Astronomers have discovered what they believe is the 10th and most distant planet in our solar system: a ball of rock that is 1 1/2 times as big as Pluto and about three times as far out.
While there are still no precise measurements of the new object, an analysis of its brightness and distance suggests that its diameter is up to 2,000 miles.
"We are 100% confident that this is the first object bigger than Pluto ever found in the outer solar system," said Caltech astronomer Michael A. Brown, who announced the discovery at a late-afternoon news conference Friday.
The size of the object, temporarily called 2003 UB313, has led Brown to declare it a full-fledged planet of our solar system -- the first discovered since Pluto was spotted in 1930.
That claim is certain to be controversial. Astronomers have long debated whether Pluto is a planet because of its small size and odd orbit.
Some scientists prefer to call the diminutive body a Kuiper Belt object after the orbiting conglomeration of asteroids, comets and other materials that extends from Neptune to the outer reaches of the solar system.
Others are content to let Pluto keep its place in the pantheon of planets, if for no other reason than tradition.
Brown said he believed the new object deserves a place as well.
"If Pluto is a planet, then anything larger than Pluto is a planet, and this is definitely larger than Pluto," he said.
The object is currently at its farthest distance from the sun, about 97 times the distance between the sun and Earth.
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.
The surface of the new object is very similar to that of Pluto, a mixture of about 70% rock and 30% 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 had probably not been discovered until now 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.
Astronomer Brian Marsden of the Smithsonian Institution's Minor Planet Center said the new object was probably just barely too faint to be detected by Clyde Tombaugh's sky survey that turned up Pluto 75 years ago.
Brown first saw the new object on Jan. 8 using the 48-inch Samuel Oschin Telescope at Palomar Observatory, along with his colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.
It had actually been photographed first in 2003 -- hence the name -- but nobody realized what it might be until its motion became apparent.
The scientists measured the brightness of the object to determine its size. Because they don't know how reflective its surface is, they could only estimate. It might be a very small object with a very reflective surface.
But Brown added: "Even if it reflected 100% of the light reaching it, it would still be as big as Pluto."
The object 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 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 website Thursday night and was planning to announce the discovery.
"We really didn't have a choice," he said.
He said he had selected a name for the new planet and had submitted it to the International Astronomical Union, but that he would not reveal it until that body had made a decision.
The new object is the third-brightest known 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% of 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 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% 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 had also been tracking another object, about the same size and distance from the sun as 2003 EL61, but brighter still.
Brown said 2003 UB313 could easily be observed by amateurs with good telescopes. It is now straight overhead in the early morning, but in six months it will be visible in the evening.