Trojans are asteroids locked in stable orbits by a gravitational balancing act between a planet and the sun. They have previously been discovered accompanying Neptune, Jupiter and Mars.
Scientists led by Martin Connors from Athabasca University in Canada suspected they may also be in stable positions orbiting with the Earth.
But Connors says finding an 'Earth-bound' Trojan is difficult because its location can only be seen from Earth in daylight.
"When you look out towards Jupiter you've got no real problems picking up its Trojans because they're in the night sky for us most of the time", says Connors.
"But for our own Trojans, they're kind of near to the sun, so they're the sort of thing you'll only see for an hour or so in the evening and in the morning, that's your only opportunity to look for them."
"That's why none have been found before."
Connors says the launch of NASA's Wide-field Infrared Survey Explorer spacecraft (WISE) in 2009 changed all that.
"WISE looked at the infrared light coming from all over the sky and was particularly good at detecting asteroids, finding about 150,000 of them, including 500 which come near the Earth."
By examining the orbits of these objects in the WISE data, Connors and colleagues identified a small asteroid called 2010 TK7 as a probable Earth Trojan. The researchers then used ground-based telescopes to confirm the sighting, calculating that it's been in a stable orbit with Earth for more than 10,000 years.
Their finding appears today in the journal Nature.
Other than its orbit, scientists know very little about this asteroid.
"Based on the amount of light it reflects we estimate it to be about 300 meters wide, about the size of a small neighborhood," says Connors.
"We know nothing (else) about it, a situation we hope will change in the future."
Connors says he has been looking for an Earth Trojan for a number of years based, out of the belief that they must exist.
"Well they do; we've got one!" he says. "The question is, are there any more?"
Scientists have suggested an Earth Trojan would be a good target for NASA's proposal to send astronauts to an asteroid.
More at Discovery News
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