ESO/M. Kornmesser

Mysterious asteroid from beyond our solar system probably came from a place with two stars

Late last year, astronomers spotted the first object to enter our solar system from interstellar space—a somewhat reddish, cigar-shaped body named ‘Oumuamua. Now, a new study hints that this exotic interloper most likely began its voyage after being cast out of a double-star system.

Astronomers first classified ‘Oumuamua (Hawaiian for “scout”) as a comet, but later observations didn’t reveal the telltale signs, including clouds of dust or water vapor. That, plus the 400-meter-long object’s high speed and odd trajectory, strongly suggested that ‘Oumuamua was an asteroid, not a comet, from beyond our solar system.

But very few single-star solar systems would be able to cast out a waterless object like an asteroid, a new study suggests. That’s because such a feat would require gravitational interactions with a planet the size of Saturn or larger, something present in only about 10% of single-star solar systems near us in the Milky Way.

But solar systems that have two suns, especially those in which the stars orbit each other tightly, are much more likely to cast out asteroids, the researchers report today in the Monthly Notices of the Royal Astronomical Society. The team’s computer simulations suggest that up to 36% of binary stars can eject asteroids. When the researchers take into account the numbers of single-star versus binary systems and the numbers and sizes of planets they’re likely to have, they estimate that more than three-fourths of the asteroids cast into interstellar space come from solar systems that have two suns.