I need some help with a couple of the planet building theories I’m playing around with. I’ve looked up most of this stuff and can follow the physics if given the time (and instructional tutuorial help book) so if possible can you provide a laymans answer for a quick read and the links to the physics so I can work out the numbers later?

Background Information:

As far as possible, the planet is earth sized if maybe slightly smaller, similar gravity, distance from the sun, atmosphere, it has two Moon's outside the Roche limit (one large, one small) etc. It is in the goldilocks zone, supporting complex life, thankfully. Unfortunately it is prone to periodic meteor showers and not always the pretty kind. It has the scars to prove it. Oh, and just to be cliché, it has planetary rings (with a few tiny shepherd moons)!

The main idea I need to keep for my story is periodic bombardment from the same source. The rings are optional, if it really isn’t feasible. However, I would really very much like to keep them as currently they are the source of my story.

Most of the settlements I want to affect by the bombardment are around the equator and low-mid latitudes. I’m working on the premise that it’s normally just a pretty sight in the sky but occasionally one or two larger events occur. And every few centuries several make it through causing chaos in the lands below. I’m very destructive! FYI not extinction level events. I’m not that destructive! Merely a few shattered cities, accidently crashing into (probably be safer to crash near rather than into) a volcano setting off a small localised lava flow, breaching natural sea walls, tsunamis and the like!

Reading up on my physics and your previous posts about planets with planetary rings, earth sized planets don’t have the gravity to maintain a ring system for any extended period of time (geologically speaking). Any small dust particles/rocks/ice that make up the thin (10-100m thick) ring within the Roche limit will ultimately end up either falling to the earth type planet or flying off into space.


So if I want the rings to exist for at least 2-3 thousand years preferably longer (e.g. 10-20 thousand years...) with some sort of periodic bombardment?

I have two possible theories:

(1) The planet’s orbit takes it near/through the very outer reaches of an asteroid belt region producing spectacular annual or decadal displays (think Anne McCaffery’s Dragons of Pern series of Thread or Stargate’s Hundred days episode – not the best but you get the idea). Could a periodic immersion in the outer dusty reaches of an asteroid belt, resupply the ring system or would that just wipe out the whole thing altogether?

(2) If an asteroid belt doesn’t work, could it be the trail of a comet tail with a large but regular orbit (more frequent than haley’s comet and much closer) that gets caught up to form the ring system?

The way I see it both these theories allow for periodic bombardment by meteor showers/comet tail fragments of the size of Tunguska 1908 event and slightly bigger to occur.

Which is more likely?

One image I have in my head: Could they dive through the rings and still hit home?; Leaving a huge tear in the ring for years to come until rotational effects and resupply by asteroid belt/comet tail fill in the gap. I wouldn’t expect every event to hit the rings, just every couple of centuries or so.

Thanks for any help!

  • $\begingroup$ Is't that overcomplication, why not just Meteor shower Perseids and Leonids but with bigger particles, let say as result of collision of 2 bodies. There plenty of such events, with different periods list. If you wish source to be on orbit of planet - then Roche limit is you friend actually, it takes time for body to be destroyed, and with decaying orbit like phobos deimos it takes geological time to begin, but when it starts it will last long as you wish. $\endgroup$
    – MolbOrg
    Commented Jun 19, 2016 at 12:43
  • $\begingroup$ I admit it could be over complication. But I think initially I was trying to avoid a large object smashing up in orbit as I wanted complex life to form on the planet without too many or too recent an extinction level event. ie, I want more than algae blooms and stromatolites on my planet! $\endgroup$ Commented Jun 19, 2016 at 13:08
  • $\begingroup$ @molbOrg, when you say 'last as long as I wish' does that mean I could have a collision in orbit and that any extinction events that may occur in the planet as a result will have 'finished' and life would be 'back to normal'...as far as it can? $\endgroup$ Commented Jun 20, 2016 at 14:28
  • $\begingroup$ @malbOrg, forgot to mention. And still have the ring system in the Roche limit throwing down a few city killers every few centuries? $\endgroup$ Commented Jun 20, 2016 at 14:35
  • $\begingroup$ posted answer for you, I hope it helps. $\endgroup$
    – MolbOrg
    Commented Jun 20, 2016 at 15:56

2 Answers 2


Probably one of good options for you is something like Phobos (moon)

  • Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. Phobos gets closer to Mars by 2 meters every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into a planetary ring.

