Why is Triton’s retrograde orbit important evidence about its origin?

Triton’s retrograde orbit—meaning it revolves around Neptune in the direction opposite to the planet's rotation—is significant because it suggests that the moon did not form in its current location alongside the planet. Instead, this unusual orbital characteristic, combined with its high orbital inclination, provides strong evidence that Triton was an independent planetesimal that formed elsewhere in the solar system and was later captured by Neptune’s gravity.1

Scientists believe that during Neptune’s early history, Triton’s path around the Sun brought it too close to the giant planet. Through processes such as gas drag or a collision with a pre-existing moon, Triton was slowed and pulled into its current orbit. Over the following billion years, tidal interactions between the two bodies gradually circularized the moon's path, a process that likely melted Triton's interior and led to its current differentiated structure.1