- The mystery surrounding Sedna - the most distant object
ever seen in the Solar System - deepened as astronomers calculated that
the planetoid's "missing" moon must belong to an entirely new
class of celestial object, and is possibly the darkest body in the Solar
System.
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- When Sedna was spotted in November 2003 it was the largest
object found since the discovery of Pluto. It has puzzled astronomers because
it rotates just once every 20 days. Slow rotation usually indicates the
presence of a moon, which would put a brake on the planetoid's rotation
by exerting tidal forces on it.
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- But no companion has been found, despite searches with
NASA's Hubble and Spitzer space telescopes. Astronomers have speculated
that Sedna once had a moon that slowed its rotation, but a collision with
a chunk of interplanetary ice knocked it out of orbit.
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- Now a UK team has shown that if Sedna has a moon, it
must be more like a gigantic, extinct comet than a planetary body.
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- Assuming the moon exists, Chandra Wickramasinghe of Cardiff
University in the UK and colleagues calculated its maximum possible reflectivity
on the basis that the space telescopes have been unable to see it. "If
there is a moon, it has to be the darkest object in the solar system,"
Wickramasinghe says.
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- "Lobster pots"
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- The only surface that even approaches this blackness
is thought to build up on a comet as its reflective ices evaporate into
space, leaving a residue of tarry carbon substances.
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- The key to their darkness is that these surfaces are
not solid but are more like frozen smoke, with vacuum comprising more than
85 per cent of their volume. This makes them "lobster pots" for
light, trapping more than 99 per cent of all photons that fall into the
surface layer.
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- But to slow Sedna down, the moon would have to be up
to a hundred times larger than any known comet, putting it in a class of
its own. "If it's real, Sedna's moon indicates that there must be
an unknown population of dark solar system objects out there," says
Wickramasinghe, whose work is published in The Observatory this August.
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- Rotation rate
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- Sedna's discoverer, Michael Brown of the California Institute
of Technology in Pasadena, is sceptical. "I'm very wary of these analyses,"
he says. "Theorists take numbers and interpret them. That doesn't
mean it is wrong, however."
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- He points out that if the observations using a 1.3-metre
telescope at Cerro Tololo in Chile, were taken at intervals close to Sedna's
true rotation rate, it could fool everyone into thinking Sedna was rotating
much more slowly. It would be like looking at a clock every 25 hours and
concluding that the hour hand had only moved between adjacent numbers.
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- Brown promises to resolve the mystery soon. "We
are going back to the Hubble Space Telescope and have optimised our observation
plan to look for a moon. If it's there, we should see it this time."
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