The “demoted” dwarf planet Pluto and its largest moon Charon make an unusual pair, and for decades, scientists have been discussing how the binary system—in which each mutually orbits the other—came to be.

Unlike Earth, where the Moon orbits the planet, Pluto and Charon orbit each other, forming a binary system that is more similar to the Earth-moon system than any other moon in the solar system.

Simulations suggest Pluto and its largest moon may have gently stuck together for a few hours before Charon settled into a stable orbit around the dwarf planet