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Question inspired by How would a primitive society react to a cataclysmic event that disproves their beliefs?

So, I need a planet which:

  • Can support life and has attributes as close to Earth as possible
  • Has a planetary ring
  • That planetary ring is observable from specific place on such planet for most of the time,

However, in rare occasions, such planetary ring "disappears" from being observable from such place for at least one week (seven rotations of such planet around its axis)

Such rare occasions should repeat in once about 100 - 200 years ( = revolutions of planet around its star)

Knowing how planets work, it is totally ok if such ring is observable from other places on such planet without any eclipses.

All I need is a place of size of about United kingdom ( = 243 610 km² of area from which is this phenomena observable)

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  • $\begingroup$ Does the ring have to be a natural thing? As opposed to something artificial/alien? (That might have some 'unintuitive behavior(s)' programmed in....) $\endgroup$ – Catalyst Jan 27 '17 at 13:25
  • $\begingroup$ is a total solar eclipse at the same time allowed, or are you trying to just eclipse the rings but not the planets surface? $\endgroup$ – Innovine Jan 27 '17 at 14:05
  • $\begingroup$ What exactly do you mean by disappear? $\endgroup$ – user31746 Jan 27 '17 at 14:09
  • $\begingroup$ @Masterzagh not observable from that area on planet $\endgroup$ – Pavel Janicek Jan 27 '17 at 14:46
  • $\begingroup$ "observable" is really wide term. By naked eye? $\endgroup$ – Mołot Jan 27 '17 at 17:31
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Magnetic ring particles

Perhaps the rings are composed of very special needle-like iron particles, which are magnetized. (how this came to be is another question...) The ring particles align to the planets magnetic field, presenting their long side towards the planet, and reflecting light towards the surface, making them visible.

Due to its turbulent, swirling liquid core, the planet has an unstable magnetic field, which reverses polarity now and then. There is evidence suggesting this has occurred in Earths past, and may even be happening now. You would need to accelerate the process quite a lot for your story though.

As the magnetic field shifts, the ring particles follow, changing their alignment, meaning that at some point they will be end-on to the viewers on the surface. The reflectivity drops off towards zero, making them invisible.

They will reappear again if the magnetic field shifts more. The advantages in this suggestion are that there are no easily observable planets or moons causing eclipses, no unrealistic orbital mechanics, and you have relatively free hands to shift the magnetic field at will.

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If a habitable planet has rings it makes for some interesting views from the planet's surface.

Here is an interpretation of what rings would look like around earth (The image assumes rings like Saturn) http://i.imgur.com/d3sjFA8.jpg

If you make the rings themselves spin around another axis than the planet's axis the view of the rings will shift with time and change from being a massive disk in the sky into a narrow stripe. It's also entirely possible for the rings to completely disappear from vision this way. (I don't know the exact speed ratios on which the rings gave to spin nor do I know how the axis of the spinning rings should be aligned towards the spin axis of the planet.)

Note that this is a gradual process, so the inhabitants of your world, more specifically, the inhabitants of the designated area from which the rings disappear from vision can see this happen gradually over the course of generations. So if you need this event to happen suddenly my method of explanation will not work.

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    $\begingroup$ How do the rings «spin around another axis»? You mean the ring plane is inclined relative to the planet’s equator? But you’d see the progression on a daily cycle. $\endgroup$ – JDługosz Jan 27 '17 at 9:23
  • $\begingroup$ I don't think that ring is realistic. It would be much farther away, and much weaker. $\endgroup$ – Feyre Jan 27 '17 at 10:37
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    $\begingroup$ You can't just "make the rings spin" in an arbitrary way. $\endgroup$ – Innovine Jan 27 '17 at 14:02
  • $\begingroup$ @jdlugosz wouldn't an inclined plane be unstable? I thought rings are formed and held in the equatorial plane only $\endgroup$ – Innovine Jan 27 '17 at 14:04
  • $\begingroup$ @Innovine you're right, that's exactly how it is. But if you have an inclined unstable moon (caught asteroid) it may cross Roche limit and become unstable inclined ring. Time events right and, while still weird as..., it's not totally impossible, for some time at least $\endgroup$ – Mołot Jan 27 '17 at 17:35
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The rings would be visible from the surface in reflected light, reflecting from mostly the primary (star). To get an eclipse event light falling towards the ring would have to be blocked by another body, probably a moon in combination with the planet itself (though another planet of significant size in an orbit close to that of your planet might work as well).

So you need a conjunction event. Those happen in reality of course simply due to orbital mechanics and are nothing special, they can be calculated (if the orbits are known precisely enough) a long time in advance.

