Pluto is the furthest planet from the sun, and has not yet been visited by any probes, what is known that Pluto has an elliptical orbit, which actually brings it inside the orbit of Neptune.
There is currently some debate over whether Pluto can be classed at a actual planet, due to its tiny size, the most likely case is that Pluto & its companion Charon, are merely large fragments of rock that are being held in orbit.
