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 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, Canada, 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 the same way 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.”

University of California at Los Angeles 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.

Chunks that float around in the zone of the giant planets are called Centaurs. Those that make it into the inner solar system, heating and vaporizing in the sun’s heat, are known as comets.

Trojans are the bits that get captured in particular locations in a planet’s orbit where gravity from the sun and gravity from the planet interact to lock them in place.

Some Trojans — around Mars, Neptune and especially Jupiter — are permanently bound to their planets, and have been for billions of years. Others, like 2011 QF99 and Earth’s Trojan 2010 TK7, are only temporarily trapped in their orbits.

“The planets are playing ball with this thing,” Jewitt said of the newly detected object. “Eventually they’ll lose control of it.”

Alexandersen and his colleagues conducted a computer simulation that showed that the Trojan — which is about 37 miles wide — is only temporarily bound to Uranus. Sometime within the next million years, it’s likely to drop out of its orbit and become a Centaur, they reported in Science.

The simulation also revealed that around 3 percent of the minor objects in the giant-planet region were likely to be co-orbiting with Uranus or Neptune at any given time. “That was higher than we thought it would be,” Alexandersen said.

The International Astronomical Union, which is in charge of naming such bodies in the solar system, may decide to give 2011 QF99 a different name. “We may get to send them a suggestion,” Alexandersen said.

For now, he said he will use the remaining few days of scheduled telescope time to continue tracking the Trojan this fall, but “probably won’t study this a lot more,” shifting gears to work on the “planned part of my thesis.”

But 2011 QF99 was “a surprise chapter” that will “certainly make its way” into the final draft, he said.


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