New Planet Discovered in the Solar System?

August 1, 2005

A chunk of rock and ice that may be a planet has been discovered in the farthest reaches of the solar system, astronomers announced Friday. The object, currently called 2003 UB313, orbits the sun and is larger than Pluto, traditionally considered the ninth planet in the solar system.

The news came hot on the heels of the announcement of the discovery of a separate planet-like object at the edge of the solar system (see "New Pluto-Size Object Discovered in Solar System")—and amid fears that a rogue astronomer may been attempting to announce the UB313 discovery as his or her own.

The discovery of UB313 is likely to reignite the debate over the definition of "planet"—and over how many objects in the solar system are deserving of the name.

The sequence of events surrounding the UB313 announcement may also change how such objects are announced in the future, according to Brian Marsden, director of the International Astronomical Union's Minor Planet Center in Cambridge, Massachusetts.

UB313 is being called a scattered disk object or an extreme member of the Kuiper belt. The belt is a ring of icy debris that floats on the fringes of the solar system beyond Neptune.

The object is currently about 97 times farther from the sun than Earth is, or about three times farther from the sun than Pluto. Its orbit is more eccentric than Pluto's, taking it from 38 to 97 times the sun-Earth distance over a 560-year period.

UB313 is the farthest known object in the solar system—even further than Sedna, a planetoid discovered nearly two years ago (see "New Planetoid Found in Solar System—Most Distant Yet"). The newfound object is also among the five brightest Kuiper belt objects, as seen from Earth.

The sheer size of the object means that it can only be classified as a planet, according to Mike Brown, an astronomer at the California Institute of Technology in Pasadena, who announced the object's discovery.

Brown made the discovery with Chad Trujillo of the Gemini Observatory on the Mauna Kea volcano in Hawaii, and David Rabinowitz of Yale University in New Haven, Connecticut.

The team had originally planned to report the discovery of UB313 in October after more detailed observations. But they decided on an early announcement after it became clear that savvy Web users could—if they were so inclined—track down UB313 observation data online and use it to claim the discovery as their own, Marsden said.

Planet Debate

Brown and his colleagues are uncertain of the exact size of the object, but its brightness and distance from the sun tell them that it is at least slightly larger than Pluto. At one-fifth the mass of our moon, Pluto is the smallest of the nine planets.

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