had a moon that slowed its rotation, but the moon collided with a chunk of interplanetary ice that knocked it out of its orbit.
鈥淲e still very strongly believe that there is or was a satellite of Sedna, and we think it鈥檚 critically important to make more effort to find it,鈥� says Brown, who announced the Hubble findings on Wednesday.
The astronomers hope that NASA鈥檚 James Webb Space Telescope, due to be launched in 2011, will get a much clearer view of distant, frozen worlds like Sedna. There are probably tens or hundreds more like it yet to be discovered, and they could help clarify how the Solar System formed.