Astronomers discover “immigrant” asteroid near Jupiter that may have been captured from an alien solar system

Nestled in the orbit of Jupiter is an alien asteroid that may have been captured from outside our solar system billions of years ago. 

Dubbed 2015 BZ509, this so-called exo-asteroid, or centaur, doesn’t move in the same direction as the vast majority of other objects in our system and may have migrated shortly after our solar system formed. Centaurs are those asteroids that roam the space between giant planets and, as a result, are subjected to “chaotic close encounters” with the planets. 

 The discovery is significant because 2015 BZ509 is believed to be the first known permanent immigrant to reside in our solar system. A mysterious object known as ‘Oumuamua, discovered last year, hit the headlines as an interstellar interloper, but it was just passing through. 2015 BZ509, by comparison, has been described as a long-term resident and could reveal clues about how our solar system, and even life itself, evolved.

To make the discovery, an international team of researchers from France and Brazil studied the orbit of 2015 BZ509 and compared it to other objects in our solar system. All of the planets, and the vast majority of other objects, travel around the Sun in the same direction. This is because they all adopted the same orbit from the cloud of gas and dust that formed them. However, within Jupiter’s orbit, 2015 BZ509 moves in the opposite direction, in what is known as a “retrograde” orbit.


These are images of 2015 BZ509 taken from the Large Binocular Telescope Observatory (LBTO). The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image

“A number of Centaurs are currently known to be in retrograde resonance between the outer planets’ orbits, but it is the discovery of asteroid 2015 BZ509, inside Jupiter’s co-orbital region, that has produced so far the most puzzling example of retrograde resonance in the solar system,” said the researchers. 

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” added Dr Fathi Namouni, lead author of the study. “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.”

To reveal when the object migrated to our solar system, Dr Namouni’s team ran simulations to trace the location of 2015 BZ509 right back to the birth of the system, 4.5 billion years ago, when the era of planet formation ended. These simulations revealed that 2015 BZ509 has always moved in this way, and so could not have been there originally, meaning it must have been captured from another system.

“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,” added Dr 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.”

Understanding exactly when and how 2015 BZ509 settled in the solar system provides clues about the sun’s original star nursery, and has important implications for the open problems of planet formation, solar system evolution, and possibly the origin of life itself.

The study is published in the journal Monthly Notices of the Royal Astronomical Society: Letters.

Image: C. Veillet/NASA 

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