Scientists In the last few years have studied the first observed interstellar visitors to our solar system. They include a comet called 2I/Borisov, spotted in 2019 and still passing through, and an asteroid called 'Oumuamua that zipped through quickly in 2017.
Now, astronomers have identified a more permanent outsider presence in our solar system. It's a group of interstellar asteroids that checked in a long time ago and never left. And they've been hiding in plain sight for billions of years, according to a new study published this week in the Monthly Notices of the Royal Astronomical Society.
These asteroids were likely around when our solar system was forming 4.5 billion years ago. They originated in a different star system. And when our solar system was forming, it was likely closer to other baby star systems as well.
"The close proximity of the stars meant that they felt each others' gravity much more strongly in those early days than they do today," said Fathi Namouni, researcher at the Observatoire de la Côte d'Azur and lead study author, in a statement. "This enabled asteroids to be pulled from one star system to another."
Namouni and fellow researcher Maria Helena Moreira Morais at the Universidade Estadual Paulista in Brazil used numerical modeling to simulate the infancy of our solar system and pinpoint the location of the asteroids billions of years ago.
The simulation placed the asteroids moving in a perpendicular orbit to the plane where the solar system's planets and asteroids orbit the sun. The asteroids were also very distant from the original disk where the planets formed around the sun.
This suggested that the asteroids were actually captured from another star system as the planets were forming in our solar system.
The 19 asteroids have been hiding in plain sight ever since, orbiting the sun along with asteroids called Centaurs that can be found in between Jupiter and Neptune.
Centaurs are unusual because they resemble and act like both asteroids and comets. This dual nature is why they're named centaurs, after half-horse, half-human creatures from mythology. According to NASA estimates, two-thirds of Centaurs came from the freezing outskirts of the solar system.
Centaurs also have orbits that are difficult to understand or predict.
"The discovery of a whole population of asteroids of interstellar origin is an important step in understanding the physical and chemical similarities and differences between solar system-born and interstellar asteroids," Morais said in a statement."This population will give us clues about the sun's early birth cluster, how interstellar asteroid capture occurred and the role that interstellar matter had in chemically enriching the solar system and shaping its evolution."
Morais also identified an interstellar "immigrant" living in our solar system in 2018.
For billions of years, it lived in our solar system without us even knowing it was there. But this object couldn't remain hidden around Jupiter forever. It was just peculiar enough to be noticed by researchers.
The researchers call this exo-asteroid 2015 BZ509. It's known as an exo-asteroid because it originated outside our solar system.
At first glance, 2015 BZ509 is just one of many objects orbiting the gas giant Jupiter in a stable configuration called a resonance. Though all of the planets and most of the objects in our solar system orbit the sun by moving in the same direction, the exo-asteroid is going its own way. With its retrograde orbit, 2015 BZ509 moves in the opposite direction.
"The asteroid and Jupiter take the same amount of time to complete one orbit around the Sun, but one moves clockwise and the other counter-clockwise so they pass by each other twice per each full orbit," Morais wrote in 2018. "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."
That orbit is the same path the object has always followed, meaning it could not have formed in our solar system. If it were native to our solar system, it would have inherited the direction from the gas and dust that formed all of the other planets and objects. Morais believed that, like the newly discovered alien asteroids, this one was captured during the early stages of our solar system as well.
The exo-asteroid serves as a warning for objects that may enter our solar system.
"If they pass by then they may also be captured in a stable orbit as it is the case of 2015 BZ509," Morais said.