And pictures from Roche limit wiki page shows different stages of that future process for Phobos.

All that process takes some time to begin, slow begin, some time and different intensity, some time to slow end, some end sorta of.

For Phobos this is one of probably futures, but for some objects in our systems this is the past, maybe one of such examples is another moon Iapetus

  • Saturn's moon Iapetus has a long, 20-kilometer-high ridge running along most of its equator. It was discovered by the Cassini probe in 2004. The ridge's origin is unknown.

and one of possible explanations is

  • Iapetus may have had a ring system during its formation due to its large Hill sphere, and the equatorial ridge could have then been produced by collisional accretion of this ring.

This could be not a ridge, if gravity would be stronger, but set of craters, and it could be not line of but scattered across entry body, if orbit of decaying body was something similar to polar orbit.

Orbit inclination of decaying body is a way to regulate how wide, starting from equator, will be field where pieces of that body will fall. From tiny equator ring to entry planet.

How fast body will decay and how long it will take and how big will be chunks in size - heavily depends on composition and how solid is that body. So it may literally fall all in dust particles, or in cubic kilometers chunks.

If you take that source of bombardment, everything else what you may need is arbitrary. Probably if you wish it to bombard only one point on planet, even that may be possible, specially if introduce another body's(moons) which influence situation. But better use some circular for bombardment it is more likely, with higher probability.

Even if you wish increase pressure to you planet, it's possible because atmospheric drag will decay orbits different for different size chunks, because surface grows like fold of 2, and mass like fold of 3 - and difference for them on the same orbit may be 10000-100000 years.

You may have not only one such object-moon, it can be more then just one, with different times of decay, different composition, they may even collide when you wish, it may take some time for them, millions millions year before collision.

Even more Three-body problem is not solved, practically it means if you may imagine some collision situation, and in same time that system have chance successfully to exists for billions years before that collision, at the moment we cant say if even our system is stable for billions of years, even in case we know it was stable for billions of years.

So after you have postulated source of problems, for your future inhabitants, you are totally free in description in wide range of possibilities.

How long ring will exists, how long it will fall is up to you actually, even tiny moons have rings

So after choosing source, everything else is up to you, you define parameters of your system by description of what happening with it, and if you do not restrict parameters of that system to some specific exact parameters, such system may exists, exactly with effects you have described, probably indefinite numbers of them, and all of them will do what you have described.

Probably there are some true limitations, but if you need just bombardment, once to 1-100-10000 years waves of such bombardment, during 100 to million years period at random places on planet surface - for typical earth-like planet you are on safe side of that reality, no handwavium needed.

  • $\begingroup$ Excellent. Thank you. That pretty much gives me everything my destructive little self wants! $\endgroup$ Commented Jun 20, 2016 at 16:13
  • $\begingroup$ good, happy destruction's) $\endgroup$
    – MolbOrg
    Commented Jun 20, 2016 at 18:31

The earth actually experiences periodic orbital bombardment every 26 million years or so, one theory for this is that there was a small star called nemesis orbiting the sun from outside the Oort cloud whose gravity might fling asteroids our way if it passed by just right. This theory was dismissed because if there was another star that close we would have found it already, but something like this, with a closer orbit, could definitely cause that sort of chaos.

  • 1
    $\begingroup$ I think that is the plot device of Anne Mccaffery's thread in her dragons of Pern series. It's been years since I read them but I remember something about a lone planet in in the stories solar system causing the thread. If so, that could possibly work, just fast forwarded a whole lot. $\endgroup$ Commented Jun 20, 2016 at 14:32

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