Neptune

The ice giant Neptune was the first planet located through mathematical prediction rather than through regular observations of the sky.

Nearly 4.5 billion kilometers (2.8 billions miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the name eye because of it's extreme distance form Earth. Interestingly, the unusual elliptical orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year period out of every 248 Earth years. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the Sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies.

Neptune's blue color is the result of methane in the atmosphere. Uranus's blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.