It turns out Uranus has a cosmic companion as it circles the sun from nearly 1.8 billion miles away. Scientists have detected a Trojan -- an asteroid-like object that shares a planet's orbit -- moving ahead of the ice giant.
The discovery of 2011 QF99 was reported this week in the journal Science. And it was found almost by accident.
Mike Alexandersen, a doctoral student in astronomy at the University of British Columbia in Vancouver, wasn't looking for a Trojan. Nor was he studying Uranus.
He and his colleagues were surveying the transneptunian region of the outer solar system, hoping to see what kinds of orbits the objects there followed. (The transneptunian region is more or less the same thing as the Kuiper Belt. Studying the patterns of objects' orbits in the region helps scientists understand how the solar system formed some 4.5 billion years ago.)
As Alexandersen and the team examined images snapped using the Canada-France-Hawaii telescope during 2011 and 2012, they noticed one object that was moving across the field of vision more quickly than the others. It was an indication that the object was closer to Earth than the rest.
That part wasn't a surprise. But seeing something that moved the way 2011 QF99 did was a shocker.
The scientists had expected to see objects known as Centaurs, which often move toward the center of the solar system along quirky paths. But over the course of a year of observations, they realized that this particular space rock was traveling in an orbit very much like that of Uranus.
That made it seem more like a Trojan, gravitationally bound to its planet. The mysterious object also oscillated as a Trojan would.
"It was, in fact, a Trojan," said Alexandersen, who added that the team members "were certainly not anticipating finding something as cool as this."
UCLA planetary scientist David Jewitt, who is credited with detecting the first Kuiper Belt object in 1992, said that the transneptunian region is the source of all sorts of objects hurtling about the solar system, providing an Armada-like "rain of stuff" cascading inward toward the sun.
As they move about, they get caught up in planets' gravity -- either getting hurled away or thrown further inward.