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.
PHOTOS: Cosmic Hotshots From Keck Observatory
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.
ANALYSIS: Uranus Pathfinder: Mission to the Mysterious Ice Giant
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.