"Neptune's moon Triton used to be paired with another object, but was torn from its companion during capture by the eighth planet's gravitational field."
http://news.bbc.co.uk/2/hi/science/nature/4758091.stm"Dr Buie explained that Pluto seemed to be very similar to Neptune's moon Triton, which is thought to be a Kuiper belt object captured by Neptune's gravity. This is despite the fact that the process of capture should have altered Triton's surface drastically through heating.
"I'm surprised Triton and Pluto aren't more different than they are," he told the BBC News website."
http://news.bbc.co.uk/2/hi/science/nature/4230918.stmseems to me, this might be the answer.