NEPTUNE
The isolated azure ice giant remains a relative mystery
Neptune was the first planet to have its existence predicted by mathematical calculations before it was actually seen through a telescope on 23 September 1846. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body was responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune. Astronomer Galileo Galilei had sketched the planet, but he mistook it for a star due to its slow motion.