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Wouldn't it be cool if giant monsters were on giant planets? Of course it would be, but unfortunately physics doesn't really like that idea. Big planets make things on them need more support, so being big sucks on big planets. It's alright on teeny ones, but bad on big ones. You'd also probably want high pressure on the planet, and low temperatures, but let's forget about that.

So how could one make it not suck to be big on such a planet.

Well....we could make the planet spin faster. Now, these big monsters, effectively, weigh a bit less. They would have to be slow, because the energy to move them (due to inertia) would be the same, so you'd probably looking at creatures with big support structures and less muscle adjacent structures?

Problem, of course, people have asked about how fast the earth would need to spin to make things weightless, of course that would destroy a planet...but what if the question was, instead,

Edit: To keep it simple, I'd like to know if a planet could (effectively) offset 10% of its gravity through rotating fast, without flinging itself into a cloud of space dust.

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    $\begingroup$ "cool if giant monsters were on giant planets", why not? Buoyancy solves a multitude of issues with size and mass - it depends what they're floating in. How do you mean "chthonian planet", do you mean that all the life is underground - or are you talking spirit beings of the underworld here? $\endgroup$ – Confounded by beige fish. May 23 at 17:54
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    $\begingroup$ Welcome to WorldBuilding.SE! There's a lot of different questions you're throwing at us here, it might be better to try and narrow this down to one specific thing that you want us to answer. $\endgroup$ – F1Krazy May 23 at 17:56
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    $\begingroup$ Hey, Ravitheravian welcome indeed, when you have a few minutes, please take the tour and read up in our help center about how we work: How to Ask. At the moment it's not clear what you're asking, could you edit the question to clarify? $\endgroup$ – Confounded by beige fish. May 23 at 18:00
  • $\begingroup$ Hi, welcome to Worldbuilding. Stack Exchange’s model is “one specific question, one specific answer” so this may be closed for being too broad. Can you edit your question to only have one question to make it clear what you’re asking? You can always ask more. Visit the help center if you’re unsure. There is a Sandbox on Worldbuilding Meta where users can develop their questions to make sure they are suitable for the main site. Also, we have a list of resources which may answer any broader questions about worldbuilding: worldbuilding.stackexchange.com/questions/143606/…? $\endgroup$ – Liam Morris May 23 at 18:03
  • $\begingroup$ hmm, yeah, I think I can give it a quick, if drastic, edit. $\endgroup$ – Ravitheravian May 23 at 18:11
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Run to your local used SF book store and find "Mission of Gravity" by Hal Clement.

Meskin is somewhat over Jovian mass, with most of the mass in a relatively small core. The planet spins once every 10 minutes giving it the shape of a poached egg and an equatorial diameter several times its polar diameter. The net result is 3g's acceleration at the equator and 700 at the poles.

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  • $\begingroup$ That sounds....very specifically near what I'm thinking of. Sounds like a good place to look! $\endgroup$ – Ravitheravian May 23 at 18:18
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Yes

Here's the non-mathy version: A planet can spin as fast as gravity will allow. That is, the "speed limit" on angular velocity is the speed at which the outward centrifugal force (rotational inertia for the nitpickers) balances the inward gravitational force. So by definition your condition that 10% of the gravity be offset is achievable and probably doesn't come close to even meeting the caveat below.

Notes and Caveats

As a planet spins it will begin to bulge at the equator and squash at the poles (Earth's radius is roughly 20km larger at the equator). This is a consequence of the aforementioned centrifugal force and the fact that planets exist in hydrostatic equilibrium (act as a fluid at rest) over long time spans. The bulging of the equator as angular velocity increases may change the point at which the planet ceases to be a planet, but this will be a secondary effect.

The centrifugal force will be at maximum at the equator and decrease with the cosine of latitude to zero at the poles. This means the apparent gravity will be lower at the equator but this effect will diminish further away. So your creatures may survive in a certain latitude band.

The direction of the centrifugal force will be directly outward at the equator and vary to being perpendicular to gravity at the poles (of course the magnitude of the force will be zero at the poles too - see above note). The most apparent effect of this is that in the mid-latitudes the ground will feel tilted because gravity will be acting downward and the centrifugal force will be acting outward at an acute angle to gravity.

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A thick, heavy atmosphere would do the trick

You don't need to resort to a spinning planet. If your giants are made of material only marginally heavier than the atmosphere, the atmosphere would support most of their weight, and very large sizes would be possibles. Think of blue whales in the ocean, the largest animal ever in existance, with a mass of up to metric 173 tons (the heaviest reported) or possibly even more.

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