Astronomers think they’ve found 19 alien objects lurking in the solar system

By tracing back the orbits of these icy objects, some researchers think they have interstellar origins. Others, however, remain extremely skeptical

Our solar system seems to be hiding a small group of alien objects that began their lives around another star before moving to our cosmic neighborhood. New research suggests that 19 Centaurs — icy asteroids with occasional cometlike behavior that usually orbit between Jupiter and Neptune — did not originate in our solar system. Instead, the researchers propose the icy rocks were stripped, individually or en masse, from a nearby star when the Sun was just a newborn in a larger stellar nursery.

"Astronomers have known for a long time that there should be asteroids circling the Sun that were not formed in the solar system, but instead were captured early on during the formation of the planets," Fathi Namouni, an astronomer at the Observatoire de la Côte d'Azur in France, tellsAstronomy. "The difficulty was that we could not tell which are solar system-born and which are extrasolar-born."

Predicting the past

Rewinding the solar system’s 4.5-billion-year history is a challenge at best. Over such large timescales, small differences in our models can produce dramatically different results. This led Namouni and his colleagues to take a statistical approach. After preselecting a group of Centaurs, they made millions of virtual clones, then rewound the trajectories of those clones back in time.

They found the objects seem to have had polar orbits around the Sun, perpendicular to the plane of the solar system. These unusual orbits, the researchers say, indicate the objects initially came from another star. The new research was published April 23 in Monthly Notices of the Royal Astronomical Society.

"They've chosen a great problem," says astronomer Kat Volk of the University of Arizona Department of Planetary Sciences. Although not involved with the new research, Volk uses simulations to study objects at the outer edges of the solar system. Regarding the apparent interstellar travelers, she says, "we don't have a great explanation for how you would make these objects."