Tenth planet as bright as fresh snow
21:12 11 April 2006
NewScientist.com news service
Maggie McKee
The "tenth planet" is only slightly larger than Pluto, new Hubble
Space Telescope images prove. It had been thought the planet, dubbed
Xena, might be 25% to 30% larger.
Intriguingly, the downsizing means the distant world must be brighter
than almost any other object in the solar system, suggesting it is
constantly being resurfaced.
Xena, officially called 2003 UB313, was first announced in July 2005
and lies about three times as far from the Sun as Pluto. Its
brightness indicated it was larger than Pluto, but it was not clear by
how much because astronomers did not know how much sunlight its
surface reflected.
Xena’s discoverers, led by Mike Brown at the California Institute of
Technology in Pasadena, US, first thought its surface might be similar
to that of Pluto, which reflects about 60% of sunlight. If so, Xena
would be 25% larger than Pluto, an estimate borne out by recent
ground-based observations of its heat, which suggested it was 30%
larger.
But these results had a large margin of error – 400 kilometres. Now,
Brown and colleagues have analysed Hubble images taken in December
2005 to reveal Xena's size to the nearest 100 kilometres. Hubble was
able to resolve Xena at visible wavelengths, showing its diameter is
2400 kilometres - about 5% larger than Pluto’s 2290 kilometres.
Frozen methane
The fact that Xena is smaller than thought means it must reflect about
86% of the light that falls on it - making it about as bright as fresh
snow and brighter than every other solar system body except Saturn's
moon Enceladus. Spectral observations suggest its surface is covered
with frozen methane, like Pluto.
But unlike Pluto, which is mottled by both bright and dark splotches,
Xena is so uniformly bright that it is impossible to tell how fast it
is rotating. "When we made the size measurement, we were thoroughly
shocked," Brown told New Scientist. "Such a high albedo is simply
unprecedented other than the very odd Enceladus."
Richard Binzel, a planetary scientist at the Massachusetts Institute
of Technology in Cambridge, US, agrees. "Space is a dirty place," he
says, explaining that particles from the solar wind alter the
structure of ices, darkening them over time. "It's very hard to keep a
surface bright and white - it requires some process to keep the
surface fresh."
Brown suggests two possible mechanisms, working together, for the
resurfacing. Xena moves in a very elongated orbit that stretches from
38 to 97 astronomical units (1 AU is the distance between the Sun and
Earth). When it is near the Sun in its 560-year orbit, it may have a
gaseous atmosphere. But when it moves away, it receives so little
sunlight that any atmosphere would freeze onto Xena's surface, leaving
it fresh and white.
But a similar freeze-thaw cycle occurs on Pluto, which moves from 30
to 50 AU over about 250 years, says Binzel. And it does not have a
blindingly bright surface.
"So it may also be that fresh methane is leaking out of the surface,"
suggests Brown. Ice particles and water vapour spew from geysers on
the snow-white Enceladus, but Brown says such violent jets are not
necessary on Xena. "Instead it would be more like a picture of a steam
vent in Antarctica, where the steam instantly freezes onto the
surface," he says.
Gravitationally stretched
But he and other astronomers had thought that Xena's interior was made
of rock and ice. For gaseous methane to survive within the planet,
"you have to have an energy source", says Binzel. Brown agrees: "The
real question is: Why would methane leak out of the surface?"
Some objects are heated when they are gravitationally stretched and
compressed by massive objects nearby. But though Xena has a moon that
might be a tenth its size, it is too small to gravitationally deform
and heat Xena, says Brown.
Similarly, the decay of radioactive isotopes could not provide the
necessary heat, says Binzel: "It's a wonderful mystery."
The Hubble observations will be published in a future issue of the
Astrophysical Journal.
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