What planet in the Solar System is not visible to the naked eye and was discovered mathematically rather than empirically?

Neptune is the eighth and furthest planet in our Solar System. It's dark, frigid, and blasted by supersonic winds. Since its discovery in 1846, Neptune completed its first 165-year orbit in 2011. Because of Neptune's distance from the Sun, the high noon on the great blue planet would appear to us like a gloomy twilight.

Asked by Vasilii Kulev · Last updated 4 years ago · 2.3K views · SOURCE