The Importance of Planetary Ring Systems as Astrophysical Laboratories
Blog report on the Seminar ‘The Importance of Planetary Ring Systems as Astrophysical Laboratories’ by Dr Phil Sutton
Most people are aware of the planet Saturn’s defining feature being its extraordinary planetary ring system. Fewer people know that Saturn is not the only planet within our solar system that harbours its own ring system - along with Saturn, Neptune, Uranus and Jupiter all have rings orbiting their atmospheres. However their rings are not as noticeable as Saturn’s, with Jupiter’s rings not being discovered until the Voyager 1 flyby in 1979 [1]. Even fewer people still know that ring systems are not limited to planets. Some asteroids are also capable of withholding and sustaining their own ring systems [2]. But what do these rings look like up close, and how did their structures even come to exist in the first place?
To answer this, we can turn our attention back to our most well-studied ring system. Saturn’s rings are so dense that they can be seen from the surface of the Earth, allowing us to observe and study them relentlessly. As Dr Phil Sutton discussed in his lecture ‘The Importance of Planetary Ring Systems as Astrophysical Laboratories’ held on Wednesday 15th October 2025, it is easier to imagine the rings as a collection of particles of various sizes orbiting the planet in a common direction. These particles are very fast-moving, and so it is difficult to measure them directly. Instead, there is another method to indirectly but still accurately measure the sizes of particles in the system. We can look at the way that light scatters off them. Smaller sized particles are more likely to forward scatter light, and larger particles more likely to backscatter light. These scattering effects can give us an insight into the composition of the system, which in turn can give us hints into how they formed.
Although they are very fast moving, orbiting in the same direction as the planet’s rotation, there is evidence to suggest that the movement is not constant throughout - parts of the rings move at different speeds. This phenomenon, known as Keplerian Sheer, means that each individual particle orbits along its own trajectory. It has been observed that the closer the ring particle to the planet, aka the inner ring, the faster it will be orbiting. Likewise, particles further away from the planet on the outer edges of the ring will orbit slower. This happens because of the gravitational influence from the planet onto the particles - if the particles were moving any slower, they would fall into the planet.
Like many other planets, Saturn has moons. However these moons are embedded within the ring system, likely forming within the rings themselves, and they affect how the rings evolve and develop over time. These embedded moons, if large enough, create gaps within the rings as they orbit, and are otherwise known as Shepherding moons as they ‘flock’ ring particles together [3]. As they follow their trajectory, they gravitationally scatter nearby ring material, leaving a cleared path in their wake. If there is asymmetry in the moon’s orbit, this can sometimes cause waves to occur within the rings around the moon. If a moon is not large enough to fully clear a gap within the rings, it instead creates a more localised interference in a shape resembling that of a propeller, thus they are often referred to as propeller structures.
Not all gaps in Saturn’s rings are created by Shepherding moons. Resonances between the moons and the rings can also cause gaps to form. For example the Cassini Division is formed by a 2:1 Mean Motion Resonance with the moon Mimas (it orbits once for every two orbits of a ring particle) [4]. Despite its small mass, Mimas is able to create such a large gap within the rings of Saturn due to large spiral density waves that allow a gap to be maintained. As well as gaps, some resonances can also form spiral-like structures within the rings. In his more recent research, Dr Sutton has been investigating the origins of a gap in the ring system of a large gas giant of the name J1407b. From obtaining the lightcurve of the star J1407, which is a graph that shows how the brightness of an object changes over time, it became clear that the planet had a ring system due to the many fluctuations in brightness - likely the result of the rings partially blocking out light. After running simulations for a resonance of 2:1, and later for a resonance of 3:1, none of the results were found to support the initial observations, and so it is unlikely that the gap was caused by orbital resonances with a moon outside of the ring system. Afterwards, Dr Sutton investigated the idea of a moon within the rings causing the gap, however the unusually high ellipticity (how elliptical, or non-circular, the orbit is) resulted in a fairly destructive environment for moon formation. Subsequent models showed that it is almost impossible for moons to form within the ring at such an elliptical orbit, and the ones that did form had very short lifetimes. Thus, Dr Sutton has entertained the idea that perhaps the initial observation was incredibly well-timed and is actually the result of a short-lived phenomenon that occurred at the time of observation. If not, perhaps it is a phenomenon that requires further study and observations of exoplanet ring-systems. Thus the study of planetary ring systems, even in nearby planets like Saturn, still remains a highly relevant topic of astrophysics.
[1] Dartnell, L. (2025). Not just Saturn! Astronomers have found rings around strange, icy worlds on the edge of the Solar System. [online] BBC Sky at Night Magazine. Available at: https://www.skyatnightmagazine.com/space-science/rings-outer-solar-system [Accessed 19 Dec. 2025].
[2] Asteroids can have rings, too. (2014). Nature. doi:https://doi.org/10.1038/nature.2014.14937.
[3] www.esa.int. (n.d.). Shepherd moon and flock. [online] Available at: https://www.esa.int/Science_Exploration/Space_Science/Cassini-Huygens/Shepherd_moon_and_flock.
[4] Goldreich, P. and Tremaine, S. (1978). The formation of the Cassini division in Saturn’s rings. Icarus, 34(2), pp.240253. doi:https://doi.org/10.1016/0019-1035(78)90165-3.
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Presentation: Date, title and name of speaker were present. Good grammar. No improvements needed.
Content: Seminar content covered in full. No improvements needed.
Context: Research context adressed. Societal context not adressed. Can be improved by explaining societal context.
Style: Not all key terms are fully explained (e.g., orbital resonance). To improve the blog in regards to lay audience suitability, key terms should be explained.
External source: Evidence of external sources but no quote. To improve, add a quote from one of your sources.
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Your explanations and analogies are great! The only minor change could be shorter sentences.