Trojan Asteroid Found Orbiting Uranus

by Tammy Plotner on August 29, 2013

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One of three discovery images of 2011 QF99 taken from CFHT on 2011 October 24 (2011 QF99 is inside the green circle). This is the first of three images  of the same patch of sky, taken one hour apart, that were then compared to find moving light-sources.

One of three discovery images of 2011 QF99 taken from CFHT on 2011 October 24 (2011 QF99 is inside the green circle). This is the first of three images of the same patch of sky, taken one hour apart, that were then compared to find moving light-sources.

What’s new in the outer reaches of our solar system? Try the discovery of a Trojan asteroid orbiting Uranus. While a plethora of puns exist for this simple fact, the reality check is that this means there are far more of these objects out there than astronomers expected. The new Trojan even has a name – 2011 QF99!

A Trojan asteroid is a transient space rock which is temporarily captured by the gravity of a giant planet. It shares the planet’s orbital path, locked into a specific position known as a Lagrange point. What makes 2011 QF99 unusual is its presence in the outer solar system. Researchers found the scenario a bit unlikely. Why? The answer is simply because of planet size. According to theory, the strong gravitational pull of the larger neighboring planets should have destabilized any captured asteroid’s orbit and shot Uranian Trojans out of the neighborhood long ago.

So just how did this 60 kilometer-wide conglomeration of ice and rock end up circling Uranus? Astronomers turned towards computer modeling for the answer. The research team, including UBC astronomers Brett Gladman, Sarah Greenstreet and colleagues at the National Research Council of Canada and Observatoire de Besancon in France, did a simulation of the solar system and its co-orbital objects – including Trojan asteroids. A short-term animation showing the motion of 2011 QF99, as seen from above the north pole of the solar system can be found here.

“Surprisingly, our model predicts that at any given time three percent of scattered objects between Jupiter and Neptune should be co-orbitals of Uranus or Neptune,” says Mike Alexandersen, lead author of the study to be published tomorrow in the journal Science.

Until now, no one had made any estimates on the percentage of possible outer solar system Trojans. Unexpectedly, the amount ended up being far greater than earlier estimates. Just over the last 10 years, several temporary Trojans and co-orbitals have been cataloged and 2011 QF99 is one of them. It made its home around Uranus within the last few hundred thousand years and will eventually – in about a million years – escape Uranus’ gravity.

“This tells us something about the current evolution of the solar system,” says Alexandersen. “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.”

Original Story Source: UBC News Release.

About 

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Jeff Ross August 29, 2013 at 1:11 PM

Do things in LaGrangian points “circle” planets?

Jeff Ross August 29, 2013 at 6:34 PM

If an object is in a lagrangian point, then it co-orbits the sun with the planet. It does not orbit the planet.

kaplanfx August 29, 2013 at 8:00 PM

The objects “circle” or orbit the LaGrange point.

Jeff Ross August 29, 2013 at 3:07 PM

The article says “circling Uranus”, not “circling near Uranus”. If we are just going for the comedy, I think “finding a trojan in front of Uranus” is equally funny and more accurate. Just saying.

Din Sel August 29, 2013 at 9:56 PM

Please no jokes about Trojan Asteroids orbiting your anus.

Eric Jablow August 29, 2013 at 10:38 PM

I thought Trojan asteroids were 60° ahead or behind a planet’s position in its orbit around the sun. They’d be at the L-4 or L-5 positions. That isn’t orbiting around the planet. What is the actual geometry?

Joe Smith September 4, 2013 at 7:28 PM

The asteroid does not actually circle Uranus; it goes around a point relative to Uranus.
The animation, “as seen from above the north pole of the solar system” shows it following a kidney bean shaped orbit around the L4 point.

IVAN3MAN_AT_LARGE August 30, 2013 at 4:20 AM

Here’s the preprint paper (PDF), if anybody is interested:
The first known Uranian Trojan and the frequency of temporary giant-planet co-orbitals

Efreet August 30, 2013 at 6:16 AM

you’re right tammy. many potential jokes, but that’s one of the reasons we love this planet so much.

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