"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."
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