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New York Times

29 July 2005


Published: July 29, 2005
Add a 10th planet to the solar system - or possibly subtract one.

Astronomers announced today that they had found a lump of rock and ice that is larger than Pluto and the farthest known object in the solar system. The discovery will likely rekindle debate over the definition of "planet" and whether Pluto should still be regarded as one.

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Forum: Space and the Cosmos
The new object - as yet unnamed, but temporarily designated as 2003 UB313 - is currently 9 billion miles away from the Sun, or 97 times as far away as the Earth and about three times Pluto's current distance from the Sun. But its 560-year elliptical orbit also brings it as close as 3.3 billion miles. Pluto's orbit ranges between 2.7 billion and 4.6 billion miles.

The astronomers do not have an exact size for the new planet, but its brightness and distance tell them that it is at least as large as Pluto.

"It is guaranteed bigger than Pluto," said Michael E. Brown, a professor of planetary astronomy at Caltech and a member of the team that made the discovery. "Even if it were 100 percent reflective, it would be larger than Pluto. It can't be more than 100 percent reflective."

The discovery was made Jan. 8 using a 48-inch telescope at Palomar Observatory in California. Dr. Brown and the other members of the team - Chadwick A. Trujillo of the Gemini Observatory in Hawaii and David L. Rabinowitz of Yale University - then found that they had, unknowingly, taken images of the planet taken as far back as 2003.

Last year, the same team announced the discovery of a distant body they named Sedna, which, until the latest discovery, had held the title of farthest known object in the solar system.

Dr. Brown said they had a name they had in mind for the planet, but did not want to disclose it publicly until it had been formally proposed to the International Astronomical Union. "We have a name we really like, and we want it to stick," he said.

Informally, the astronomers have been calling it "Xena" after the television series about a Greek warrior princess, which was popular when the astronomers began their systematic sweep of the sky in 2000. "Because we always wanted to name something Xena," Dr. Brown said.

The astronomers were not able to see 2003 UB313 using NASA's Spitzer Space Telescope, which looks at longer-wavelength infrared light. That means the planet is less than 1,800 miles in diameter.

What is most surprising is that the orbit of the planet is sharply skewed to most of the rest of the solar system. The orbits of the most of the planets lie close to the same plane as the Earth's, known as the ecliptic plane. The orbit of 2003 UB313 is tilted by 44 degrees.

"That blows my mind," said Harold Levison of the Southwest Research Institute in Boulder, Colo., who was not involved in the discovery. "Getting something up that high is very hard."

The object is also the third brightest in the Kuiper Belt, a ring of icy bodies that circles beyond the orbit of Neptune. The new planet could have been easily discovered much sooner if anyone had looked at that part of the sky.

"It's because no one looks that far off the ecliptic," Dr. Brown said. "No one expects to have an inclination that high."

Another group of astronomers led by Jose-Luis Ortiz at the Sierra Nevada Observatory in Spain announced that they had found a large Kuiper Belt object, designated 2003 EL61, that they thought could be Pluto-size or larger. Dr. Brown's group had been observing the same body but had not announced it, and their observations had already pinpointed a moon circling 2003 EL61, which constrained the size of the body to about 70 percent the diameter of Pluto.

On his Web site, Dr. Brown wrote that the Spanish group deserved credit, saying they had gambled that no one else would find the planet. "We were wrong!" he said.

Dr. Brown had still hoped to hold back announcements of 2003 UB313 and another large Kuiper Belt object, 2005 FY9, until October, but his hand was tipped by Brian Marsden, director of the Minor Planet Center, who said that he was worried about hanky panky.

Dr. Marsden said that it was possible by looking on the Internet at the logs of one of the telescopes Dr. Brown's team had been using to find out where they had been pointed. He had evidence, he said, that someone had done that and computed crude orbits of the two unannounced planetoids, "presumably" in preparation for their own observations.

"I was shocked to find this kind of information was available on the Web," Dr. Marsden said. He urged Dr. Brown to announce his findings. "I was suspicious and I warned him," he said. "We try to give credit where credit is due. Brown's team deserves a lot of credit for carrying out this program."

Astronomers suddenly have three large bodies at its outskirts, and with one of them larger than Pluto, the debate over what is a planet will likely revive.

Astronomers do not have a formal definition for planet, and many have said that if Pluto had been discovered today, it would not have been called a planet.

The first of the smaller Kuiper Belt objects were discovered in 1992, more than half a century after Clyde Tombaugh found Pluto.

The Minor Planet Center proposed in 1999 that Pluto, while maintaining its position among the major planets, also be given an official designation among the Kuiper Belt objects. The center dropped the proposal after outcry from those who saw "dual status" as a demotion.

Mr. Williams said he still supported bestowing dual status to Pluto. But thought that 2003 UB313 should not be added to the registry of major planets. "Leave it as a minor planet permanently," he said.

Mark V. Sykes, one of the defenders of Pluto's planetary status, said his initial inclination would be name 2003 UB313 a planet, too. He wondered whether it had an atmosphere and what sort of geological processes generated its apparently bright surface. "The kinds of questions we would ask about this object would be planet-like questions," said Dr. Sykes, director of the Planetary Sciences Institute, a private research institution based in Tucson, Ariz.

Astronomers will also have to figure out how the body made it to its current skewed orbit. "It makes Pluto seem less weird," Dr. Sykes said.

Dr. Brown said he used to support Mr. Williams's view of Pluto as a minor planet, but "I've given up on that." After if Pluto, for historical reasons, has been grandfathered in as a planet, "This one, I would say, counts out as the 10th planet," Dr. Brown said.

Five and a half years ago, Dr. Brown bet an astronomer friend, Sabine Aireau, five bottles of good Champagne that he would find a Kuiper Belt object larger than Pluto by the end of last year. In December, having failed, he bought the five bottles of Champagne to send her. Then 2003 UB313 turned up on Jan. 8. "I lost the bet by eight days," Dr. Brown said, but "she graciously decided she would let that window slide and I would win the bet."

He added, "That means I get to drink 10 bottles of good Champagne, and I think I will."


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