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.

“Because Pluto is rotating rapidly prior to the collision, and because Charon lies mostly outside of their corotation zone, it is able to ‘push’ Charon off, and Charon starts to slowly migrate out,” first author Dr. Adeene Denton, a planetary scientist from the University of Arizona, told The Guardian.

Pluto and its largest moon Charon could have come together via a 10-hour “kiss-and-capture” encounter after a grazing collision