Interstellar asteroid captured by Sun in distant past

A negative image from the Large Binocular Telescope Observatory shows the asteroid 2015 BZ509 (circled in yellow) as it moves in a retrograde direction against background stars. It is the only known body orbiting the Sun that is thought to be from another star system. Click the image for a larger gif animation. Image: C. Veillet / Large Binocular Telescope Observatory

Astronomers have found what appears to be the first known asteroid from another star in permanent orbit around the Sun.

Another interstellar interloper, known as Oumuamua, made news last year but it was just passing through the solar system, on its way to points unknown. The newly analysed body – (514107) 2015 BZ509 – is a permanent fixture, orbiting the Sun at the distance of Jupiter, moving in the opposite, or retrograde, direction as the known planets and asteroids.

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” said Fathi Namouni, lead author of a study in the Monthly Notices of the Royal Astronomical Society: Letters.

“If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them,” he said.

But it does not. Computer simulations tracing the body’s location back 4.5 billion years show it has always moved in the retrograde direction and must be an object from another star system that was captured by the combined gravity of the sun and its retinue of planets.

“Asteroid immigration from other star systems occurs because the Sun initially formed in a tightly-packed star cluster, where every star had its own system of planets and asteroids,” said co-author Helena Morais. “The close proximity of the stars, aided by the gravitational forces of the planets, help these systems attract, remove and capture asteroids from one another.”

She said figuring out where 2015 BZ509 came from and how it was captured could shed light on the Sun’s original stellar nursery and whether elements and chemicals brought in from other star systems might have contributed to the evolution of life on Earth.