neptune the planet
Neptune's cloud has an especially vivid blue tint that is partly due to an as-yet-unidentified compound and the result of the absorption of red light by methane in the planets mostly hydrogen-helium atmosphere. Photos of Neptune reveal a blue planet.
The eighth planet from the sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.)
When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Joseph, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus's orbit.