Meet 2003 UB313, our newest planet

Astronomers at California Institute of Technology have discovered what they believe is the 10th and most distant planet in our solar system, a ball of rock about twice as big as Pluto and about three times as far away from the sun.

The new object, temporarily called 2003 UB313, is currently at its farthest distance from the sun, about 97 times the distance between the sun and Earth.

It lies in the fringes of the Kuiper Belt, a conglomeration of asteroids, comets and other materials circling the sun well beyond the orbit of Pluto.

When it gets halfway through its orbit in 280 years, 2003 UB313 will be about 36 times the Earth-sun distance – or nearly as close as Neptune.

“If Pluto is a planet, then anything larger than Pluto is a planet, and this is definitely larger than Pluto,” said Caltech astronomer Michael A. Brown, who announced the discovery Friday.

Brown’s claim of planethood for 2003 UB313 is certain to be controversial. Astronomers have long debated whether Pluto is a planet because of its small size and odd orbit, although many scientists are content to retain its current designation.

The surface of the new object is very similar to that of Pluto, a mixture of about 70 percent rock and 30 percent water ice. It is very cold, probably about minus 400 degrees Fahrenheit.

“It’s not a very pleasant place to live,” Brown said.

Brown said 2003 UB313 probably has not been discovered before because its orbit lies at a 45-degree angle from the plane, known as the ecliptic, in which the nine known planets circle the sun.

“Nobody looks that high up in the sky,” he said. He only looked there, he said, because he could not find any more objects in the ecliptic.

Brown, along with his colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University, first saw the new planet Jan. 8 using the 48-inch Samuel Oschin Telescope at Palomar Observatory.

It had actually been photographed in 2003 – hence the name – but nobody realized what it was until its motion became apparent.

The planet 2003 UB313 has since been observed with a variety of other telescopes. Researchers have attempted to measure its heat output with the Spitzer Space Telescope, but the orbiting observatory could not find it, putting an upper limit on its size of twice Pluto’s diameter.

Brown said that he had not intended to announce the finding until he had pinned down the exact size, but someone “with more cleverness than scruples” hacked into his restricted Web site Thursday night and was planning to announce the discovery.

“We really didn’t have a choice,” he said.

He said he has selected a name for the new planet and submitted it to the International Astronomical Union, but he would not reveal it until that body makes a decision.

The new object is currently the third brightest body in the Kuiper Belt. The discovery of the second-brightest object was announced Thursday by a group headed by Jose-Luis Ortiz of the Sierra Nevada Observatory in Spain.

That object, called 2003 EL61, is about 60 percent the size of Pluto – about the same as another body, called Sedna, that Brown discovered last year.

Brown and Trujillo also discovered another large Kuiper object, called Quaoar, in 2002. Like Sedna, however, Quaoar is smaller than Pluto.

The object detected by the Spanish astronomers is about 52 times the Earth-sun distance at its farthest point from the sun. Its closest approach to the sun also lies near Neptune’s orbit.

It is different from the previously discovered Kuiper bodies in that it has a moon that circles it every 49 days in a highly elliptical orbit. That moon has about 1 percent of the mass of the parent body.

Brown had also been tracking 2003 EL61 and was surprised when the Spanish astronomers made their announcement Thursday.

He said he has also been tracking another object, about the same size and distance from the sun as 2003 EL61, but brighter still.

Brown said 2003 UB313 can easily be observed by amateurs with good telescopes. It is currently straight overhead in the early morning, but in six months it will be visible in the evening.