'Tenth Planet' found to be a whopper

news@nature.com ^ | 1 February 2006 | Mark Peplow

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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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[SunkenCiv]

URL corrections:

http://news.nature.com/news/2006/060130/060130-7.html
http://www.nature.com/news/2006/060130/multimedia/060130-7_m1.html

[SunkenCiv]

Correction to the corrections:

http://www.nature.com/news/2006/060130/full/060130-7.html

[SunkenCiv]

Mike Brown (co-discoverer) suggested an already-used name from the asteroids (Persephone I think), so you're in good company. :') Ceres may eventually make it to the domain of the planets (maybe not soon, but it could happen) because it is big (until recent Kuiper Belt discoveries, Ceres had as much material as all other asteroids combined) and is basically spherical.

[SunkenCiv]

I feel kinda sorry for the astronomer who discovered what he thought was a planet in orbit around Barnard's Star. While he may have wound up with bupkis, there's some possibility that he'll be vindicated. Dunno if anyone has tried with the latest and greatest technology (other than the Hubble ST). Gliese 793 was found to have a similar wobble using the same equipment, so Barnard's Star's wobble is generally believed to have been a problem with the mechanism of the instrument itself, rather than an actual slight wobble of the star. It would be amusing if Gliese 793 and Barnard's star both wound up having planets detected around them. :')

Google

[ModelBreaker]

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.

It'll have to be named "Kwanza" after the loonies start whining that Neptune, Jupiter etc are all dead white guys.

[SunkenCiv]

"To Pluto And Far Beyond" By David H. Levy, Parade, January 15, 2006 -- We don't have a dictionary definition yet that includes all the contingencies. In the wake of the new discovery, however, the International Astronomical Union has set up a group to develop a workable definition of planet. For our part, in consultation with several experienced planetary astronomers, Parade offers this definition: A planet is a body large enough that, when it formed, it condensed under its own gravity to be shaped like a sphere. It orbits a star directly and is not a moon of another planet.

[MillerCreek]

A very great deal (is "out there...very large...lurking"), is my best guess.

[piasa]

Of course, Ceres was the goddess of motherly love and agriculture (or something), which kinda clashes with a cold, barren, icy rock floating around millions of miles away from the sun...

Are you sure there isn't a Helen Thomas in ancient mythology? She's old enough.

[Robert A Cook PE]

Let's see ......

We go from calling the sun Greek and Roman the god itself.

To naming the visible planets after other Greek and Roman gods.

To naming new planets and moons after classic Greek and Roman gods from mythology.

To naming mythological TV and movie characters after classic Greek and Roman gods.

To naming newly-created mythological TV and movie characters after made-up Greek and Roman characters named similar to the classic manner.

To naming newly-discovered planets and moons after mythological TV and movie characters named after made-up Greek and Roman characters named similar to the classic manner.

[Robert A Cook PE]

It'll have to be named "Kwanza" after the loonies start whining that Neptune, Jupiter etc are all dead white gods.

[Robert A Cook PE]

Any moon/dirt/rings around Ceres?

[Michael Goldsberry]

Rupert!

[Rebelbase]

Planet Brokeback.

[johnny7]

It could use some shrubbery.

[Allegra]

Finbar!

[theDentist]

How long does it take to orbit our Sun?

[Squantos]

Is this bigger than Uranus ?

[Allegra]

I wonder how one of the "Brokeback Mountain" guys would answer that question?

[Squantos]

Eaker will know he has a glass belly button !