# A planet with changing gravity?

## A planet with changing gravity

On earth gravity stays constant (besides negligible fluctuations), but it may be possible to change that. What kind of geology, natural satellite, or other anomaly could cause periodic changes in gravity? (And what magnitude of difference would they have). The ideal goal is to map the change to a kind of sine wave, so that the bounds of the changes stay constant. The periodicity of the planet's gravity can range from days to years.

• Unless my understanding of science is off, the planet would have to be an artificial construct with a gravity regulator for this to work. – Frostfyre Jun 9 '15 at 16:18
• If you mean gravity varying with time, I agree with Frostfyre, physics (to our knowledge and short of magic tech) doesn't work that way. You could have a planet with gravity that varies by location, especially latitude for very rapidly spinning bodies. – Jim2B Jun 9 '15 at 17:06
• To add to naysayers: while it is possibly to have planet with variable gravity, a state like that is not stable, the planet will eventually reach equilibrium. And while the planet is doing that, the resulting seismic activity and extreme weather will pretty much make certain the planet is inhabitable. Not only would no one want to live there, it would be impossible even if you wanted to. – Ville Niemi Jun 9 '15 at 18:08
• I recall a short story which featured exactly this. bowlturner below describes how a very dense object within the planet could cause this, and I recall that turned out to be the solution in the story. I think the story was part of a series about extreme engineers who went in to similarly wierd situations and solved them, but I can't remember. – Paul Johnson Jun 9 '15 at 20:18
• Depends on torque applies on rotating Earth to discount some or all gravity by unknown artificial or natural means. – user6760 Jun 9 '15 at 23:53

The gravity on Earth is changing. We see it in the tides of our oceans. The tides are caused by the Moon and the Sun which exert a gravitational pull on all the mass of our planet, but exert a greater pull on the parts of our planet closest to them because gravitational attraction decays with distance. The Moon isn’t very massive and the Sun is a long way away so on Earth the tidal forces aren’t very large, but if our planet were to orbit something very large and very close these tidal effects could become significant. If a moon orbits a large enough planet too closely, the gravitational gradient can overcome the gravitational self-attraction of the satellite and cause it to disintegrate. This distance is called the Roche limit. A body near the Roche limit will have reduced gravity on the sides facing directly towards and directly away from the body it orbits, but will have normal gravity elsewhere.

Any bodies in orbit will exert tidal forces on one another. These forces can range from nearly imperceptible, as on Earth, to making anything on one side of the planet that isn’t held down float off into space. The magnitude of these forces will depend on the size and density of both bodies and the distance between them. As long as the planets are not tidally-locked (rotating so only one side of the planet faces the other as our moon does) the gravity will change periodically, as the tides change on Earth. Additionally, an eccentric orbit could also introduce changes to these effects with the desired periodicity.

• Absolutely not. In the case of the earth example you provided, the moon does not influence gravity but (partially, thank God) negates the effect by pulling in the opposite direction! – Stephan Bijzitter Jun 9 '15 at 21:07
• @StephanBijzitter Gravity is a field at least in classical physics, and the field IS modified by the fact that there is mass other than the earth, and it is also variable because the moon moves with respect to the earth. In fact someone could interpret the first sentence incorrectly, I proposed an edit. – Formagella Jun 9 '15 at 22:12
• Actually, the "anything not bolted down floats off" is a bit overdone. At any level of negative force, water, rocks and dirt will all float off. Even the underlying bedrock starts to come apart under very little (relatively speaking) force. – WhatRoughBeast Jun 9 '15 at 22:22
• In fact, "anything not bolted down floats off" is the rigid Roche limit for a body with no tensile strength. You experience apparent zero-gravity when "standing on" the surface of the body, and can easily drift away. If the body is rotating, you won't go with it. The tidal force will also strip the atmosphere. – Steve Jessop Jun 10 '15 at 2:49
• @SteveJessop that is the limit yes. But for the body to coalesce and form a body in the first place, it would have to be well within the Roche limit to have formed a planet within the lifetime of the star. It is of course possible that the size/shape/n-body-gravity-field changed since then. – Aron Jun 10 '15 at 6:13

A very massive planet with an extremely high rotation speed would have an huge gravity gradient between the poles and the equator. The classic sci-fi book "Mission of Gravity" by Hal Clement exports such a world.

