<p>The rings lose energy due to the Poynting-Robertson effect. Quoting from <a href="https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect#Source_of_the_effect" rel="nofollow noreferrer">Wikipedia</a>:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>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</p>
<ol>
<li>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.</li>
<li>Continuously sweep out small particles but let the large ones stay.</li>
<li>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.</li>
<li>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.</li>
</ol>
<p>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.</p>

CC BY-SA 4.0
2023-07-25T21:31:16.190Z
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
CC BY-SA 4.0
2023-07-25T21:33:55.127Z
That goes along with removing the small particles and not letting the large ones turn into small ones.
eshaya
CC BY-SA 4.0
2024-08-13T19:49:13.653Z
@eshaya I was reading again this interesting answer. I had some questions on it: are the 4 options that you mentioned required to be met simultaenously or just applying one of them would be enough? Also, would rings emit gravitational waves? Or there could be some rings where their symmetry would not emit any as they orbit a central point of mass?
vengaq
CC BY-SA 4.0
2024-08-14T15:37:00.693Z
@eshaya So getting the planet out of their star to avoid Poynting-Robertson effect wouldn't be enough? Then, unless we continually replenish the rings they will fall to the planet inevitably, even if we do not consider gravitational waves emission?
vengaq
CC BY-SA 4.0
2024-08-14T15:39:52.297Z
Constant replenishment will work on its own.  All accelerated objects emit gravitational waves.  You emit gravitational radiation each day starting when you get out of bed in the morning.  But, the energy involved in gravity waves under normal circumstances is so many orders of magnitude less than the many other forces going on in the Solar System that one can safely ignore it.
eshaya
CC BY-SA 4.0
2024-08-14T16:09:54.407Z
But I still have somewhat of a trouble understanding if rings would still gradually disappear due to its particles falling into the planet if we omit gravitational waves emission and any form of Poynting-Robertson effect. So, if we consider a ring around a planet that doesn't radiate and that is isolated (without a star) and we ignore for a moment gravitational waves emission, would then the rings still gradually fall towards the planet? @eshaya
vengaq
CC BY-SA 4.0
2024-08-14T16:16:57.643Z
The rocks in the rings grind each other into dust and the dust grains fall into the planet in a few million years.  If nature does not replenish them, then the rings will probably disappear.  But, tidal disruption of passing comets and asteroids or collisions with the rings and moons may be replenishing the rings.
eshaya
CC BY-SA 4.0
2024-08-14T16:30:26.267Z
@vengaq Ah, you asked if the planet did not radiate would the ring last?  If we ignore Poynting-Robertson and spin-orbit coupling with the planet, then there are dynamical forces, especially if there is any magnetism around, that will expand the ring.  The inner parts move inward and the outer parts move outward.  Without replenishment the density becomes lower and lower.
eshaya
CC BY-SA 4.0
2024-08-14T16:34:46.783Z
@eshaya would that be due to a magentic field of the planet? If it didn't have a magnetic field as well, would rings still be moving inwards and outwards? Would that movement ever stop or the parts going inwards would fall into the planet and the ones going outwards will be expelled from the orbit? Also, If the density of the rings was high enough from the outset will the ring be stable?
vengaq
CC BY-SA 4.0
2023-07-25T20:59:24.070Z
2023-07-26T12:21:20.487Z
2023-07-26T12:21:20.487Z
eshaya
<p>Apparently, Saturn is losing its rings (<a href="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" rel="nofollow noreferrer">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</a>) 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?</p>

CC BY-SA 4.0
2023-07-19T16:17:07.737Z
Related: https://astronomy.stackexchange.com/questions/34849/are-saturns-rings-stable https://astronomy.stackexchange.com/questions/8616/recent-origin-of-saturns-rings https://astronomy.stackexchange.com/questions/8112/how-long-do-planetary-rings-last
planetmaker
<planet><astrophysics><orbital-mechanics><planetary-ring><saturn-rings>
Can rings last around planets indefinitely?
vengaq
# Can rings last around planets indefinitely?

(Asked by: username_0 on 2023-07-19)

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?

**Tags:** planet, astrophysics, orbital-mechanics, planetary-ring, saturn-rings
> **Comments:**
> **username_1**: Related: https://astronomy.stackexchange.com/questions/34849/are-saturns-rings-stable https://astronomy.stackexchange.com/questions/8616/recent-origin-of-saturns-rings https://astronomy.stackexchange.com/questions/8112/how-long-do-planetary-rings-last  

---

## 1 answers

### Answer by username_2 on 2023-07-25. Score: 1

The rings lose energy due to the Poynting-Robertson effect. Quoting from [Wikipedia](https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect#Source_of_the_effect):

> 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.

> **Comments:**
> **username_3**: 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?  
> **username_2**: That goes along with removing the small particles and not letting the large ones turn into small ones.  
> **username_0**: @eshaya I was reading again this interesting answer. I had some questions on it: are the 4 options that you mentioned required to be met simultaenously or just applying one of them would be enough? Also, would rings emit gravitational waves? Or there could be some rings where their symmetry would not emit any as they orbit a central point of mass?  
> **username_0**: @eshaya So getting the planet out of their star to avoid Poynting-Robertson effect wouldn't be enough? Then, unless we continually replenish the rings they will fall to the planet inevitably, even if we do not consider gravitational waves emission?  
> **username_2**: Constant replenishment will work on its own.  All accelerated objects emit gravitational waves.  You emit gravitational radiation each day starting when you get out of bed in the morning.  But, the energy involved in gravity waves under normal circumstances is so many orders of magnitude less than the many other forces going on in the Solar System that one can safely ignore it.  
> **username_0**: But I still have somewhat of a trouble understanding if rings would still gradually disappear due to its particles falling into the planet if we omit gravitational waves emission and any form of Poynting-Robertson effect. So, if we consider a ring around a planet that doesn't radiate and that is isolated (without a star) and we ignore for a moment gravitational waves emission, would then the rings still gradually fall towards the planet? @eshaya  
> **username_2**: The rocks in the rings grind each other into dust and the dust grains fall into the planet in a few million years.  If nature does not replenish them, then the rings will probably disappear.  But, tidal disruption of passing comets and asteroids or collisions with the rings and moons may be replenishing the rings.  
> **username_2**: @vengaq Ah, you asked if the planet did not radiate would the ring last?  If we ignore Poynting-Robertson and spin-orbit coupling with the planet, then there are dynamical forces, especially if there is any magnetism around, that will expand the ring.  The inner parts move inward and the outer parts move outward.  Without replenishment the density becomes lower and lower.  
> **username_0**: @eshaya would that be due to a magentic field of the planet? If it didn't have a magnetic field as well, would rings still be moving inwards and outwards? Would that movement ever stop or the parts going inwards would fall into the planet and the ones going outwards will be expelled from the orbit? Also, If the density of the rings was high enough from the outset will the ring be stable?  

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