Astronomers discover 'Trojan asteroid' in Uranus' orbit

This image is one of three discovery images of 2011 QF99 taken from Canada-France-Hawaii Telescope on Oct. 24, 2011 (2011 QF99 is inside the green circle). 2011 QF99 is the first Trojan asteroid discovered around Uranus. (Canada-France-Hawaii Telescope / Coelum)

The seventh planet from the sun was once considered too distant to host asteroids, but that concept was blown away with the discovery of a new triangle in the sky – its points being the sun, the Uranus and a 36-mile-wide body of rock and ice in its orbit.

Researchers at the University of British Columbia (UBC) said they believe the Trojan asteroid, which has been given the official tag 2011 QF99, belongs to a larger-than-anticipated number of transitory objects of the kind presently trapped by the gravitational pull of the Solar System's larger planets.

The astronomers presented their findings in the Aug. 30 issue of the journal Science.

Trojan asteroids are objects that share their orbit with a planet, but do not collide with it. The sun, Uranus and the newly discovered asteroid form a triangle whose sides are currently about 1.7 billion miles (2.8 billion km) long.

Astronomers had considered Trojans at Uranus a low possibility since the gravitational pull of larger neighboring planets would eventually eject any Trojan asteroids that may be circling the planet’s gravitational field.

"Our model predicts that at any given time three per cent of scattered objects between Jupiter and Neptune should be co-orbitals of Uranus or Neptune," lead study author Mike Alexandersen told UPI. 

This image shows the motion of the Trojan asteroid 2011 QF99 at Uranus over the next 59,000 years. Shown here is the trajectory of 2011 QF99, according to the best fit to the observations. The current position is marked by a red square, and the black line shows the trajectory 59,000 years into the future. L4 and L5 are the triangular Lagrange points. (UBC Astronomy)

This percentage surprised the researchers as it is much higher than previous estimates, says a UBC press release.

In 2011 and 2012, astronomers were tracking objects in the vicinities of the giant planets for 17 months in order to better understand the evolution of the outer solar system.

"Our search was focused on finding Neptunian Trojans and trans-Neptunian objects," study lead author Mike Alexandersen, an astronomer at the University of British Columbia in Vancouver, told SPACE.com."One is always surprised and excited to find something other than what one was expecting and searching for." 
QF99 ranks as a ‘temporary’ Trojan, according to the Canadian astronomers, because the object was only recently (within the last few hundred thousand years) captured by Uranus. The asteroid is predicted to escape the planet's orbit in about a million years.

"This tells us something about the current evolution of the Solar System," Alexandersen said. "By studying the process by which Trojans become temporarily captured, one can better understand how objects migrate into the planetary region of the Solar System." 
Asteroid QF99 is about 19 astronomical units from the sun — that is, 19 times that between the Earth and sun. The Earth is typically about 93 million miles (150 million kilometers) from the sun.