For every mystery science solves, an equally baffling one takes its place. Case in point: the rings of Saturn, first seen by Galileo in 1610, and still not completely understood by astronomers.

When Galileo saw the rings, he likened them to “cup handles” attached to the sides of the planet. It was Dutch astronomer Christian Huygens who first recognized a ring around Saturn, in 1659. Then in 1675, Italian-born astronomer Jean Dominique Cassini realized there were two rings, now called A and B. We know Saturn has several rings; its ring system is the most complex in the solar system. Now, NASA’s Cassini mission has discovered that these familiar rings, known to humankind for hundreds of years, are changing. In fact, they differ radically from what the Voyager spacecraft observed in 1980 and 1981.

Here’s what we knew about Saturn’s rings: the seven rings seem to consist mainly of water ice, and extend hundreds of thousands of miles beyond the planet. Particles within the rings vary in size from a few micrometers to tens of meters, and within the rings are dark areas sometimes called ringlets or spokes, that seem to orbit Saturn at a different speed than other material in the rings. A few of Saturn’s 46 known moons orbit the planet within the rings.

Our long-standing image of Saturn’s moons may be incomplete, however. NASA’s Cassini mission, launched in 2004 to study Saturn, has yielded surprising results, which garnered widespread news coverage after being announced in early September. Saturn’s rings, it appears, are constantly changing. According to NASA’s Cassini webpage, the A ring, which is Saturn’s outermost main ring, seems to consist mainly of empty space. Particles within the ring cluster together in groups that change in response to Saturn’s gravitational forces. Also, the D ring has grown dimmer and moved 125 miles closer to the planet since it was observed by Voyager 25 years ago. In addition, astronomers found a spiral ring that circles the planet and crosses into the F ring. Scientists aren’t sure yet of the implications of this new information, but it does suggest one thing: that what we know about Saturn is only a small portion of its story, a story that seems to be continuously changing.