First Planet Found by Telescope Was Uranus

Uranus was the first planet found with the aid of a telescope. English astronomer William Herschel noticed it in 1781. He first took it for a comet or a star, and it was not accepted as a planet until two years later.

The older planets, from Mercury to Saturn, are bright enough to see without a telescope, so people knew them for thousands of years. Uranus orbits about 2.9 billion km from the Sun, roughly 19 times farther than Earth, which is why it took a telescope to reveal it.