"We are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system," Brown said in a media statement.
Marsden, of the Minor Planet Center, said there "isn't much doubt" that 2003 UB313 is bigger than Pluto. However, he rejects the idea that the newly discovered object deserves planet status.
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"If anything, it suggests we really seriously should go back to eight [planets in the solar system]the traditional 19th-century ones," Marsden said.
Pluto was discovered in 1930. At the time, astronomers believed Pluto's mass was equal to Earth's. Since then refinements in measurement techniques have revealed the planet's relatively diminutive size.
Marsden said he proposed in 1999 to include Pluto in the catalog of what is now almost a hundred thousand sequentially numbered small bodies. The move would have effectively stripped Pluto of its status as a planet, at least in some astronomical circles.
The proposal was dropped, however, after objections from a minority of astronomers who wanted to preserve Pluto's historical significance.
Now that an object bigger than Pluto has been discovered, Marsden said, this is an ideal time to rectify matters.
If 2003 UB313 is to be called a planet, then a couple of dozen other Kuiper belt objects should be too, as well as the asteroid Ceres and perhaps a dozen other objects in the asteroid belt, Marsden added.
Discovery and Announcement
Brown, Trujillo, and Rabinowitz first observed 2003 UB313 on October 21, 2003, using the Samuel Oschin Telescope at Caltech's Palomar Observatory in San Diego County, California.
At the time, the object was so far away that its motion went undetected. It wasn't until January of this year that the object's orbital path was uncovered.
Since then the team has been studying UB313 with other telescopes. They planned to announce the discovery in October with more detailed information on its size and composition.
The team was also planning to announce the discovery of another large Kuiper belt object, 2005 FY9, in October.
The announcement of both objects was suddenly moved up to July 29 after an unexpected announcement from another group of astronomers, Marsden said. On July 28 a team led by Jose-Luis Ortiz at the Instituto de Astrofísica de Andalucía in Granada, Spain, had announced to the Minor Planet Center the discovery of 2003 EL61, another planet-like object at the edge of the solar system.
Brown's team also had independently observed EL61 and had posted on a Web site an abstract of a paper on the object on July 19. They plan to present their EL61 paper at an astronomy conference this September in Cambridge, England.
The U.S. astronomers, though, had not reported their EL61 discovery to the Minor Planet Center, which is the official governing body for the recognition of such discoveries.
The Minor Planet Center's Marsden said the timing of the Spanish team's discovery "struck me as a little odd," given that Brown's paper on EL61 had been posted online just ten days earlier. In e-mail discussions with Brown about the issue, the Caltech astronomer told Marsden about the two other objects and asked for advice.
"While he didn't care if the other object might [be] scooped, he did care about the big one [2003 UB313]," Marsden said.
Before Marsden replied to Brown, he learned from a colleague that someone had already calculated the orbits of 2003 UB313 and 2005 FY9. This other party had based his or her calculations on Brown's observation data from a Chilean telescope, which is readily available on the Web.
Marsden shared the information with Brown and urged him to make the announcement July 29.
"We had to assume that the [stranger's use of Brown's data] was malicious and that the person was going to use the information to attempt to claim he had discovered the objects himself. Thus we had to announce late on a Friday afternoon with no preparation," Brown wrote in an e-mail interview.
Marsden does not suspect that Ortiz's team is responsible for the suspected foul play. He also noted that Internet insecurity has made it tougher for scientists to check and double-check their findings before going public.
"It's a little startling to realize that the records of telescope pointings are available on the Internet. There's so much stuff out there that it's hard to keep something you found secret while you really study it and then prepare a proper publication about it," Marsden said.
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