'Tenth Planet' found to be a whopper

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Published online: 1 February 2006; | 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.

Mark Peplow

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.

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.

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.

Imagine how you'd feel if your baby didn't have a name for seven months.  Mike Brown, Caltech

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 |

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[HamiltonJay]

'Tenth Planet' found to be a whopper....


Its got a malted milk center?

[SunkenCiv]

No known moons, but I'd be a little surprised if none are ever found. :')

[Malsua]

2003 UB313 and its moon - currently nicknamed Xena and Gabrielle

Great, now we've got a lesbian planet.

[Onelifetogive]

Let's call it Reagan.

The moon would have to be called "Nancy."

[Young Werther]

I suggest that we name it "Werther" which is a more worthy name.

Wink! Wink!

[beezdotcom]

"What would make it'frosting on the cake' would be to find we are living in a binary star system with a brown dwarf secondary star orbiting our primary star."

Ahh...the 'Nemesis Hypothesis'

[beezdotcom]

Great, now we've got a lesbian planet.

Maybe it looks like this....

[Eaker]

I sure am glad that you asked Dasher that question instead of me!

[Squantos]

I was afraid I'd misspell planet....

[Robert A Cook PE]

See, finding any sort of "moon" around Ceres would call for a wholesale "renaming" of several planet-like bodies - which allows Pluto to be kept, add Ceres, add Xena ... etc.

This would keep more people happy, since 9 planets into 11 planets doesn't break any "rules" ....

[SunkenCiv]

Other asteroids are already known to have moons, but the asteroids are not generally spherical, they're just irregular chunks of rock. Having a moon would not help. Mercury and Venus are moonless, and until 1978 (I think that's the year) Pluto was believed to be moonless.

[Dashing Dasher]

Uranus is bigger than Pluto's.


Butt, Richard Simmon's is the biggest.

[edcoil]

You want fries with that?

[Paleo Conservative]

Super sized!

[devane617]

I want to name it "GSpot".

[ChadGore]

Re: 'Tenth Planet' found to be a whopper

The Burger King people must be proud.

[COBOL2Java]

Ah, that Burger King Whopper's a piker! Check out this baby!

[patricktschetter]

Heh. Even that pales in comparison to this burger 20 miles from where I now live: ...The record for the largest hamburger is held by Rutland, North Dakota. In 1982 the town made the World's Largest Hamburger, 3,591 pounds, which was consumed by some 8,000 people.

[Darksheare]

Welcome.

[Irish_Thatcherite]

Yeah, What would make it'frosting on the cake' would be to find we are living in a binary star system with a brown dwarf secondary star orbiting our primary star..

Isn't there theory about a companion star in the Oort Cloud?