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Uranus has a 'Trojan asteroid,' astronomers say

Astronomers have spotted a 36-mile-wide asteroid leading ahead of Uranus, in an orbit thought to have been impossible.

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

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Astronomers have discovered an unexpected, novel kind of triangle in the sky — one whose points are the sun, Uranus and the first "Trojan asteroid" ever seen near the tilted planet.

The discovery of the Trojan asteroid for Uranus suggests Uranus and Neptune could have far more such asteroid companions than previously thought, scientists say.

In astronomy-speak, objects that share their orbit with a planet — but do not collide with the world — are known as Trojans. Such objects have been seen around several planets in our solar system, including the Earth, but the newfound asteroid 2011 QF99 near Uranus is the first ever seen for that planet. [Photos of Uranus, the Tilted Planet]

Trojan asteroids explained

To picture, say, Earth's Trojan asteroid, imagine the sun and Earth as being two points in a triangle whose sides are equal in length. The other point of such a triangle is known as a Trojan point, or a Lagrangian point, named after the mathematician who discovered them. At such a point, the gravitational attraction of the sun and Earth essentially balances out, meaning they are relatively stable points for asteroids or other objects.

The sun and Earth have two Trojan points, one leading ahead of Earth, known as the L-4 point of the system, and one trailing behind, its L-5 point. The sun and other planets have Lagrangian points also,  with asteroids seen at those the sun shares with Jupiter, Neptune and Mars.

Scientists thought the Trojan points of Uranus, the seventh planet from the sun, were too unstable to host asteroids. Now astronomers using the Canada-France-Hawaii Telescope have discovered the first Trojan for Uranus.


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