First confirmed object from another star was cigar shaped, 400 metres long, and a bit red

Tuesday, 21 November 2017

Astronomers have confirmed an object spotted in space last month is the first known visitor from outside the solar system.

They found the asteroid was cigar-shaped up to 400 metres long and had a somewhat reddish hue, Nasa said.

Observations suggest the object had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years.

An artist's impression of the first confirmed interstellar asteroid.

Possibly 10 times longer than it is wide, its elongated shape is unlike that of any asteroids seen in our solar system.

* NASA's asteroid chaser swings by Earth on way to space rock

* Asteroid shaves Earth, hours after astronomers spotted the space rock**

A team of astronomers led by Karen Meech of the Institute for Astronomy in Hawaii found the asteroid varied in brightness by a factor of ten as it spins on its axis every 7.3 hours.

'This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,' Meech said.

'We also found that it had a reddish colour, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.'

Those properties suggested the asteroid was dense, comprised of rock and possibly metals, had no water or ice, and that its surface was reddened due to the effects of irradiation from cosmic rays over hundreds of millions of years, Nasa said.

It is travelling about 138,000km per hour (38.3km per second) relative to the Sun, and is now about 200 million km from Earth - the distance between Mars and Jupiter.

Its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars' orbit around November 1 and will pass Jupiter's orbit in May 2018. It will travel beyond Saturn's orbit in January 2019, and will head for the constellation Pegasus.

Discovered by the University of Hawaii's Nasa-funded Pan-STARRS1 telescope - used to find and track asteroids and comets - the asteroid was named 'Oumuamua, which is Hawaiian for 'a messenger from afar arriving first'.

'Oumuamua is thought to have come from the direction of where the bright star Vega is now, although when the asteroid was there 300,000 years ago Vega was not actually near that position. The asteroid made the trip to our solar system at a clip of around 95,000kmh.

Astronomers estimated an interstellar asteroid similar to 'Oumuamua passed through the inner solar system about once per year, but they were faint and hard to spot and had been missed until now, Nasa said.

Survey telescopes had only recently become powerful enough to have a chance to discover them.