Bad News for Pluto as "10th Planet" Is Sized Up

For more than seven decades, schoolchildren have been taught that tiny, lonely Pluto is the outermost of the nine planets that orbit the Sun.

Now, thanks to German astronomers, a big hole has now been bashed into that cherished piece of learning. A team led by Frank Bertoldi of the University of Bonn in Germany has determined that Pluto is much smaller than an enigmatic object, 2003 UB313, whose discoverers claim is the solar system's 10th planet.

UB313, found some 15 billion kilometers (nine billion miles) from Earth, ignited a huge row after its finding was announced last July 30 by an American team. Pluto's defenders blasted UB313, saying it was not a planet... but a vulgar rock.

The polite term for such abuse is Kuiper Belt Object (KBO), which is used to describe the estimated 100,000 pieces of icy, primeval debris that slowly encircle the Sun on the outskirts of the solar system, far beyond the orbit of Neptune.

Astronomy geeks fighting for prestige

The UB313 supporters' club responded tartly, claiming that if anything deserved the moniker of KBO, it was Pluto. For one thing, Pluto, discovered in 1930 by American Clyde Tombaugh, has a weird, unplanetary orbital plane. It is 17 degrees off the horizontal plane taken by the eight other planets.

In addition, its path around the Sun is so egg-shaped that, for 20 years of its 248-year orbit, it is inside the track of Neptune itself.

Now, weighing powerfully in this group's favor are the first detailed measurements of UB313's size. If the data are accepted by a special 19-member panel set up by the International Astronomical Union (IAU) to determine what constitutes a planet, Pluto faces being booted out of the solar system's elite club and downgraded to a mere KBO.

Either that, or the planetary list will have to expand to include UB313 and possibly many others.

Size does matter, after all

In a study published on Thursday in Nature, astronomers led by Frank Bertoldi of Germany's University of Bonn say they have measured reflected solar radiation from UB313, using a 30-metre (100-feet) telescope in Spain, to get a yardstick of its size.

The result: UB313 has a diameter of about 3,000 kilometers (1,900 miles), which would make it the largest Solar System object to be spotted since the discovery of Neptune in 1846.

And in a presentation reported last Friday by ScienceNow, a website run by the American Association for the Advancement of Science (AASA), Michael Brown, a California Institute of Technology (Caltech) astronomer, estimated that UB313 is roughly one percent larger than Pluto. Brown, who led the team that discovered UB313, bases this on an image taken last December by NASA's orbiting Hubble Space Telescope.

Exciting times

For Bertoldi, time is running out for Pluto as the solar system's outermost 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," said Bertoldi.

Pluto's humiliation is knowledge's gain, though.

"The discovery of a solar system object larger than Pluto is very exciting," said Wilhelm Altenhoff of the German Max Planck Institute for Radio Astronomy, a veteran watcher of asteroids and comets.

"It tells us that Pluto, which should properly also be counted to the Kuiper Belt, is not such an unusual object. Maybe we can find even other small planets out there, which could teach us more about how the solar system formed and evolved," Altenhoff said.

Bob's your uncle?

UB313 has yet to be given a name.

Brown, a fan of TV's own Warrior Princess, informally calls it Xena, but if the object is confirmed as a planet, he will be under pressure to name it, like the nine others, after a figure from Greek or Roman mythology.

In a naming competition run by the British magazine New Scientist, readers' suggestions included Persephone, Pax, Galileo and Cerberus -- as well as Rupert and Bob.