[Edward64]

Pretty interesting.

https://www.cnn.com/2018/05/21/world...tem/index.html

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.

In 2017, the first observed interstellar visitor, an asteroid named 'Oumuamua, was found traveling through our solar system. Now, for the first time, astronomers have discovered a permanent resident that moved in during the early stages of our solar system's development, about 4.5 billion years ago.

The researchers call this exo-asteroid, known as 2015 BZ509, an "interstellar immigrant." 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. 2015 BZ509 moves in the opposite direction in a retrograde orbit.

"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.

"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.