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 suggested 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.
The fact that Xena is smaller than initially 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.
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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