We are not alone

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Artist's impression of the obit of the Trojan asteroid

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Astronomers using Nasa's Wide-field Infrared Survey Explorer (Wise) have spotted Earth's first "Trojan" asteroid.

Trojans are asteroids that share the same orbital path as a planet (see the Nasa animation), but because the constantly lead or follow on behind, they're incredibly difficult to spot from the ground.

The side-on view we have of the rest of the solar system means that trojans orbiting other planets are much easier to spot. Neptune, Mars and Jupiter all have them, and scientists have speculated that the earth too might have its own shadowy doppelganger.

Using the perspective offered by the Earth-orbiting Wise telescope, the team - lead by Martin Connors at the Athabasca University in Canada - set out to confirm that theory by scanning the entire sky in infrared light. They spotted two candidate asteroids, one of which has now been confirmed as Earth's first true trojan.

"Wise is a game changer" says Dr Connors, "these asteroids dwell mostly in the daylight making them very hard to see. But we finally found one."

A lump of rock and ice some 300 metres across, asteroid 2010 TK7, follows a complex spiral path some 80 million kilometres from Earth.

"It's as though earth is playing follow my leader" says Amy Mainzer, the project's principal investigator at Nasa's Jet Propulsion Laboratory in Pasadena. "Earth is just chasing this asteroid around."

Although close in astronomical terms, the team say there's no chance 2010 TK7 will collide with Earth. The asteroid's orbit has been plotted over the next 100 years, showing the closest it will come is 24 million kilometres from our planet.

Still, now at least we know its there.