The discovery of what might be our solar system's 10th planet could change the way astronomers label the objects they find in the skies.
The key question, still unresolved: Just what is a planet?
Until the discovery of 2003 UB313 -- they'll choose a catchier name soon -- astronomers had loosely defined what objects they classified as planets, based on factors such as whether they (a) orbit a star, (b) are shaped into spheres by gravity and (c) are at least as big as Pluto, the smallest and most distant planet in our solar system.
But that may change, thanks to a team from Caltech, Yale and the Gemini Observatory in Hawaii, who have spent three years looking for planetary-sized objects using a new generation of robotic telescopes and sophisticated cameras.
The heavenly body they found is bigger than Pluto, is farther from the sun, revolves on an ecliptic orbit far different from that of any other planet, and is the largest object found in the solar system since the discovery of Neptune in 1846.
Many astronomers believe that the same region of the solar system -- a vast, icy area where some comets originate and which is known as the Kuiper Belt -- probably holds similar objects waiting to be discovered. And experts wonder whether they should all be labeled as planets.
"I think there's probably a thousand Pluto-sized planets out there," said Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colo.
A decision by the International Astronomical Union on granting the Kuiper Belt object planetary status could take months or years. "We're all going to have to argue about that," said Chad Trujillo, a Gemini astronomer and co-discoverer of UB313.
Researchers announced their findings at a hastily arranged news conference a week ago, after they began to worry that posting the specific locations of their surveys on the Web -- a standard practice -- could allow competitors to replicate their work and "scoop" them.
The team actually spotted UB313 and two other objects smaller than Pluto as early as 2003, but didn't realize they had such a large object until they took a closer look earlier this year. They had planned to break the news at the American Astronomical Society conference in Cambridge, England, next month and publish additional details in a scientific journal later this year.
But given the interest in a new planet, they realized they couldn't keep mum any longer. A Spanish team was already finding objects in the same area of space.
"We had been studying three sites with different telescopes, and we realized that if someone found what we saw on this one, we'd lose out on years of work," Trujillo said.
Trujillo and David Rabinowitz, the co-discoverer at Yale, wouldn't say what name they are proposing to the IAU, the group responsible for naming and classifying heavenly objects.
Informally, they named the object Xena, after the warrior princess of television fame. But Trujillo said that's no indication of the group's real preference. (The designation 2003 UB313 is based on IAU rules saying astronomers should initially identify space objects by the year of the discovery, followed by a code used to identify the month of the discovery and the order in that year in which the discovery was reported.)
Because it's bigger than Pluto, UB313 should be classified as a planet or Pluto should be demoted from planetary status, Trujillo argues. But he doubts that will happen -- in part because it would mean rewriting what's been taught in schools since Pluto's discovery in 1930.
"My feeling is this new one won't be called a planet, but Pluto will retain its status," he said. Which still leaves astronomers arguing over just what makes a planet a planet.
Stern serves on an International Astronomical Union committee formed to come up with specific criteria and expects a proposal in the next few months. "I think this discovery's energized us," he said.
Pluto's status debated
The discovery also is rekindling a debate over Pluto's planetary status that began at least a decade ago and reached a crescendo as astronomers discovered similar objects in the Kuiper Belt. Some argue that Pluto's small size (just 1,500 miles in diameter) and its methane ice surface make it more like the objects rotating in the Kuiper Belt than the solar system's outer gas giant planets, such as Jupiter and Neptune.
When New York's famed Hayden Planetarium reopened in 2000, scientists there put only eight planets on display -- classifying Pluto instead as a Kuiper Belt object. News of that decision prompted an outcry from Plutonian enthusiasts.
"People even said it was un-American because Pluto was the only planet discovered by an American," said Ben Oppenheimer, an astrophysicist at the American Museum of Natural History, which runs the Hayden. "I didn't see it as a demotion, I just saw it as a better way to classify it."