When an object is 16 billion kilometres away but probably only half the width of the United States, how do you work out precisely how big it is? That was the challenge faced by teams estimating the size of UB313, the potential ‘tenth planet’ in our Solar System, which was first spotted in 2003.

So far two teams have tried, and come up with different answers. The first group, led by Frank Bertoldi, an astronomer at the University of Bonn, Germany, measured how much heat UB313 emits, and compared it with the body's predicted temperature, based on its distance from the Sun. The team calculated that UB313 has a diameter of around 3,000 kilometres (F. Bertoldi et al. Nature 439, 563–564; 2006).

Now a team led by Michael Brown, the planetary scientist at the California Institute of Technology whose group discovered UB313, has made its own estimate using the Hubble Space Telescope. The researchers took 28 images of UB313 and inferred its size using its distance from Earth and the width of the object in the image — just 1.5 pixels across. They reckon it is around 2,400 kilometres in diameter, closer to the size of Pluto (see graphic).

That would make UB313 unusually bright for its size. Brown's team suggested on 11 April that the body reflects 86% of the light that falls on it. The only object more reflective in the Solar System is Enceladus, a saturnian moon that gets its shiny surface from geysers, which spew water into the object's freezing atmosphere, creating ice on the surface.

Analysis of light reflected by UB313 shows that its high reflectivity is probably due to a coating of frozen methane. But that prompts questions about how the methane got there, and why cosmic rays, which split hydrogen atoms from methane molecules, have not added a layer of soot to the surface. This process is thought to be responsible for red patches on Pluto's surface, which reflects a maximum of 60% of the light that falls on it.

“Why does it have higher ice coverage than Pluto? We don't understand it well,” says Bertoldi. The body may be leaking methane from its warmer interior. Or it could be because UB313 used to be much closer to the Sun, so it may have had a methane atmosphere that froze to its surface as it moved farther out. The same fate probably awaits Pluto as its elliptical orbit takes it away from the Sun.