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.
This color photo of Neptune's largest moon Triton was obtained by NASA's Voyager 2 probe on Aug. 24, 1989, from 330,000 miles away. The resolution is about 6.2 miles, sufficient to begin to show topographic detail. CREDIT: NASA/JPL
Tritón creciente. Es una luna retrógrada (gira en sentido contrario al resto de los satélites de Neptuno), lo que hace pensar que se trata de un "cuerpo capturado" por la gravedad de Neptuno, y que quizá se formo en otro sitio (¿el cinturón Kuiper?).
Triton (Voyager 2) [Wikipedia] is the largest natural satellite of the planet Neptune. 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.