Triton is, with a diameter of 2,700 km, the largest of Neptune’s 13 moons. It is the only large moon in the Solar System with a retrograde orbit (an orbit in the opposite direction to its planet’s rotation), which cannot have formed out of the same region as Neptune, so it must have been captured from elsewhere. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been captured from the Kuiper belt.
Historic Milestone: Pluto Mission Reaches Neptune's Orbit AUG 25, 2014 // BY IAN O'NEILL: Discovery News NASA New Horizons' view of distant Neptune and Triton. NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE