"Alien" Asteroids Found "Living" in the Solar System

Recently, astronomers published a paper in the journal Monthly Notices of the Royal Astronomical Society about the discovery of an asteroid group from another system that arrived long ago and have stayed since. For billions of years, they have apparently been hidden in plain sight.

The asteroids have likely been here during our solar system's formation about 4.5 billion years in the past, and scientists think that during that time, our star system's location was probably close to other young systems, too. The asteroids came from such a star system.

Observatoire de la Côte d'Azur researcher and lead author of the study Fathi Namouni says that the closeness of those stars mean that each of them was affected by the others' gravity more strongly than in the present. This affinity pulled asteroids from one system to the other.

The researchers made use of numerical modeling for simulating our solar system's birth and to determine the asteroids' location at that time. The results they obtained put the asteroids in a perpendicular orbit with our solar system's planets. In addition, they were very distant from the disk where our planets originally formed around our sun, suggesting that these asteroids have actually been captured by our star from another system during the period that our planets were forming.

There are a total of 19 of these 'alien' asteroids now orbiting our sun. These include other asteroids that astronomers call Centaurs, which are found between Neptune and Jupiter.

Centaurs are celestial bodies that act and look like comets and asteroids at the same time. They were named Centaurs due to their dual nature, after the half-man, half-horse, creatures of Greek mythology. NASA estimates that two-thirds of known Centaurs originated from the freezing outer fringes of our solar system. The orbits of Centaurs similarly have unpredictable orbits.

Brazil's Universidade Estadual Paulista researcher and study author Maria Helena Moreira Morais said that discovering an entire population of 'alien' asteroids is important in understanding the chemical and physical differences and similarities between asteroids native to our solar system and those with another origin. It provides clues about the birth of the star cluster where our sun belongs, and the roles of interstellar objects in enriching our solar system's chemical  composition and influencing its evolution.

In 2018, Morais had identified another exo-asteroid called 2015 BZ509 residing in our system. It has lived with us for billions of years without being detected, hiding around Jupiter. Its peculiarity was instrumental in its discovery. Exo-asteroids are asteroids that came from another system.

2015 BZ509 looked just like the other objects that orbited Jupiter. It had a retrograde orbit that went opposite the orbital direction of our planets and celestial bodies.

Morais wrote in the 2018 paper that the Jupiter and the asteroid move in opposite directions, so that they pass by one another two times for every full orbit. It's a stable configuration that does not vary.

The object has always followed that same path, which reveals that it was formed in another solar system. An object that originated from our own system would have had the orbit that is congruent to all other native objects and planets. Morais thus believed that 2015 BZ509 was also captured during the early formation of our system.

Morais concludes that other objects that will enter the solar system may be captured into a stable orbit as well.