Imagine an earth-like planet on which gravity is reduced by 20%-30% every 15 months. The low-grav-effect should start slowly, reaching its peak/bottom of round about 0.7 g after 6 weeks and should slowly diminish again for another 6 weeks, so that it lasts for 3 months in total. The effect exists since the emergence of the planet.The planet itself is located in our galaxy, but much nearer to the center.

Which astronomical or cosmological event would explain such an effect? (It preferably should be visible to observers from the planet´s surface.)

This is serving for fictional worldbuilding. But I want the most scientific approach possible and only stretch or ignore empirical data, where absolutely necessary to fulfill my conceptions, in order to make the whole story function. For example: The 20%-30% grav-relief is only guessing. What I want is a considerable effect on intelligent lifeforms on the planet, so that they could think about the effect like a fifth season, with all of it´s implications.

  • $\begingroup$ Aertemis: the underpinnings of a fluctuating gravity situation is interesting to think about. Keep only your first paragraph and delete the rest. Actually I think I will just do this myself, now. We will see if I get in trouble somehow. $\endgroup$
    – Willk
    Nov 14, 2017 at 18:54
  • $\begingroup$ Do you mean a cosmological (i.e. dealing with the large-scale behavior of the universe) effect or an astronomical (i.e. on smaller scales) effect? $\endgroup$
    – HDE 226868
    Nov 14, 2017 at 20:38
  • $\begingroup$ Thanks for the remarks. @Will: totally OK to delete the follow-up-questions. I will figure out, how to post them correctly, so that the whole bundle is linked. $\endgroup$
    – Aertemis
    Nov 15, 2017 at 10:15
  • $\begingroup$ @HDE-226868:I am by far not qualified enough to estimate the consequences of saying it´s astronomical or cosmological. Just used the term "cosmological" and didn´t know, what it implies. But naively spoken, I don´t care as long as it is visible and it works somehow. $\endgroup$
    – Aertemis
    Nov 15, 2017 at 10:21
  • $\begingroup$ I've tried to work out a binary star scenario for this - I'm no astrophysicist, but it looks like it's impossible (stable planetary orbit around primary requires [distance from primary] < 0.2x[min distance from secondary] (see here)), but for that minimal distance, the secondary must be around x10,000 more massive than the sun (the primary) to negate 30% of the planet's gravitational pull. This makes the secondary a super-massive black hole, which (probably?) makes the whole system non-viable... $\endgroup$
    – G0BLiN
    Nov 15, 2017 at 16:10

5 Answers 5


Binary planet

http://farm1.staticflickr.com/164/358228043_d07837333f_z.jpg?zz=1binary planet

Our own moon has enough gravity to tug up a bulge in the ocean, producing tides. But what if our planet's partner were even bigger? If the partner were as big or even bigger than the planet in question, its gravity could be felt by people on the ground. When overhead it would pull on objects to some degree, effectively making them lighter. When on the far side its pull would be additive to that of the planet, making things heavier.

One would need to work jiggery pokery with the masses, distances and velocities of these 2 planets such that the orbital period was as requested by the OP and the mass of the companion planet was correct to make the 20%-30% decrease when overhead. The OP did not ask for a 20-30% increase in gravity also but that is what she gets.

As regards noticeable, a giant planet sized satellite would be noticeable to people on the planet's surface. I am sure basketball season would be timed for when it is high in the sky.

ADDENDUM from comments

I'm trying to crunch the numbers for this scenario, and I'm failing to get any "sane" results - may be an error with my calculations, though - @Will (or anyone else, for that matter), would you mind suggesting distance and mass for the satellite that produces the 20-30% reduction in perceived surface gravity (let's ignore the seasonality of the effect for simplicity)? – G0BLiN 2 days ago

OK @G0BLiN with your tricky to spell username. Math is not my strong suit. But there are calculators to help. I used this one. https://www.omnicalculator.com/physics/gravitational-force

First: Earth Plugging in my own 100 kg weight standing at Earths radius 6371 (6371 km from the center of mass) I get:

earth gravity 981.7 N / 100 kg = 9.8 m/s2 which is Earths surface gravity. Looks good.

