Neptune ♆

Neptune's atmosphere is composed of hydrogen, helium, and methane, similar to Uranus. However, Neptune appears a deeper, more vivid blue, suggesting an additional unknown chromophore contributes to its color. The planet has the strongest sustained winds of any planet in the solar system, with speeds exceeding 2,100 km/h. The Great Dark Spot, observed by Voyager 2 in 1989, was an anticyclonic storm similar to Jupiter's Great Red Spot, though it had disappeared by the time Hubble observed Neptune in 1994.

Neptune is the most distant known planet in our solar system, orbiting at an average distance of 30 AU from the Sun. A year on Neptune lasts 164.8 Earth years. The planet has a strong magnetic field tilted 47 degrees from its rotational axis and offset from the center. Neptune has 16 known moons, the largest being Triton, which orbits in a retrograde direction -- suggesting it was captured from the Kuiper Belt. Triton has active geysers that eject nitrogen gas, making it one of the few geologically active moons in the solar system.

Voyager 2 is the only spacecraft to visit Neptune, making its closest approach on August 25, 1989. The flyby revealed a dynamic atmosphere with enormous storms, discovered six new moons and several ring arcs, and provided detailed images of Triton's geologically young surface. No follow-up missions to Neptune have been approved, though several concepts have been proposed, including a Neptune orbiter with a probe to study the atmosphere and a Triton lander.