Earth is not alone in its orbit around the Sun - a small 'Trojan' asteroid sits in front of our planet and leads it, according to British science revue Nature, which published the discovery Thursday.

This diminutive asteroid has a diameter of just 300 metres but is called a Trojan because of its particular position in a stable spot either in front of a planet or behind it. Because the asteroid and planet are constantly on the same orbit, they can never collide.

Jupiter, Mars and Neptune also have Trojan asteroids accompanying them, as do two of Saturn's moons.

NASA scientists discovered the asteroid, which lies 80 million kilometres (50 million miles) from Earth, using its Wide-field Infrared Survey Explorer (WISE) telescope.

Astronmers have long thought that Earth did have some Trojans but their discovery has proved elusive because of the difficulty of seeing them in daylight.

"WISE was a game-changer, giving us a point of view difficult to have at Earth's surface", said Martin Connors, a professor at Canada's Athabasca University and the lead author on the Nature paper on the discovery.

Our Trojan -- which is officially called 2010 TK7 -- has an unusual orbit that takes it further away from the sun that most Trojans go, moving above and below the line of the orbit, which is what attracted scientists' attention.

Connors and his team scanned the sky from Jaunary 2010 to February 2011 using additional data about near-Earth objects (NEOs) and the Canada-France-Hawaii telescope in Mauna Kea, Hawaii to try and pinpoint a Trojan.

But this asteroid is keeping its distance: its orbit is "stable for at least 1,000 years", says Connors, and it won't be coming nearer than 24 million kilometres (15 million miles) from Earth over the next 100 years, says NASA.

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