Triton, the largest moon of Neptune. Because Triton has a retrograde orbit (opposite that of it's planet), it was likely captured by Neptune's gravitational pull from the Kuiper Belt. It's about the same size and has a very similar makeup to Pluto, another Kuiper Belt object. Triton's orbit is slowly shrinking, and it is expected that in about 3.6 billion years, it will either collide with Neptune or be destroyed, forming a Saturn-like ring system.
Dark, cold & whipped by supersonic winds, Neptune is a hydrogen & helium gas giant in our solar system. Over 30 times as far from the sun as Earth, it takes almost 165 Earth years to orbit our sun. On Neptune, one "day" (time it takes to rotate once on its axis) takes 16 hours. It has 13 moons & 6 rings.