# Can rings last around planets indefinitely?

Apparently, Saturn is losing its rings (https://weather.com/en-IN/india/space/news/2023-05-04-saturn-is-losing-its-rings-webb-may-tell-us-how-long-they-have-left) However, is there any way or conditions where the rings around a planet would last indefinitely? Do rings emit gravitational waves that would relax their orbit and eventually make them crash into the planet? Or an axially asymmetric ring around a planet would avoid the emission of gravitational waves?

• Commented Jul 19, 2023 at 16:17

## 1 Answer

The rings lose energy due to the Poynting-Robertson effect. Quoting from Wikipedia:

From the perspective of the grain of dust circling a star, the star's radiation appears to be coming from a slightly forward direction (aberration of light). Therefore the absorption of this radiation leads to a force with a component against the direction of movement.

Basically, small particles lose velocity if you shine light on them. I suppose the small particles then put a drag on the larger particles. You ask, how to keep rings around planets going

1. Move planet away from any starlight. You not only have to remove the planet from the star, but you need to cool the planet, so that it emits no micron sized infrared light.
2. Continuously sweep out small particles but let the large ones stay.
3. Guide all large particles away from each other so there are no collisions (which can generate more small particles). Or cover them with rubber bumpers.
4. Replenish the rings by bringing in asteroids at the outer edge and let them crash into each other creating fresh ring material. This is probably the best option, since nature may be doing this all along.

Another option is to untilt the planet so the rings are always edge-on to the sun. This reduces the effect by self-shading, but it does not eliminate it completely.

• I see (+1!) it's the smallest particles that experience angular momentum loss and subsequently remove angular momentum from larger particles via collisions? That explains 1. and 2. But what is the purpose of the "rubber bumpers" in 3.? They change inelastic collisions to elastic collisions between large particles, but how does that reduce angular momentum loss rather than simply lower the "temperature" of the particle motion?
– uhoh
Commented Jul 25, 2023 at 21:31
• That goes along with removing the small particles and not letting the large ones turn into small ones. Commented Jul 25, 2023 at 21:33