We have discussed before that all the gas planets in the solar system have rings. Even through a small telescope Saturn has visible rings, but Jupiter, Uranus and Neptune do not. So how did astronomers discover their rings in the first place?
Hubble image of Uranus and its rings
The rings around Uranus were discovered in 1977. Astronomers knew that Uranus was going pass in front of a distant star in the night sky, from Earth's perspective. They pointed their telescopes to towards the planet each night, and expected to see the planet block the light from the star only when the star was directly behind the planet. What they actually observed was the star flickering right before and right after is passed behind the planet. This meant that there must be some unseen object near the planet blocking the starlight! The only plausible explanation was that Uranus has very thin, dim rings that are not visible from telescopes here on Earth. In 1986, Voyager flew by Uranus and imaged the rings for the first time, proving their existence. Since then, we have discovered rings around Jupiter and Neptune in similar ways.