The largest rings around planets are expected to form moons, and not stay as rings. The largest ring in our solar system evidently does not care too much about scientific rules.
By using the NASA spacecraft WISE, Wide-field Infrared Survey Explorer, astronomers were able to capture images of Saturn’s largest ring. It was discovered in 2009, and was named Phoebe. Phoebe is also one of Saturn’s most distant moons, which is similar in composition to the ring Phoebe.
Previous theories have been that such large rings have to form moons eventually, so when Phoebe turned out to be full of smaller objects and dust, astronomers had to rush back to their calculations and start finding other explanations.
The study lead author, Douglas Hamilton, a planetary scientist at the University of Maryland, explains a test scenario they did to understand how the ring was even possible: “I constructed theoretical models for how the dust particles of different sizes would move and then I constructed artificial rings. What would the ring look like if it was made only of large particles? What about mixture? The data is good enough to constrain those models so we rule out the possibility of the ring being composed entirely of large particles.”
It’s not easy to detect the ring with the help of visible light, which could be solved by capturing infrared light with WISE instead. This is because the grains that form the Phoebe ring absorb sunlight, so looking for heat is a much easier detection system to use.
Phoebe is 270 times larger than the radius of Saturn, and can with good margins count itself as one of the “large, moon-forming rings” previously described as the norm in educational textbooks.
The study was published in the journal Nature.