8. NEPTUNE Neptune is the eighth planet from the Sun and like the other gas giants Jupiter, Saturn and Uranus, is composed chiefly of hydrogen and helium. No human has ever lived as long as it takes Neptune to orbit the Sun – about 165 Earth years. This is Proteus – Neptune’s second largest moon. There are thirteen in all. Proteus was discovered in 1989 when Voyager II flew by it. It’s still the only spacecraft to have visited Neptune so far. This is Triton – Neptune’s largest moon. It was discovered seventeen days after Neptune in 1846. It’s one of the only moons in our solar system that’s still geologically active and contains geysers that spit nitrogen gas. Neptune is a stormy place. Here’s the Great Dark Spot discovered in 1989 which seems to have vanished since then.