The problem which arises here is your requirement that the eclipse lasts for several weeks. Realistic conjunctions and eclipses last a few minutes to hours at best. To get the event you want you'd need to have a large moon around your planet that circles it very slowly, so slowly that when you have a solar eclipse that eclipse lasts for a week or more, and large enough that such an eclipse is not just total, but more than total (it needs to block light not just from striking the planet but also the ring around it which tends to be several times the planetary diameter in size). When your natives get struck with such an event, them not seeing the planetary ring would be the least of their problems. The sun has been eaten by a space monster, eternal night is coming, the world is coming to an end!

Mind that I've not done any of the calculations about what such a system would look like, whether it could even exist (the gravitational equations might make a moon that large impossible, though its existence could well be the cause of the rings).

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  • $\begingroup$ Large planet in close orbit isn't a stable thing. $\endgroup$ – Mołot Jan 27 '17 at 17:41
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A volcano eruption might add enough dust to cover the view of the stars and rings. It will also reduce the sunlight if the dust eclipses the sun. The dust does not need to cover the whole sky, because the winds might move the dust to cover only the view to the rings.

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  • $\begingroup$ You might also consider that the rings are a single color (say, red), and that the dust just happens to absorb light mostly of that frequency (its all colors but that red). So it'd still pass regular starlight mostly unchanged, and leave the sky blue, but hide the rings. $\endgroup$ – Innovine Jan 27 '17 at 21:00
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Here's one mechanism. I think the gap in the rings would last more than 7 days... more like 7 years. But you said that was flexible, so maybe you can work with it.

These rings are made almost entirely of ice. Ice bounces sunlight well, sparkles in the sky plainly. Rings are orbiting at the same rate as planet rotation so ground observers always see same portion of the ring. The rings are refreshed by outgassing from a moon, as happens on Saturn. http://www.space.com/12432-saturn-water-rain-enceladus-moon.html

The planet exists in a binary star system. It orbits its primary. The second star is more distant.

Every N hundred years, the two binary stars move briefly closer together, providing a slight uptick in heat for the planet, just enough to melt the rings. It's not a stable system in the long run, but it could be for many human generations. The close pass of the stars might only last hours depending on speed. I don't know how close to knife's edge you can cut it.

The rings melt, and steam doesn't reflect sunlight as well as ice, especially as it thins out. No more rings. But over time, any remaining larger chunks will redistribute around the ring, and the moon will refresh the smaller ice crystals, making the rings fade back into view.

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    $\begingroup$ If the rings were to rotate at the same rate as the planet, they would need to be infinitely thin and located in geosynchronous orbit (far away). No outgassing will place particles into this particular orbit. $\endgroup$ – Innovine Jan 27 '17 at 15:13
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    $\begingroup$ it's also impossible that stars can just get a little bit closer together for a few hours every few hundred years. they'll either be a little closer for a hundred years then a little further, or be radically closer for a few hours (days,years would be more realistic) then extremely far away for the rest. trinary systems do have complex behaviors and unpredictable motion, but the motion you describe here is not realistic afaik. $\endgroup$ – Innovine Jan 27 '17 at 15:15
  • $\begingroup$ @innovine it is extremely improbable but not impossible. You can have an orbit that brings them close enough for years. You can have an orbit where they never get close enough ever. Orbital mechanics is continuous functions. By First Fundamental Theorem of Calculus, you can work out that there must therefore be some orbit that doesn't last very long. Improbable though that may be! :-) $\endgroup$ – SRM Jan 27 '17 at 23:28
  • $\begingroup$ I disagree.. coming closer together for just a few hours every hundred years implies an incredibly eccentric orbit, which rules out that they only come a little bit closer. they can only come closer for a very short time if they immediateny fly very very far apart again thereafter. $\endgroup$ – Innovine Jan 28 '17 at 7:35
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If the planet is small, and perhaps needs to have a large moon that occasionally blocks the ring due to small discrepancies in the orbiting of the planet, the planet would need a very dense core so that it does not have to be enormous to have such a large moon. Osmium would be an interesting element to have as the core.

Also, if the civilization of the planet is based on small islands scattered throughout a gigantic ocean, and each has never had contact with the others, then for them it will happen more rarely, while as a whole it occurs on the planet commonly. This would limit storytelling, but perhaps conquest of other islands and realization of the commonality of the solar eclipse could be fun to use ("our great eclipse is very often! The world is plunging into darkness! It's the apocalypse!").

TLDR: Dense core for large moon, small islands in large ocean.

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