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 August 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 trans-Neptunian region of the outer solar system, hoping to see what kinds of orbits the objects there followed. (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 in 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.
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 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 team members “were certainly not anticipating finding something as cool as this.”
University of California at Los Angeles planetary scientist David Jewitt said that the trans-Neptunian region is the source of all sorts of objects hurtling about the solar system, providing a “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.
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