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I have a planet about 2 Earth Radii large, and even at a lighter Density this puts it squarely in the 10 Earth Mass Range and about 2.5g. It's about 90% ocean, has a thick atmosphere, and near constant storms.

I don't need this to be a specifically human-habitable world, it's intended for big hexapodal aliens, however I would like to see if I can tweak the gravity to be more Earth comparable, somehow, like 1.25-1.5 g instead for more dynamic mountains/geological features, and ideally also megafauna and creatures that can fly.

Are there any (with some wiggle room) scientific ways I can lower the gravity/mass without compromising the planet's stability (making sure it has plate tectonics and oceans and weather etc.) - Highly improbable but possible is very much welcome.

Barring that, any absolutely wild sci fi concepts would be handy too.

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  • $\begingroup$ Gravity is obviously relevant to a conversation about flight, but denser air also creates more to fly in. Mars has less gravity than Earth, but its super thin atmosphere make it very difficult to fly in. With the right atmostphere your world may be very well suited to flight even with strong gravity. In Earth's atmosphere insects essentially swim! $\endgroup$
    – John McD
    Mar 30, 2023 at 15:42
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    $\begingroup$ @JBH The inverse square law is very much the basis of Newtonian gravitational force. It is the granddaddy of all inverse square laws. $\endgroup$
    – biziclop
    Mar 30, 2023 at 16:22
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    $\begingroup$ @JBH Exactly. So for the same M double the R means 1/4 acceleration. The inverse square is right there in the equation :) $\endgroup$
    – biziclop
    Mar 30, 2023 at 16:25
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    $\begingroup$ @biziclop I apologize, you're correct. For the gravity to be equal GM(earth)R(earth)^2 = GM(planet)(2R(earth))^2. So the planet must be 4X the mass of Earth in 8X the volume which is 0.5X density. $\endgroup$
    – JBH
    Mar 30, 2023 at 16:29
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    $\begingroup$ Mostly I feel this StackExchange is an opportunity to discuss inverse square law in a variety of interesting contexts. $\endgroup$
    – John McD
    Mar 30, 2023 at 16:32

2 Answers 2

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It has a thicker crust and mantle, but an Earth size core.

To achieve 1.25-1.5 G your planet needs a mass somewhere between 5 and 6 Earths, but it has a volume of 8 Earths. Earth has a density of ~5.51 g/cm³, but you need a density of ~3.44-4.13g/cm³. This is right in the same approximate density as Mars which is 3.93 g/cm³; so, we know for a fact that rocky planets can form at this density.

The Earth is more dense than Mars because it has a proportionally larger iron core, but Earth and Mars's mantle and crust are largely composed of silicates and oxides with much have much lower average densities in the 2–3 g/cm³ range.

Since the Earth's magnetosphere extends 6 to 10 times the radius of Earth, it means that if you put an Earth sized core in a super Earth with twice the radius, that it would still be protected from solar radiation while significantly increasing the planet's mantel-to-core ratio. In fact, if you kept the core at exactly Earth size, you would actually drop plant's density to ~3.2g/cm³ giving you a surface gravity of just 1.16g making it a quite comfortable planet for humans to walk on.

enter image description here

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    $\begingroup$ The core dynamo is driven by convection, and with more than twice as thick a mantle sitting on top of it, you're not going to get as much of a temperature gradient to drive that convection. But, again, you probably don't need one anyway. $\endgroup$ Mar 30, 2023 at 21:15
  • $\begingroup$ @ChristopherJamesHuff The gradient should be about the same, but it will get a lot hotter towards the core. If anything I would expect a stronger field because of the lower magma viscosity. $\endgroup$
    – Nosajimiki
    Mar 30, 2023 at 21:41
  • $\begingroup$ The gradient only exists because of heat loss through the mantle. That will be greatly reduced. It'd be hotter, yes, but more uniform. $\endgroup$ Mar 30, 2023 at 23:14
  • $\begingroup$ This is handy, thanks - to clarify, if using this method how thick could you approximate the crust to become? I would imagine this might affect tectonic activity $\endgroup$
    – Rexotec
    Apr 1, 2023 at 6:15
  • $\begingroup$ @Rexotec The crust could be the same thickness. The abundance of radioactive material in the mantle has more to do with how molten it is than anything else; so, any difference that added mass and volume would make could be offset with different concentrations of Uranium. So you could still have a very Earth like crust and upper mantel. $\endgroup$
    – Nosajimiki
    Apr 1, 2023 at 20:23
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Changing the metal buildup of the core

When considering the overall mass of a planet (especially when it's solid), it's what's under the crust that matters. This is because the crust of Earth relative to the core is comparable to the thickness of a skin of an apple compared to the inside of the apple.

Assuming that your Super Earth has a similar core build up to our Earth, it's going to have mainly iron & nickel. Iron is a pretty necessary metal to have in your core, as it's what produces the magnetic field that keeps us safe from the Sun's solar winds. Along with that, iron isn't too dense of a metal, with a density of 7.87 g/cm^3.

However, the nickel in our core doesn't provide enough benefit to argue it staying, along with it being denser than iron, at 8.9 g/cm^3. Thus meaning, we could change out this nickel for a much less dense metal, such as Titanium (4.51 g/cm^3).

This would make your planet a lot lighter, along with not having much detriment on the integrity of the planet.

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    $\begingroup$ Unfortunately nickel is a relatively rare element overall, barely consisting of over 1% of the Earth's total mass. The largest contributors by far are iron and oxygen. $\endgroup$
    – biziclop
    Mar 30, 2023 at 16:15
  • $\begingroup$ I see, then perhaps replace some iron with titanium, as well as all of the nickel? The OP said they were okay with science fiction ideas. Perhaps that might allow an entirely titanium core with the inhabitants making the magnetic field? $\endgroup$ Mar 30, 2023 at 17:20
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    $\begingroup$ The magnetism of iron at the surface conditions of Earth has nothing to do with planetary magnetism, which is a product of convection patterns in an electrically conductive fluid. And titanium is highly reactive and only found in tiny amounts in elemental form. Sulfur would be a more likely element, the core of Mars is surprisingly large and low density, and is suspected to have a large sulfur content. At any rate, with an escape velocity in the area of 25 km/s and a dense atmosphere, it doesn't really need a magnetic field. $\endgroup$ Mar 30, 2023 at 21:09

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