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I'm looking for a scientific explanation as to why smaller pebbles and rain would slowly fall upwards without the planet itself orbiting far too close to fry every living thing on the surface. A way for this water and pebbles to slowly replenish would be helpful, but not immediately important for the story I'm trying to develop.

The biology of such a world could be some form of magnetism for the ground dwellers, while most creatures would thrive in the sky. Any other suggestions welcome.

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  • $\begingroup$ Welcome to worldbuilding. Please take the tour and visit the help center to better understand how this community work. $\endgroup$ – L.Dutch - Reinstate Monica Nov 24 '18 at 5:51
  • $\begingroup$ It's a whole lot easier to do this with rain than with pebbles (unless they are positively tiny). Would you settle for an answer that only covers the rain case? $\endgroup$ – a CVn Nov 24 '18 at 8:14
  • $\begingroup$ Hmmm...possibly a whopping great magnetic field for the rain, but that doesn’t cover the rocks... $\endgroup$ – Joe Bloggs Nov 24 '18 at 9:07
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    $\begingroup$ So I'm tempted to write an answer explaining the physics issues that arise with this, but first... what are you actually trying to do. Is this nothing more than a little bit of flavor that you want to make an exotic planet feel... well...exotic? Or are these falling pebbles part of some crucial plot device that needs to work or your world falls apart? $\endgroup$ – Cort Ammon Nov 24 '18 at 17:56
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    $\begingroup$ mechanically i need to have aerodynamic battles using hand to hand combat, sky sails, unique evolution, etc. bonus for me to write and explore a unique planet of alien nature. i would preferably it be built around hard science as this is the second book i’m writing following the same character. the first one is about space battles done with swords and other primitive technologies. set thousands of years after the end of humanity. My aim is to give my lead character something familiar to his skill but new avenue and properties to learn and work with. $\endgroup$ – Sebastian Nov 25 '18 at 15:39
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Gravity- and spin-based explanations don't work very well here, because if gravity/centripetal forces are strong enough to send small pebbles into the sky, they're also strong enough to send the entire surface of the planet into the sky, and this is not a survivable scenario, especially since it probably requires a nearby black hole or neutron star.

A better option might be electrorepulsive effects. Like charges repel; if the surface of the planet has a negative charge and the upper atmosphere has a positive charge, then small objects at the surface will tend to pick up a negative charge. The ground then repels them, the sky attracts them, and if they're light enough and the charge is strong enough then they can float upwards.

On our planet, this is how spiders fly, and also why your hair stands on end if you play with a Van de Graaf generator. Apply a bit of handwaving to explain why your planet has a particularly strong charge difference and you can rationalise small rocks floating upwards. Meanwhile, heavier objects don't float, because square-cube considerations mean that they don't carry as much charge per unit mass.

Rain is tougher to explain, because it doesn't usually form at ground level - and if it forms in the sky and then falls upward, nobody's going to see it!

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    $\begingroup$ Well, I thank you for that link on electrically-powered spider flight. Wow. Today-I-learned, indeed. $\endgroup$ – David Heyman May 15 at 20:26
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1. Very small, porous planet

Small mass, weak gravity. Need to be more like a spongy asteroid, but with small inhabitants, a small world can still be arbitrarily large and rich in proportion.

A disadvantage of small bodies, though, that they can't really have an atmosphere. You may not need one, but then make sure it rotates slowly, otherwise things lifting off would not fly up, but away.

For how exactly that lifting-off might work, just one example: thousands of small geysers erupting from the pores, when facing the sun, could shoot rocks up, and also provide your rain falling up.

2. Twin (double) planet

Gravitationally locked binary systems of even Earth-like size, in extreme proximity, are possible. These have a low-gravity zone in between. The idea of a binary planet has been explored e.g. in Robert Forward's Rocheworld, including the peculiarities of their gravity. E.g. the two bodies even share an atmosphere. A quote from the plot, quite in line with your up-falling water idea:

[the twin planet was] approaching a period where the configuration of the star and planets of the system allow for a phenomenon where the ocean on the water lobe of Rocheworld can partially flow to the rocky lobe, due to the change in the gravitational equipotential.

3. Centrifugal, but safely below the escape treshold.

As pointed out by others, centrifugal force won't work, as soon as it actually balances out gravity. So, it shouldn't.

There could be an angular velocity, where the planet doesn't disintegrate yet, but most of its gravity is cancelled out at the equator.

This would make the planet highly unstable, magnifying the effect of all sorts of transients caused by environmental fluctuations, which couldn't make a difference in a slower-rotating case. These extremities may genertate some very useful chaos, with just about any surprising and unpredictable phenomena you could imagine.

Not sure about an atmosphere, though. Again, I can imagine that it could still hold one that's higly layered, with ring-like streams of matter (following up on the Roche-limit pointed out by @Laurel), possibly circulating back toward the poles.

