Astronomers say they've learned more about the first known object to enter our solar system from deep space, including its size and colour.

New data from the European Southern Observatory's telescopes and others around the world have revealed that the asteroid — spotted last month, already speeding away from the sun — is rocky, cigar-shaped and about 400 metres long.

It's believed the interstellar interloper could be one tenth as wide as it is long.

The researchers were even able to determine its colour: it has a reddish hue.

The discovery was an exciting one for the astronomical community as it was the first recorded visitor from beyond our solar system.

"For decades we've theorized that such interstellar objects are out there, and now — for the first time — we have direct evidence they exist," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in a statement about the new findings published Monday in the journal Nature.

The object was discovered by Canadian astronomer Robert Weryk at the University of Hawaii Institute for Astronomy. He came across the image that was taken as part of the Pan-STARRS survey, which is looking for near-Earth objects.

Initially it was believed that it came from within our solar system, but a look back at an image from the night before in the same general area revealed a trajectory that took the asteroid far outside our solar system, originating in the direction of the star Vega. (Though because the stars are moving, Vega wouldn't have been there when the asteroid started its journey).

This diagram shows the orbit of the interstellar asteroid Oumuamua as it passes through the solar system. (ESO/K. Meech, et al.)

The object, named 1I/2017 U1 — or 'Oumuamua a Hawaiian word for scout or messenger from the distant past — varies in brightness by a factor of 10. It's believed that this is caused as it rotates on its axis once every 7.3 hours. It's the first time an asteroid has varied so much in brightness. 'Oumuamua is likely made up of rock and metals, but has no water or ice.

Astronomers are still tracking the asteroid as it travels about 38.3 km/s. It will pass Jupiter's orbit — though on an inclined plane compared to the orbits of the planets — in May 2018.

"Certainly this is a new type of object. It'll go a long way to improving our understanding as to how the solar system formed and evolved," Weryk told CBC News at the time of discovery.

"I'm certainly interested in finding more of these."