Answer: Neptune

When it comes to human understanding of our solar system, first came direct sight, then came optically-assisted sight via telescope, and finally we began mathematically predicting the presence of planets based on the laws of physics.

The first such planet to be predicted by math was Neptune. In 1846, calculations by French mathematician Urbain Le Verrier were used to locate the planet Neptune by telescopic observation. The observation was a huge event within the scientific community as it was a dramatic confirmation that Newtonian physics (gravitational theory) and the accompanying field of celestial mechanics could be used to predict the existence and position of planets.

What’s interesting about the “discovery” of Neptune, as it were, is that the observation in 1846 was not the first time that the distant and dim planet had been observed by telescope. There is evidence of multiple observations over the preceding two hundred years, but prior to 1846, no one suspected it was a planet; Galileo himself merely thought it was a fixed star far outside our solar system.