• Wouldn't the planet flatten? My intuitition tells me it would flatten until there was equilibrium, or until the planet broke apart into a disk. – Brian Woodbury Apr 27 '17 at 3:20
• @BrianWoodbury I believe it was flattened. The gravity at the equator was something like 3 Gs, which meant that the humans could survive it to communicate with the natives. – David Thornley Feb 1 at 18:53

If your planet had a stable micro black hole orbiting inside it (well below the mantel) gravity would vary by location depending on the orbit of the black hole. Of course, such a planet would eventually be doomed, as the black hole would absorb more and more mass until it had absorbed enough to do catastrophic damage to the planet.

Just how a black hole would form a relatively stable orbit within a planet is left as an exercise for the reader, but unexpected results from a super-collider might be a good start.

• "Thrice upon a time" by JP Hogan explains that pretty well. ;-) – cfischer Jun 9 '15 at 21:11
• I think I got the idea from a David Brin book, but that could easily be the source; I read a lot science fiction. – aslum Jun 9 '15 at 21:13
• @aslum, that would be "Earth" by David Brin. – Paul Johnson Jun 11 '15 at 18:24
• A black hole would have nothing to orbit inside a planet. The force of gravity due to a shell planet inside of the shell is 0. If you try and solve this problem by adding something else massive enough for the black hole to orbit, then you end up with a point source of gravity so close to the middle that you've cancelled the desired effect. – Brian Woodbury Apr 27 '17 at 3:15
• A black hole could certainly orbit a planet from below the mantle. Gravity doesn't stop below the surface... and a small black hole might have an event horizon that's only millimeters across meaning very little mass gets absorbed as it orbits... – aslum Apr 27 '17 at 12:41

My best guess would be some how an ultra dense piece in the mantle is circulating in the planet. Say a (very large) clump of lead (or even something more dense) that from some reason is moving around in the mantle of the planet. As it orbits the core, it changes the 'center' of gravity and affects gravity on the surface.

I would expect if this is even possible, that it would in reality be a very slow rotation, maybe once in a humans lifetime or once in a thousand years for a circuit to complete, but maybe a very active planet might have it more often, and that might be why it's a lot more active, say something large collided with the planet and everything hasn't completely settled down several million years later.

• But it wouldn't really change the gravitational force, just the point of origin would change. – Stephan Bijzitter Jun 9 '15 at 21:09
• Wouldn't being closer to a larger mass have greater gravity, than being farther away? So when it's under your feet, wouldn't it have more pull, than on the far side of the planet? – bowlturner Jun 9 '15 at 21:14
• @bowlturner: Yes but it would have to be a very big planet for you to notice that difference. – Lightness Races in Orbit Jun 10 '15 at 0:33
• Places like the Himalayas have a huge gravitational anomaly. But of course, we're talking "huge" as in "huge for gravity", it's not noticeable for the unaided human - the anomaly is about 250 mgal, 0.025% of the total gravitational force. Gravity is just an incredibly weak force with respect to the mass involved :) And yeah, the "clump" would move incredibly slow - we're talking tens of millions of years to get anywhere (it can't really flow faster than the surrounding mantle). Oh, and of course - it would sink. Rapidly. – Luaan Jun 10 '15 at 6:59
• Well to be honest, I was thinking the mass would have to be at least as large as the Indian subcontinent, not the little pimple of mount Everest... – bowlturner Jun 10 '15 at 13:16

For the exact formulation you'd need a lot of maths and simulation, but if you only look for a feasible explanation, what Mike says is the easiest way to explain a changing gravity.

Big orbiting planets can stay at equilibria even when their orbits go one across the other, even when planets are almost nex to each other for a period of time. Therefore, you could explain the changes on gravity that way: When another orbiting planet goes nearby, gravity is way stronger towards that planet. Also, a big "star" (it's a planet, not a star, but people might be unable to differentiate) appears on the sky for that duration of changed gravity, which is really interesting from a mythological aspect.

