Earth has a surprise, long-time companion - a 300 metre-wide asteroid that travels ahead of the planet in its journey around the sun.
The cosmic chum, known as 2010TK7, had been elusive because it spends most of its time in the daylight sky, making it difficult to see from Earth.
It belongs to a group of asteroids known as Trojans, and was discovered by a Canadian-led research team using a NASA satellite launched two years ago.
In 1772, a French mathematician, Joseph-Louis Lagrange, first predicted that asteroids could share the same orbit as planets, provided they were at gravitationally stable points just in front of, or just behind, the bigger heavenly body.
"Such Trojan asteroids have been found co-orbiting with Jupiter, Mars and Neptune [but] they have not hitherto been found associated with Earth, where the viewing geometry poses difficulties for their detection," the researchers, led by Martin Connors of Athabasca University in Alberta, said.
The discovery was reported in the journal Nature.
David Coward, an astronomer and associate professor at the University of Western Australia, said it was an important find that could shed light on the history of the solar system.
Some theories suggest Trojan asteroids migrated into their orbits; others say they were formed there along with the planets.
"The only way we can test this is to have real observations," Dr Coward said.
He said it was also important to know about asteroids near the earth, in case they were likely to hit the planet.
Earth's relatively large companion does its own dance, spiralling back and forth in front of the planet as they move around the sun together, but Dr Connors's team said it appeared to be in an orbit that would be stable for 10,000 years.
It was identified using NASA's wide-field infra-red survey explorer, which has detected more than 500 near-Earth objects since its 2009 launch.
The asteroids are called Trojans because the first one - discovered preceding Jupiter in 1906 - was named Achilles, and it became a tradition to name subsequent ones after participants in the Trojan War.
* What do you think is a better name for 2010TK7?
- Sydney Morning Herald
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