Much like Uranus, Neptune has only been explored by Voyager 2. With the passing of Voyager 2, a very large amount of scientific information about Neptune was gained such as it's physical makeup. It was discovered by Urbain Le Verrier around 1845. The planet it surrounded by a scarce series of rings that are barely visible. Neptune is technically the furthest planet from the sun. This distinction was made after Pluto was declassified to a dwarf planet in recent years. Regardless of the Voyager mission, there is still much more to be discovered about the planet. Being the furthest proper planet from the sun, it takes years to get to so exploration is not easy.
Neptune from Voyager 2