Asteroid 2015 BZ509 Came from Interstellar Space 4.5 Billion Years Ago


2015 BZ509, an object in Jupiter’s co-orbital region with a retrograde motion around the Sun, is the first known asteroid to have been captured from the interstellar medium, according to a team of astronomers from France and Brazil.

An artist’s impression of a rocky asteroid. Image credit: Mark A. Garlick, / University of Warwick / University of Cambridge.

An artist’s impression of a rocky asteroid. Image credit: Mark A. Garlick, / University of Warwick / University of Cambridge.

All of the planets in our Solar System, and the vast majority of other objects as well, travel around the Sun in the same direction.

However 2015 BZ509 is different — it moves in the opposite direction in what is known as a retrograde orbit.

“We’re sure 2015 BZ509 is retrograde, co-orbital and stable,” said Dr. Fathi Namouni of the Cote d’Azur Observatory and Dr. Helena Morais of the Universidade Estadual Paulista.

“The asteroid is retrograde, as its orbit around the Sun counters the prograde norm of following the direction in which the Sun rotates on its own axis; and co-orbital, as it shares Jupiter’s orbital path albeit backwards; and in resonance with Jupiter, as its orbit is perfectly synced with the planet by taking one Jovian year — nearly 12 Earth years — to complete.”

“Indeed, the asteroid crosses the giant planet’s path by tracing what is known as a trisectrix curve, alternately weaving inside then outside Jupiter’s orbit while the planet’s gravity tugs help keep the asteroid on a stable course.”

“How 2015 BZ509 came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” Dr. Namouni said.

“If this asteroid was a native of our Solar 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.”

The team ran simulations to trace the location of 2015 BZ509 right back to the birth of the Solar System, 4.5 billion years ago when the era of planet formation ended.

These show that the object has always moved in this way, and so could not have been there originally and must have been captured from the interstellar medium.

“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,” Dr. Morais said.

“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.”

The discovery, reported in the Monthly Notices of the Royal Astronomical Society: Letters, has important implications for the open problems of planet formation, solar system evolution, and possibly the origin of life itself.

“Understanding exactly when and how 2015 BZ509 settled in the Solar System provides clues about the Sun’s original star nursery, and about the potential enrichment of our early environment with components necessary for the appearance of life on Earth,” the astronomers said.


F. Namouni M.H.M. Morais. 2018. An interstellar origin for Jupiter’s retrograde co-orbital asteroid. MNRAS 477 (1): L117-L121; doi: 10.1093/mnrasl/sly057