Britannica Sources used in this answer

How might Triton have been captured by Neptune?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Moons of Neptune: Triton — Triton Image of Triton, Neptune's largest satellite, captured by a camera aboard Voyager 2, 1989.

Triton is widely considered to be a captured object that originally formed as an independent icy planetesimal in the outer solar system, likely within the Kuiper belt.123

During Neptune’s early history, Triton’s orbit around the Sun may have brought it too close to the planet. It is hypothesized that Triton was slowed sufficiently to be captured through one of the following mechanisms:

Gas drag: Friction from Neptune’s extended atmosphere may have slowed the moon.1
Collision: An impact with one of Neptune’s existing moons could have reduced Triton's velocity, allowing it to be trapped.1

Following this initial capture into an elongated, retrograde, and highly inclined orbit, tidal interactions between Triton and Neptune gradually circularized its path. This process, which may have taken over a billion years, likely caused massive internal heating and melting within Triton. The capture event would have severely disrupted Neptune's original satellite system, potentially ejecting or destroying other moons and leaving the current inner moon system as fragments of the original bodies.13