Answer: Uranus

Out of all the planets in our solar system, only three of them have been officially discovered. The rest of the planets in the solar system are observable, unaided, by the human eye. Although early man may not have had any idea that Mars was a planet and not a very bright red star, it certainly wasn’t discovered by any specific person.

The introduction of the telescope in the 17th century changed all of that though. People began looking towards the sky with an enhanced eye, probing the secrets of the solar system. Early astronomers observed the same things their ancestors had, but they simply did so with a radically more powerful gaze that revealed planetary rings and orbiting moons.

It wasn’t until the late 18th century, however, that an astronomer chanced upon an intrasolar object that had never been recognized as a planet before (it was generally mistaken for a star by previous observers). In 1781, Sir William Herschel—not only a famed astronomer, but also a talented musician and prolific composer—was the first person to see the distant planet of Uranus as something more than a star and, in doing so, became the first person to officially discover a planet in our solar system. For the curious, the other discovered planets are Neptune (discovered in 1846 by Johann Galle and Urbain Le Verrier) and Pluto (discovered in 1930 by Clyde Tombaugh and downgraded to “dwarf planet” in 2006).

Herschel was a dedicated astronomer and invested a significant portion of his life into the science. He cast, ground, and polished more than 400 mirrors for telescopes, and with his assistants, built and sold at least sixty complete telescopes of various sizes. He also built a 40-foot reflecting telescope with a diameter of 49.5 inches. Shortly after the 40-foot “monster” was operational, he discovered a previously unobserved moon (Mimas) in orbit around Saturn.

In addition to discovering Uranus, Herschel also discovered two of Saturn’s moons (Mimas and Enceladus) and two of Uranus’ moons (Titania and Oberon). Herschel also laid the foundation, through his observations and calculations, of modern binary star astronomy. In addition to his work discovering Uranus and other intrasolar objects, Hershel discovered that the Martian ice caps changed size with that planet’s seasons, discovered infrared radiation, and was the first to propose a model of the galaxy based on observation and measurement, concluding that it was in the shape of a disk. Due to the vast reach of his work and the contributions he made to the foundations of astronomy, Herschel is widely regarded as the greatest astronomer of the 18th century.