A lost interstellar asteroid has entered the solar system after wandering between the stars for hundreds of millions of years, scientists believe.
The elongated, dark-red object, named Oumuamua, is the first space rock from outside the solar system ever observed by astronomers.
A telescope in Hawaii designed to spot Near-Earth Objects (NEOs) picked up the 400m-long asteroid on October 19 as it was heading back into interstellar space.
Moving at 95,000km/h, Oumuamua - which is Hawaiian for "messenger from afar arriving first" - was at first thought to have travelled from the bright star Vega, 25 light years away in the constellation Lyra.
But Vega was not even close to its present position 300,000 years ago, when Oumuamua's journey would have started.
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That has led scientists to speculate the asteroid is an interstellar wanderer that has stumbled across our solar system.
Dr Karen Meech, of the Institute for Astronomy in Hawaii, said: "Oumuamua may well have been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with the solar system."
Observations showed the asteroid's brightness varied dramatically by a factor of 10 as it span on its axis every 7.3 hours, the scientists say in the journal Nature.
The asteroid's properties suggest it could have a high metal content and lacks significant amounts of water or ice.
Astronomers estimate that interstellar asteroids pass through the inner solar system about once a year but they are faint and hard to spot.
Scientists had to act quickly with Oumuamua, which was detected as it was on its way back into interstellar space after passing the sun and the Earth.
Science magazine reports it came as close as 23 million kilometres to Earth and is heading towards the constellation Pegasus.
