Mini-moons producing propellor-shaped wakes in Saturn’s rings, hundred of times larger than similar objects discovered in 2006, have been identified in the planet’s A-ring by NASA’s Cassini probe. The existence of the ‘propellors’ could also point to behaviour exhibited by protoplanetary discs, including the disc that formed the Solar System 4.6 billion years ago.

Four years Cassini scientists announced that they had discovered small propellors in the inner region of the A-ring, but since then they have been using Cassini to track even larger propellor features in the outer part of the ring. The propellor shapes are wakes in the ring carved out by the gravity of small moonlets, the gaps extending several thousand kilometres in length and kicking ring particles half a kilometre above and below the plane of the rings (which are, on average, only ten metres thick). Although the moonlets at the heart of the propellors are too small to be resolved even by Cassini, based on the size of the propellors they have been estimated to be about one kilometre across. Eleven such giant propellors, which have been given nicknames such as Earhart and Bleriot after famous aviators, have been identified by Cassini, and there could be dozens more. But where do they come from, and why are they only found in the A-ring?

One theory, proposed in 2007 by Miodrag Sremcevic of the University of Colorado at Boulder, suggests that they are the remnants of a shattered asteroid or small moon. This would explain why the small propellors are found in narrow regions, or belts, of the ring, rather than spread all over. Matthew Tiscareno, a research associate at Cornell University in New York who led the Cassini observations of the propellors, thinks that the propellors may have once existed all over the A-ring, which is a better environment for forming moonlets compared to the B- and C-rings because it is farther from Saturn and the planet’s gravitational tides that could pull moons apart. “They may have either been destroyed in regions other than the propellor belts, or have for some reason migrated to the propellor belts,” Tiscareno tells Astronomy Now. Even in the last four years, Cassini has noticed some movement in the orbits of the large propellors, possibly because they are encountering denser patches of the A-ring, or are being perturbed by the gravity of larger moons nearby.

The extra attraction of the propellors is that they may help tell us about the processes in protoplanetary discs that form planets around other stars, and which formed the planets in the Solar System. Saturn’s rings have often been held up as an analogy of a protoplanetary disc in miniature, and if we could look in-depth at these discs we might see similar propellor-shaped features.

“Scientists have never tracked disc-embedded objects anywhere in the universe before now,” says Tiscareno. “All the moons and planets we knew about before orbit in empty space. In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects. With this new discovery, we can now track disc-embedded moons individually over many years.”