New Pluto-Size Object Discovered in Solar System

John Roach
National Geographic News

July 29, 2005

Astronomers in Spain and the U.S. have discovered a large object
skirting the fringes of the solar system. Research teams from both
countries announced the discovery separately.

The object, designated 2003 EL61, is about 930 miles (1,500 kilometers) across, according to Mike Brown, an astronomer at the California Institute of Technology, who led the U.S-based team. The team announced its discovery today.

The object also has a moon.

2003 EL61 and its satellite are located in the Kuiper belt, a region beyond Neptune that includes Pluto and the recently discovered large planet-like objects Quaoar and Orcus.

Preliminary reports from the Spanish scientists suggested it may be twice as big as Pluto, but those reports now appear to be incorrect.

"They were just guessing a size, because they didn't know it has a satellite," Brown wrote in an e-mail to National Geographic News.

The astronomers are classifying the newly discovered object as a "scattered Kuiper belt object." Such objects are believed to have had a close encounter with Neptune, which then scattered them with its gravitational force into eccentric orbits.

Awkward Situation

Today's announcement makes for "sort of an awkward situation," said Brian Marsden, director of the International Astronomical Union's Minor Planet Center in Cambridge, Massachusetts. There were some early questions about which team would receive credit for the discovery.

Brown's team first noticed the object on May 6, 2004. The team will present details about the find at a conference in Cambridge, England, this September.

The presentation will include measurements of the object's size from new observations made with the Spitzer Space Telescope. The observations were made July 22.

The Spanish group, led by Jose-Luis Ortiz at the Instituto de Astrofísica de Andalucía in Granada, Spain, found the object in images taken in 2003. They announced their discovery Thursday.

"We were able to compute a preliminary orbit, run it backwards, and find the object in old images of several archives," Ortiz said in an e-mail to National Geographic News.

"We communicated this to the Minor Planet Center and provided coordinates for amateurs to be able to observe it now and report it, and some did," he added.

Brown said the Spanish group gets credit for the discovery, as they announced it first. "Those are the rules that everybody plays by," he said.

Eluding Detection

According to the Minor Planet Center, 2003 EL61 is about 51 astronomical units from the sun. An astronomical unit is the distance between the Earth and the sun. The object's elliptical orbit, which takes 285 years to complete, brings it as close as 35 astronomical units to the sun.

"The surprising thing is, why was it found only recently?" Marsden said. "That's because it's rather far from the ecliptic [the plane of Earth's orbit]. It's got a 28 degree inclination, and it's near its furthest extent at the moment, and [it] has been for the last several decades."

Inclination is the tilt of the object's orbit in relation to Earth's orbital plane.

According to Ortiz, another reason 2003 EL61 has eluded detection until now is that very few astronomers are looking for objects with orbits beyond Neptune.

"It takes a very long time to complete a survey around the ecliptic—typically years," he said. "Analyzing the data also takes very long, so it is not a very big surprise that this object has eluded detection."

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).