Published online 1 February 2006 | Nature | doi:10.1038/news060130-7


'Tenth Planet' found to be a whopper

Large size of 2003 UB313 fuels debate over what is and isn't a planet.

2003 UB313 and its moon - currently nicknamed Xena and Gabrielle - take time off from their sword and sorcery shenanigans.2003 UB313 and its moon - currently nicknamed Xena and Gabrielle - take time off from their sword and sorcery shenanigans.© W. M. Keck Observatory

The recently discovered 'tenth planet' of our Solar System is substantially larger than Pluto, astronomers have found.

For many, the discovery that object 2003 UB313 is about 3,000 kilometres across will remove any doubt that it deserves to be called a planet.

"Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," says Frank Bertoldi, an astronomer at the Max Planck Institute for Radioastronomy in Bonn, Germany, and part of the team that reveals UB313's size in this week's Nature1.

When astronomer Mike Brown of the California Institute of Technology in Pasadena unveiled 2003 UB313 to the world in July 2005, his team was already confident that the new object was at least as large as Pluto, and deserved the status of 'planet'.

But UB313's elongated orbit takes it almost twice as far away from the Sun as Pluto ever gets, making it very difficult to measure its diameter precisely. One clue to its larger size came from the fact that it is slightly brighter than Pluto; a larger mirror would reflect more of the Sun's light. But an alternative explanation could have been that UB313 is simply made of a more reflective material than Pluto.

Ice maiden

How does the 'tenth planet' measure up against other bits of the Solar System? Click here to find out.How does the 'tenth planet' measure up against other bits of the Solar System? Click here to find out.

Using the Institute for Millimetre Radio Astronomy (IRAM) 30-metre telescope in Spain, Bertoldi's team has now studied the radiowaves coming from UB313, which reveal how much of the Sun's rays are absorbed and re-radiated as heat. Because very little reflected sunlight is emitted at these wavelengths, the object's brightness in radiowaves depends only on its size and surface temperature.

Based on its enormous distance from the Sun, UB313 is calculated to be tremendously cold: a staggering -248 °C. Bertoldi and his colleagues combined this value with their measurements of UB313's radiation to determine its reflectivity and size.

Although this first estimate of 3,000 kilometres may be out by as much as 400 kilometres, this still puts UB313 well ahead of 2,300-kilometre-wide Pluto in the size stakes, making it the largest body found in the Solar System since the discovery of Neptune in 1846.

“Imagine how you'd feel if your baby didn't have a name for seven months.”

Mike Brown,

The research also shows that UB313 has a reflectivity, or albedo, of about 60%. This is roughly the same as Pluto's, suggesting that the two objects' surfaces are made of very similar materials, such as frozen methane and nitrogen snow. Only a very frosty world could produce an albedo of 60%, says Brown.

Brown has also been trying to measure the size of UB313 by using the Hubble Space Telescope. Although he released preliminary findings on 25 January at a public meeting at Foothill College in Los Altos Hill, California, suggesting that UB313 was just a few percent larger than Pluto, he now says that measurement is wrong. "It was an extremely preliminary estimate," he explains.

A planet with no name

2003 UB313 is not the catchiest name, but unfortunately this temporary designation will have to stick until the International Astronomical Union (IAU) decides whether it is indeed a planet that warrants a name from classical mythology.

Since 1992, more than 1,000 similar, albeit smaller, objects have been found in the region around Pluto known as the Kuiper Belt, and astronomers estimate that there may be more than half a million still waiting to be discovered. As more of these icy remnants from the Solar System's birth turn up, Pluto blends into the crowd and its claim to be a unique planet grows slimmer and slimmer.


Some astronomers argue that Pluto should be stripped of its title, to become a Kuiper Belt Object like its orbital fellows. Others suggest that anything larger than Pluto found in the outskirts of the Solar System should also be called a 'planet', which would include UB313. "I'd prefer to keep Pluto as a planet, for historical reasons," says Bertoldi.

The IAU set up a committee of 19 top astronomers to come up with a workable definition for a planet that would rule UB313 in or out, but in November 2005 the group finally admitted defeat after failing to reach a clear consensus. The IAU has promised action later this year, but Brown is already impatient. "Imagine how you'd feel if your baby didn't have a name for seven months," he says.

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  • References

    1. Bertoldi F., Altenhoff W., Weiss A., Menten K. M.& Thum C. . Nature, 439 . 563 - 564 (2006). | Article | PubMed | ISI | ChemPort |