Look at this picture of Saturn. Can you see the biggest ring? Are you sure? Saturn's rings have always had something of an air of mystery -- but astronomers had at least been fairly confident in their observations of those rings and their relative size. The brightest and most opaque sections were assumed to be the largest, while the parts that were dimmer and more transparent were assumed to be sparser. Then researchers figured out how to weigh those rings, and found something surprising.
It's not as simple as stacking everything on a scale. Rather, researchers had to come up with a formula to get measurements of spiral density waves taken by NASA's Cassini mission from inside of the ring, to get a series of weights. Then they compared the heavier-looking portions of the rings with the lighter ones. They found that, although parts of the rings certainly looked less transparent and considerably brighter than their surroundings, their weight was more or less the same throughout.
What's going on here? Although researchers are fairly comfortable pointing to the fact that it is an illusion, the cause of it remains unclear. What is much clearer is that it's not just about ring size, it's also about age. The estimates of the age of Saturn's rings have been based in a large part on their size. Now those may have to be revised, perhaps by quite a lot.
When all the interplanetary dust has settled, NASA estimates that the age estimate could drop from somewhere in the billions to closer to a relatively youthful couple hundred million years.
Top image: A downward view of Saturn's rings / NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic; Middle image: Detailed cross-section of Saturn's rings / NASA/JPL/Space Science Institute