https://www.vice.com/en_us/article/v7dana/mysterious-instability-in-the-cosmos-puzzled-scientists-for-decades-now-we-have-an-answer

Our solar system is relatively tranquil at this point in time, with planets that follow predictable orbits around the Sun, but these cosmic surroundings weren’t always so calm.

 Scientists have long suspected that during the infancy of our solar system, tumultuous instabilities dramatically shifted the orbits of the gas giants—Jupiter, Saturn, Uranus, and Neptune—and may have even straight-up kicked a fifth mysterious planet out into the interstellar wilderness. However, the cause of this instability, which has also been observed in other star systems, has remained a matter of debate