If you want even more variation, add more planets to the "equation". That way, the mythology around that goes even deeper. Every planet, with it's distinguishable bright and color (since star's light are reflected differently) brings a different gravity and for a different duration of time.

How about the following.

1. The planet has three layers, one is the rocky mantle, one is the metal core.
2. The third layer is intermediate between the two others, and somehow has low friction. I am thinking along the lines of a liquid here, so we will probably need to postulate high temperatures for an appropriate material to remain liquid.
3. The mantle rotates at one speed, e.g. 1 rotation per 24 hours.
4. The core rotates at a different speed, e.g. 1 rotation per 30 hours.
5. The core is unevenly distributed density.

We could postulate the core has absorbed a particularly large incident body some time back, that for some reason has never quite migrated to its geometrical center.

I am not saying this situation would be stable forever, but why not over a geologically insignificant and biologically large period, like 10 million years?

• Well, we're pretty sure this is how the Earth works - the core is rotating faster (or slower? Not sure...) than the surrounding material, and it is indeed formed of a liquid and solid part. The tricky part about your scenario is - how did the incident body (massive enough to cause noticeable gravitational distortion) get to the core without killing all life on the planet? Even just accounting for the gravitational potential energy released during the sinking, we're talking about gigantic amounts of energy. At the very least, this should cause tons of earthquakes and eruptions. – Luaan Jun 10 '15 at 7:03
• @Luaan: the OP did not specify the planet was inhabited - or even habitable. ;-) I agree it would be complex to have two large bodies collide without much dissipation of energy. It is not impossible, if their initial orbits are just right (they meet with zero radial speed). Not very probable, though. Unless somebody made it happen ... – ALAN WARD Jun 10 '15 at 7:41
• The speed is not the only problem - you always have to release at least as much energy as is the gravitational potential energy, no matter the initial relative speed. That's even before the sinking starts - the sinking is quite hard to model, actually. – Luaan Jun 10 '15 at 7:53
• Right. So, basically, we need to scratch the idea of a collider if we want the resulting planet not only to have a variable gravity, but also avoid boiling off seas and atmosphere and in general making the planet non-habitable. – ALAN WARD Jun 10 '15 at 8:04
• Yeah - and as for the sinking of something comparable to the mass of the Earth's core, it's thought that the sinking of the actual core managed to melt the whole planet (the parts that weren't molten already, of course) -this is known as the Iron Catastrophe, and the only energy source was the gravitational potential energy of stuff that was already on or below the surface of th earth. That's already much less than from any impactor, and it was still enough to melt most of the planet :D Gravity is weak, but that doesn't help us when we explicitly want extra gravity. – Luaan Jun 10 '15 at 8:17

Along similar lines to the black hole scenario, you might have some weakly interacting dark matter orbiting inside the planet.

You need the dark matter to clump together and to have minimal interaction with normal matter.

A candidate for this would be Weakly Interacting Massive Particles. Wikipedia references a paper suggesting they would tend to clump together - I am not sure how well validated this is, but good enough for a story.

Note we still have to worry about the Roche limit, whether inside or outside the planet. I think that if it is dense enough, it will be stable.

A easy and fun way to do it is to place several moons orbiting your planet. They have different trajectories in space, for example almost comet like for one, thus coming only once a decade. The other moons could be orbiting in whatever way you want.

• Chaotic tides, magnetic filed perturbation if one moon is highly metal.
• Extreme events might occur when all celestial bodies align creating strange gravity impact (high jumps or whatever).

I would keep it simple but if you are ready to push yourself, nothing better than the usual duo planet orbiting one star with a moon or 2 to gear things up. There you got retard gravity.

• Interesting idea, but I keep wondering just how big and close should a moon be to change gravity on the surface of a planet by a meaningful amount. The planet system should also be stable in order to allow for life evolving, could, for example, changing gravity affect plate tectonics and create inhospitable regions all over the surface? – zovits Jun 10 '15 at 8:31

A few of the answers here propose a very high-density mass located off-center in the planet's core, along with the different rotational period of the core to the surface, leaving the problem of how this mass came to be there in the first place. What about the opposite scenario?

