Oumuamua: Lost interstellar asteroid enters solar system
The dark red rock, named Oumuamua, is the first space rock from outside the solar system ever observed by astronomers.
A lost interstellar asteroid has entered the solar system after wandering between the stars for hundreds of millions of years, scientists believe.
The dark red object named Oumuamua, is about 400 metres (1,312 ft) long, scientists reported in the journal Nature.
It 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) spotted the asteroid on 19 October as a faint point of light moving in the sky.
After further observations and orbital calculations, there was no doubt the object originated from outside the solar system.
Moving at 95,000 kilometres per hour (59,030 mph), Oumuamua was at first thought to have travelled from the bright star Vega, 25 light years away in the northern constellation of Lyra.
But Vega was nowhere near its current position 300,000 years ago, when its journey would have started.
That has led scientists to speculate that the asteroid is an interstellar wanderer that happens to have stumbled across our solar system.
"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," Dr Karen Meech, from the Institute for Astronomy in Hawaii, said.
"This unusually large variation in brightness means that the object is highly elongated," she added.
"We also found that it has a dark red colour, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it."
The rock's properties suggest it could have a high metal content and lacks significant amounts of water or ice, and its surface has become dark and red by the impact of cosmic rays over millions of years.
Astronomers estimate that interstellar asteroids pass through the inner solar system about once a year, but they are difficult to see because they are so faint.
Oumuamua was discovered by the 1.8 metre Pan-STARR telescope in Hawaii, which is part of a system set up to track potentially threatening NEOs.