Now let us consider Jupiter at 317 Earth masses and radius 69911 km. How far off the surface can on object in orbit exist? Metis is the closest moon of Jupiter, orbiting at 128,000 km. 128000 + 69911 = 197911 km from Jupiter’s center of mass.

enter image description here

322.5 N / 100 kg = 3.2 m/s2. Which is about a third of Earth’s gravity.

So: if I were standing on Earth’s surface looking at Jupiter 197911 km overhead, I would be lifted up by Jupiter with a force of 30% of Earth’s own pull. I would feel 30% lighter. On the far side of Earth I have the diameter of the Earth between us too; if it is 12742 then the force is 2.8 N in addition to the 9.8 of earth.

A problem: to orbit as closely as Metis does a body must move very fast. It is falling towards Jupiter at a good clip and it has to reliably miss. A fix: the Earth in this scenario orbits in a highly elliptical orbit.

elliptical orbit Ha! Gifs work on WB stack!

This can address the periodicity also – for much of the 18 month orbit the Earthlike body is at some distance from its large Jupiteresque partner. It swoops close for the specified period in the OP during which the proximity allows the above described gravity reduction due to the large partner.

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    $\begingroup$ I'm not sure you could get 20-30% reduction without violating the Roche Limit and having tidal forces destroy the planet $\endgroup$
    – Josh King
    Nov 14, 2017 at 21:47
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    $\begingroup$ Wouldn't such planets be tidally locked to each other? And if they would, effect would not be periodic. Also, Roche limit gets to play its tricks. $\endgroup$
    – Mołot
    Nov 15, 2017 at 10:52
  • 2
    $\begingroup$ Note you'd get the 30% lighter effect on both sides (the side closest to the other planet and the side further) and also oceans and even the planets crust would be trying to flow into that area. $\endgroup$
    – Tim B
    Nov 15, 2017 at 11:18
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    $\begingroup$ @JoshKing I think you will find that the Roche limit is much less significant when two bodies are of similar size en.wikipedia.org/wiki/Roche_limit $\endgroup$
    – Slarty
    Nov 15, 2017 at 11:57
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    $\begingroup$ Even if the roche limits are not exceeded, wouldn´t the gravitational force completely change the planet´s usual rotation speed an thus the alternation and length of day and night? Or could that just be an effect limited to the length of the event? $\endgroup$
    – Aertemis
    Nov 15, 2017 at 12:36

Exiting an unusually dense dark matter field.

This is, essentially, equivalent to decreasing the planet's mass, as suggested by Luke's answer, but it's a reasonably plausible mechanism, especially in a science fiction context.

Cold dark matter would tend to increase in density around concentrations of normal baryonic matter. If the background density of dark matter is already fairly high, this could produce a noticeable increase in the total mass hanging around in the interior of a planet, thus increasing its gravity. Note, however, that this would not generically be a static blob of dark matter that's gravitationally bound to the planet--it's a dynamic structure, more of an event than a thing, consisting of whatever dark matter particles happen to passing through the planet at any given time, like a standing wave in a stream where water bulges as it flows over a submerged rock. Thus, if the planet moves out of the background dark matter cloud, or if the cloud itself moves, this standing wave of extra mass would disperse, and the planet's gravity would decrease.

There are all kinds of ways to make that periodic. Maybe the cloud is associated with the planet's sun, and the planet has an eccentric orbit that periodically takes it closer to the middle (denser) part of the cloud and then out towards the (thinner) edges. Maybe the cloud itself regularly expands and contracts (which could happen if each individual particle of the dark matter cloud is on a highly eccentric solar orbit, and they all have similar phases), thus periodically changing the background density where the planet orbits. Maybe there's a coherent cloud of dark matter orbiting the same sun like another (very large and diffuse!) planet which periodically intersects the baryonic planet. And so forth--I'm sure you can come up with lots more possibilities for periodically varying the dark matter density surrounding the planet.

