Asteroid in Jupiter's orbit was captured from another star system

Retrograde orbit indicates it did not originate in our solar system.
By Laurel Kornfeld | May 23, 2018
While interstellar asteroid 'Oumuamua briefly visited our solar system late last year, another asteroid, now in orbit around Jupiter, originated in another star system and was subsequently captured by our own.

Designated (514107) 2015 BZ509, the once exo-asteroid is in a retrograde orbit around the Sun, meaning it circles the Sun in the opposite direction of that traveled by the planets, asteroids, and comets in the solar system.

Like 'Oumuamua, this asteroid was discovered by the Pan-STARRS 1 telescope in Hawaii. Its unusual orbit led scientists to study it in an effort to determine its origin.

"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery. 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," noted Fathi Namouni of the University of Cote d'Azur in Nice, France, and lead author of a paper on the discovery published in Monthly Notices of the Royal Astronomical Society.

Helena Morais of Sao Paulo University in Brazil commented that while the asteroid and Jupiter take the same amount of time to orbit the Sun, one orbits clockwise and the other counter-clockwise, meaning they actually pass each other twice in their 12-year solar orbits.

"This pattern is repeated forever--it is a stable configuration--in a simplified model with only the Sun, Jupiter, and the asteroid. We saw that when we include the other planets, it is still very stable, over the solar system's age," Morais explained.

Computer simulations going all the way back to the solar system's beginning confirm the asteroid always moved the way it is moving now.

2015 BZ509's retrograde orbit is the clue that it originated outside of our solar system. All objects native to the solar system orbit in the same direction because they inherited that direction from the gas and dust that formed solar system objects.

Like many stars, the Sun formed in a cluster or stellar nursery. Each star in this cluster likely had its own system of planets, asteroids, and comets. Eventually, the individual stars dispersed, but before this happened, the gravitational forces of individual star systems and planets could have caused systems to attract and capture asteroids from neighboring systems.

Morais estimated the solar system captured 2015 BZ509 approximately 4.5 billion years ago.

Further observation of this asteroid, along with identification of its composition, could help scientists learn about its parent star and provide insight into the ancient stellar nursery in which the Sun formed.

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