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.

Billions of years ago, two icy worlds—Pluto and Chron—collided in the farthest reaches of our solar system. But rather than smashing into pieces, Pluto and its largest moon, Charon, briefly spun together like a celestial snowman before separating, forever linked in orbit.