Why it's so special: Only one spacecraft has ever visited the eighth and most distant planet from the sun. On Aug. 25, 1989, NASA's Voyager 2 spacecraft took the first-ever close-up images of Neptune.

James Webb found Neptune's moon Nereid has an unusual icy surface, suggesting it formed alongside the planet instead of being captured later.

But so far, none of the big space agencies have picked up the idea as a fully-fledged mission concept. Though we will eventually send another probe to Neptune at some point, unless one of them does ...

Before the Voyager 2 flyby of Uranus and Neptune, these two distant planets were thought to be cold, dead worlds on the outskirts of the solar system. The historic spacecraft visit, however, revealed ...