Saturn. Saturn's rings are the most extensive ring system of any planet in the Solar System, and thus have been known to exist for quite some time. Galileo Galilei first observed them in 1610, but they were not accurately described as a disk around Saturn until Christiaan Huygens did so in 1655.
Explanation:
That's right, Venus (and the planet Mercury) are the only two planets that don't have a single natural moon orbiting them. Figuring out why is one question keeping astronomers busy as they study the Solar System.
