I was more than a little sceptical when I first heard this question. How do you define what makes a separate ring?
However, plenty of websites will tell you that Uranus has 13 rings, Saturn has seven, Neptune six, and Jupiter three.
Saturn's rings are of course the most spectacular, and were discovered in 1610 by Galileo Galilei.
William Herschel described a possible ring around Uranus in 1789. This sighting is generally considered doubtful, because the rings are quite faint, and it was nearly 200 years before anyone else observed them (using vastly superior technology). But Herschel made an accurate description of what's now known as the epsilon ring: its size, its angle relative to Earth, its red colour, and its apparent changes as Uranus travelled around the Sun.
Uranus's ring system was definitively discovered in 1977 using the Kuiper Airborne Observatory (which operates from an aeroplane). A team of American astronomers were studying the atmosphere of Uranus, by observing its occultation of a particular star. When their observations were analysed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind Uranus. They concluded that there must be a ring system around Uranus. Later they detected four additional rings (making a total of nine).
In 1986, Voyager 2 confirmed the existence of 11 rings around Uranus. Then in 2005, the Hubble Telescope detected two more; these are so far from the planet that they are called the 'outer' ring system. The outer one is twice as far from Uranus as the previously known rings. The outer rings are brightly coloured (one blue and one red), while the inner ones are dark grey.
Seven years before its observation of Uranus, Voyager 2 had discovered a faint ring system around Jupiter. This is made up of three parts: the gossamer rings, the halo ring and the flat main ring. The flat ring is 4,000 miles wide, and the halo ring is approximately 12,400 to 25,000 miles thick. (Note that one of these parts seems to have more than one ring; this would seem to justify my initial scepticism, as it seems that Jupiter may have more than the three rings it's commonly credited with.)
After passing Uranus, Voyager 2 proceeded to Neptune, where it confirmed the presence of six rings in 1989. The three prominent arcs have been named Liberty, Equality and Fraternity. The rings extend 38,525 miles from the planet – compared to a planetary radius of about 15,000 miles).
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