Earth has one. Jupiter has more than 5,000. And now astronomers know that Uranus, despite its odd, sideways orbit around the sun, has a tag-along companion too.
This type of object, known as a "Trojan," compatibly shares a planet's path around the sun, thanks to a neat trick of orbital mechanics.
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Trojans are fortuitously positioned in precise locations - called Lagrangian points - where gravitationally tugging by two larger bodies balances out, creating safe harbors for smaller, third objects to fly undisturbed.
Earth's Trojan, for example, is a 1,000-foot diameter asteroid that orbits in a complicated pattern some 50 million miles away. It was discovered in 2010 with NASA's Wide-field Infrared Survey Explorer, or WISE telescope.
Trojans also have been found sharing orbits with Jupiter, Mars, Neptune and two of Saturn's moons.
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Scientists didn't think Uranus, the only planet that is keeled over on its side, relative to the sun, had the gravitational stability to support Trojans. But a 17-month study survey by the Canada-France-Hawaii Telescope has proven otherwise.
The newly found object, known as 2011 QF99, won't stay with Uranus forever. Computer models show the Trojan will escape Uranus' gravitational leash in about 700,000 years.
The research appears in this week's Science.
Image: Uranus and its family of moons: Now it looks like the planet has one extra. Credit: NASA