The climate would certainly be hostile (e.g. due to the lateral speed differences between layers), with thick, hot, whirling winds of smoky-dusty matter blown off the surface, but may perhaps help your creatures gradually form their own half-natural "planetary Dyson sphere" (actually: a belt) out of debris: a shell, or shield in the sky they may inhabit and maintain (e.g. because the weather would kill them on the surface).

Examples of effects lifting things off:

  • Solar winds, ionized clouds generating strong electrostatic fields (e.g., periodically, synced to the orbit), attracting oppositely charged chalky, porous, light minerals with peculiar electrical properties (constant, heavy dust storms due to the fast rotation can charge objects on the surface, as well as polish them to pebbles). Then the flying pieces would eventually discharge up there, and fall back down, closing the cycle (-> your replenishment requirement), while also providing energy to your sky-dwellers as a bonus.

  • As already described, at the boundaries of centrifugally separated layers/rings of floating matter all sorts of interactions could really spice up the weather. Winds can lift things up even here, on Earth, and warm air can do it, too. In your world, with much less pull from gravity, strong currents could blowing vertically upward and easily carry small objects.

  • Whirling ionized matter in that thick, layered atmosphere, could also generate high electric currents (e.g. thunderbolts) and thus strong magnetic fields (fitting that other aspects of your story, too), lifting certain metallic rocks, as others have already mentioned.

  • It may even be some partly artificial effect (depending on your story): the guys in the sky may exploit and control the instabilities, deliberately triggering certain features of the climate, precisely to get some stuff up there.

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  • $\begingroup$ Ahh, only when I finally posted this (after sitting on it for half a day), and the page reloaded, could I see that @GeoffreyBrent also came up with the electrostatic idea. $\endgroup$ – Sz. Nov 25 '18 at 6:34
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When another large planetary object passes close gravity pulls in both directions and small objects can fall skyward, to the other planetary object. Our ocean tides are a function of the pull of our moon. In your hypothetical world, the planet could have an eccentric orbit which passes near a black dwarf star, which is the cold, dense remnant of a white dwarf star.

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  • $\begingroup$ I was thinking something along these lines, but i'm not clear on if this is technically possible in a stable orbit with these interactions. Additionally, the way tides work on earth, they are pulled sideways together with the spin of the planet as well as orbit (i assume) so it kinda feels like this much gravity would just cause a storm and mayhem instead of a lift from the surface. $\endgroup$ – Sebastian Nov 24 '18 at 6:15
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    $\begingroup$ You'd notice quite a bit more flying skyward if this happened, namely large chunks of the planet as it is ripped apart by tidal forces. See the Wikipedia page on the Roche limit for more details. $\endgroup$ – Laurel Nov 24 '18 at 6:41
  • $\begingroup$ @Laurel thanks, that's a problem xD $\endgroup$ – Sebastian Nov 25 '18 at 16:14
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The planet's moon could be magnetic and attract the metal in the pebbles. It would have no effect on non-metallic objects like animals, plants, etc.

The "rain" you see rising are in fact small metallic or other magnetic sands that drag a bit of the water with them that they are pulled from. If the ground is dry, you only see magnetic pebbles, stones, and dust rising. But if the ground is wet or if you are over a body of water, the magnetic moon will pull metallic sand and dust from the puddles and the ground of the rivers and lakes and these grains of sand will be surrounded by the water that sticks to them, making it appear as if there was a rising rain.

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Developing the answer by Mike Wilco and addressing some of the objections

The centrifugal force will be greatest at the equator. Suppose that at the poles and intermediate places, the force isn't sufficient to throw anything off the surface.

Water will tend to move towards the equator and then be thrown off. It will carry loose pebbles with it. This could happen within a narrow band, perhaps only a kilometre wide that encircles the planet.

I'm thinking of asking a follow-up question about how the planet losing mass in this way would affect rotational speed. Would it spin faster or slower? I don't know.

As noted by Renan, this loss of mass would have to happen over geological time.

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  • $\begingroup$ For centrifugal force to be powerful enough to do this, the planet's surface at the equator needs to be spinning at orbital velocity. For an Earth-like planet, this translates to one complete revolution (i.e. one planetary "day") every 80 minutes or so. At this speed, the planet will spin itself to pieces - not just small pebbles but EVERYTHING will fly off. $\endgroup$ – Geoffrey Brent Nov 24 '18 at 23:45
  • $\begingroup$ @Sz. Conservation of angular momentum applies to the entire system - both what remains of the planet and the material that flies off. It does not require that the smaller planet-remnant has the same angular momentum as the larger planet originally did. Since the matter that's flying off doesn't exert torque on the planet as it goes, there's no obvious reason why rotation would accelerate. $\endgroup$ – Geoffrey Brent Nov 25 '18 at 13:32
  • $\begingroup$ True, my argument was faulty; deleted. I just hastily followed up the idea in the reverse process that spins up stars and planets when the lose matter collapses during formation. $\endgroup$ – Sz. Nov 25 '18 at 16:01

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