Unfortunately, this sort of thing, by definition, would not be (directly) visible to the inhabitants! But it's the only option I can think of that could be stable over geologic timescales and wouldn't kill everyone on the surface.

  • $\begingroup$ @Logan-R-Kearsley: I really like that one. But as you said, it´s not visible. $\endgroup$
    – Aertemis
    Nov 19, 2017 at 15:56

The strength of a gravity field is given by (G * m) / r^2. To decrease the strength of the gravity field, you can decrease m, the mass of the planet, increase r, the distance from the center of the planet, or decrease G, the gravitational constant.

Decrease mass of planet

Your planet is some how loosing mass without loosing size. Where is it going? Who is taking it? How are the people on the surface surviving the process? These are questions you'd have to answer.

Increase the distance from the center

Your planet is getting bigger without changing mass. I can't imagine how this happens. Someone is inflating a planet sized balloon inside the planet?

The gravitational constant is diminishing

We named it constant because it's not known to change. That being said, we don't know why it should be at the value it apparently is, so we have no idea what might cause it to change. If the change isn't local to the planet, then everything in the solar system gets farther away as their orbits are no longer stable. Eventually things start flying away into interstellar space to freeze in darkness forever.

If the strength of the gravitational field isn't decreasing, then maybe there is a force that counteracts it. I suggest: centrifugal force (I know it's not a real force, but it seem lit it is to anyone on the surface of the planet)

Centrifugal force

If you planet is spinning ever faster and faster, then people near the equator would experience lower and lower gravity. People near the poles will not experience the same thing nearly as strongly. If the Earth reaches a speed where the equator is moving at orbital velocity (about 1 rotation every 90 minutes) then people living there would experience 0 gravity. If the earth speeds up any more than that, then it will break apart and everyone on earth will die.

  • $\begingroup$ "it will break apart and everyone on earth will die." [Citation Needed] $\endgroup$ Nov 15, 2017 at 13:27
  • $\begingroup$ This answer focuses only on the planet's gravity, not the total perceived gravity (which may different due to the effects of other celestial bodies). None of the given options (mass loss, growing radius, change of gravitational constant) address OP's request for a recurring event, and, none seem to be survivable to the inhabitants. $\endgroup$
    – G0BLiN
    Nov 15, 2017 at 14:47

As cosmological event I think the flyby of a very large astronomical object can cause this kind of effect. But then your problem will be that its strong gravity will induce very strong tidal effects which will make all other effects you required pretty useless.

  • 1
    $\begingroup$ And throw his planet out of orbit, preventing it from being periodic as op wants. $\endgroup$
    – Mołot
    Nov 14, 2017 at 20:10
  • $\begingroup$ There cloud be a really complex set of planets that would achieve the desired result.... but I don't think it is likely il units or is by design $\endgroup$
    – Pliny
    Nov 14, 2017 at 21:09

I believe that any significant change to a planets gravity as described by the OP would inherently result in a destabilizing orbit (which eventually means crashing or flying off somewhere).

That being said I'm going to play with the fringes of physics.

I propose your planet orbits a blackhole that periodically produces Gravitational waves, which I think if you are close enough to the source would have a perceivable effect on gravity. As we have learned from the science of Interstellar its possible to have a habitable world near a blackhole.

This is sort of handwavium answer because gravitational waves and their effects aren't fully understood let alone what would be the conditions needed to have the right balance of not getting ripped to pieces while receiving enough em radiation to support life.

  • $\begingroup$ Gravitational waves don't do that. $\endgroup$
    – JDługosz
    Nov 18, 2017 at 7:06
  • $\begingroup$ well so much for my handwavium $\endgroup$
    – anon
    Nov 20, 2017 at 15:03
  • $\begingroup$ Handwavium is very much audience dependent. Especially in the last few decades, sci-fi fans have gotten very knowledgeable. Generally they are more aware of physics and math than screenwriters. $\endgroup$
    – JDługosz
    Nov 20, 2017 at 23:22

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