Neptune’s largest moon, Triton, is the only large moon in our solar system that orbits in a direction opposite to the rotation of its planet. This is known as retrograde orbit. Scientists believe that Triton is a ‘captured moon’; that is, Triton did not form locally but was captured by Neptune’s gravity as it passed by. In about 3.5 billion years Triton will wander too close to Neptune and break up, resulting in a ring system.
(https://bit.ly/2CZt9Zw, https://bit.ly/2D2ZnTH)