The mysterious, cigar-shaped object now called ’Oumuamua was found crossing the solar system last October by robotic telescopes on Hawaii. The trajectory showed it had come from another star system and was already on its way back into interstellar space. This sparked a race against time. Astronomers had just a week before it faded from view.

Identifying its home star system seemed like a hopeless task. Our galaxy contains hundreds of billions of stars. Now, however, a new study narrows things down a bit. It concludes that ’Oumuamua, meaning “scout” in Hawaiian, probably came from a binary star.

Binary stars are systems in which two stars are in orbit around one another. Together with systems containing three or more stars, they make up about a third of the stars in the galaxy.

Alan Jackson, of University of Toronto at Scarborough, Canada, and collaborators used computer models to find that rocky objects like ’Oumuamua are far more likely to come from binary than single star systems.

Mysterious object confirmed to be from another solar system Read more

They found that single star systems, like the sun, are better at ejecting icy comets than asteroids. This is because the comets live much further from the sun, and so are more weakly bound by gravity than the nearer asteroids.

In a binary star system, however, there are stronger gravitational fields at play because of the two stars orbiting each other. According to the new work, these would be sufficient to eject as many asteroids as comets into interstellar space.

Asteroids in our own solar system are valued as they are leftovers from the formation of the planets, around 4.5bn years ago. As such, they act as time capsules preserving the conditions in which Earth formed. With ’Oumuamua, astronomers have seen one that relates to a whole different star system.

“It’s remarkable that we’ve now seen for the first time a physical object from outside our solar system,” says lead author Jackson.

It was roughly 400 metres long and composed predominantly of rocky material. It reflected just 4% of the light than fell on it, making it as dark as asphalt. Although devoid of ice it was covered in the kind of organic molecules that are thought to be the building blocks of life.

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It may also be the tip of the celestial iceberg. An earlier study by another group of astronomers, led by David Jewitt, of University of California, Los Angeles, estimated that another 10,000 interstellar asteroids could be closer to the sun than the eighth planet, Neptune, which lies 30 times further from the sun than the Earth. Yet these are currently undetected.

New survey telescopes and upgrades to existing ones are expected to begin seeing them in the coming years.

Stuart Clark is the author of The Unknown Universe (Head of Zeus).