Credit: Voyager 2/NASA
Beautiful blue Neptune is, it seems, a shy planet, being very far out in the solar system. Discovered in 1846, its existence was deduced from mathematical prediction, not from being directly observed. Irregularities in the orbit of Uranus led astronomers to think that another planet must be gravitationally affecting Uranus. Their prediction turned out to be correct.
Credit: Voyager 2/NASA
The methane in Neptune's atmosphere absorbs red light, giving the planet its lovely blue hue. In 1989, Voyager 2, the only space craft to visit this distant world, recorded the Great Dark Spot in Neptune's atmosphere (upper left in the image above). But unlike Jupiter's persistent Great Red Spot, the dark spot was gone a few years later, based on Hubble observations, and a new spot had appeared elsewhere.
Credit: Voyager 2/NASA
Neptune in a gas giant planet and has the strongest winds in the solar system; they reach supersonic speeds of 1,300 mph. The image above shows light-colored clouds high in the atmosphere. Most of the atmosphere is hydrogen and helium, but scientists think that methane closer to the interior of the planet may condense out into diamonds, not only on Neptune but Uranus as well: "Once these diamonds form, they fall like raindrops or hailstones toward the center of the planet," says one of the researchers. 
Credit: Voyager 2/NASA
Neptune has about a dozen moons, and pictured above are Neptune and Triton as crescents, a view never seen from Earth. The unusual angle of the photo yields a slightly reddish planet. What can't be seen are Neptune's rings, which were only discovered in the 1960s.
Credit: Voyager 2/NASA
Triton is Neptune's largest moon, and this color mosaic image shows its surface in great detail. Triton has the coldest surface temperature of any body in the solar system, minus 391 degrees Fahrenheit. It's so cold that nitrogen exists as frost.Many of these facts about Neptune and Triton were new to me, but maybe you remember a PBS special from 1989, Neptune All Night. It was a live, seven-hour show featuring data and images from Voyager's fly-by as they were coming back to Earth. It's up on YouTube (that's where the link goes), although the video quality isn't great. Still, I love the idea of staying up all night to watch brand-new images from the very edge of the solar system. Slowly the planets are giving up their secrets, even the shy one.
