Why do scientists think Triton was captured by Neptune?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Scientists believe that Triton was captured by Neptune rather than forming in orbit around the planet. This conclusion is primarily based on the moon's unusual orbital characteristics, specifically its retrograde orbit—meaning it moves in the opposite direction of Neptune’s rotation—and its highly inclined orbital plane.1

Researchers hypothesize that Triton originally formed as an independent icy planetesimal in the outer solar system, similar to the dwarf planet Pluto. During Neptune's early history, Triton likely passed too close to the planet, where it was slowed by gas drag in Neptune’s extended atmosphere or by a collision with a pre-existing moon. This loss of energy placed Triton 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 geological differentiation.1