A moderately-sized cavity in the planet's solid core, off-center - which could either be filled with a significantly lower-density material, (helium perhaps?), or simply be a vacuous void in the core. As with the other explanations, with the core's rotational period differing from that of the surface thanks to the liquid outer core between them, the position of the planet's center of mass would shift relative to the surface geography. But a region of lower density material is, perhaps, easier to explain than one of highly dense material - a mining operation, a chemical reaction (the other reactant being less abundant then the core material, it eventually ran out), or even being scooped out by a rogue wormhole.

# an industrial accident

In a universe with something like Tensor–vector–scalar gravity, a Type II civilisation (in its early days) created wormholes or warp drive. Sean Carroll has a few youtube videos that cover Dark Matter: one in particular is more in-depth for TeVeS, but I can find it right now. He uses the less-technical version in several talks.

I find it interesting that TeVeS implies a preferred reference frame for gravity— exactly my favorite way to make FTL travel safe for the arrow of time. He explains how the Ve and Te fields would allow interesting gravitational phenomena to exist (if we can find a way to express it).

So, it all fits: a wormhole hub ruins or an "accident" leaves behind something that makes the additional fields mess with gravity. That civilisation is long gone, and new intelligences are dealing with it. Or maybe it's current and some people have to put up with that as, say, an inherent part of doing business at a major shipping hub.

Expanding on the satellite solution and the issue with the Roche limit.

The Roche limit goes as the cube root of the relative densities. If $M$ refers to the planet and $m$ to the satellite:

$d_{Roche} = R_M(2\rho_M/\rho_m)^{1/3}$

It follows that if the satellite is twice the density of the primary, the Roche limit is not a problem.

So, make your primary have low density and the satellite nickel-iron.

• The satellite can have a highly elliptical orbit.
• Have more than one satellite.

It is really a planet?

I'm thinking of David Weber's Mutineer's Moon. (There are other stories in the series but this is the relevant one.) It turns out Earth's moon was actually destroyed 50,000 years ago--what we see in the sky is a starship on long term picket duty that was concealed by peeling off the outer layers of our moon and covering the ship with them. There was a mutiny, the ship was severely damaged and remains there to this day inhabited only by the now-sentient computer core.

The gravitational anomalies we see on the moon are really the result of the mass distribution of the underlying starship. While it does nothing that changes the gravity of the moon it certainly would be capable of doing so.

If you want to play around with what's going on inside your planet, feel free. Gravity is defined as $F=\frac{Gm_1m_2}{r^2}$, where $m_1$ and $m_2$ are the masses of the objects in question, $G$ is a constant, and $r$ is the distance between the two objects. Since $G$, $r$, and $m_1$ are all constant, you can have the inside of the planet change its mass by some chemical reaction (unobtanium?). As $m_2$ increases, $F$ increases; as $m_2$ decreases, $F$ decreases.

Well, with gravity you mean probably gravitational pull at the surface. So there are some possibilities:

• the planet gains and loses mass somehow;
• the planet expands and contracts (thus changing the pull on the surface, the differences have to be big to be noticeable);
• some extraplanar very heavy and near astronomical body causes tides;
• big rotational changes (while not really influencing gravitational pull the centrifugal force would work in the opposite direction and would so lessen the weight of things on the surface).

And naturally: the world simply has different laws of physics.

It is hard to think about a scenario how the planet loses and gains mass. Something I think of: the planet has an enormous part of its mass in the form of water, that is stored underneath the crust. Some events lead to evaporation of it, so the main mass of the water is leaving to higher atmosphere and raining later back to the planet. Still complicated.

The other possibilities are also difficult. Expansion and contraction of the crust in big style is something I have no idea about how it could happen without breaking suspension of disbelief. Same thing about changes in rotation speed. Some big mass (say a pretty heavy moon) might sound believable, but it seems something that big induces strong tidal forces that will lead to reduction of rotation speed around each other fast (as happens with the moon of the earth, it shows already the same side to the earth at all times, because of the tides induced from earth on the moon and the tides the moon induces on earth will other time lead to the moon being at the same place over the surface of earth at all times).

So I would go with a world of different physics. The force of gravitation is changing regularly.

## protected by ArtOfCodeJun 11 '15 at 8:45

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