Found! Weird Asteroid in Jupiter's Orbit Is 1st Interstellar Immigrant
2015 BZ509 was discovered in November 2014, also by Pan-STARRS (which is short for "Panoramic Survey Telescope and Rapid Response System," in case you were wondering). The space rock is an oddball: Its orbit is "retrograde," meaning 2015 BZ509 moves around the sun in the opposite direction of Jupiter, Earth and most other bodies in the solar system.
"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery," astronomer Fathi Namouni of the Côte d'Azur Observatory in France, who led the new study, said in a statement. "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."
Namouni and study co-author Helena Morais, who's based at São Paulo State University in Brazil, performed computer simulations that traced 2015 BZ509's movements backward through time.
"We showed that 2015 BZ509 has been a retrograde co-orbital of Jupiter over the age of the solar system, i.e., since 4.5 billion years ago," Morais told Space.com via email. "The solar system could not produce retrograde orbits so far back in time, so the only option left is that of capture from another system."