First planet discovered by telescope

The first planet to be discovered using a telescope was Uranus, which was recognized as a planet in 1781, based on telescope observations by Sir William Hershel (UK) and others.

The observations that established Uranus as something other than a regular star were made on 13 March 1781 by Sir William Herschel. He was using a state-of-the-art 2.1-m-long (7-ft) reflecting telescope with a 15.2-cm (6-in) mirror, which he made and installed at his home in Bath, UK. He realized that the point of light known in older star catalogues as "34 Tauri" was in fact something much closer.

He published his findings in a letter to the Royal Society the following month, initially reporting it as a probable comet. Later that year, Anders Johann Lexell (working in St Petersburg, Russia) and Johann Elert Bode (working in Berlin) made follow-up observations that led to the conclusion that the object was in a near-circular orbit, and therefore almost certainly a planet.

Herschel initially suggested calling the new planet Georgium Sidus (George's Star) after the British King, but unsurprisingly this was not a popular choice internationally. Most astronomers called the planet "Herschel" until 1782, when Bode suggested naming the planet after Ouranos, the Greek god of the sky. The name was quickly accepted, although the British government's Nautical Almanac insisted on referring to the planet as Georgium Sidus until 1850.