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Many people are excited with the idea of super-Earths--rocky, habitable exoplanets greater in mass, density and diameter (because I'd prefer to go the whole way than stop at the halfway point) than our Earth--being also more habitable than our Earth. That would make sense climatically, geologically and magnetically speaking.

But gravitationally speaking, I'm sorry, but I don't buy the excitement.

Anyone would know that the bigger a body, the greater the gravity, and therefore the likelier we'd end up getting crushed upon contact with either the atmosphere or the surface. You won't be seeing any spine as mighty as the Andes, the Himalayas or even the Mid-Atlantic Ridge anywhere anytime soon. On a similar note, the ocean floor will be just one uniform abyssal plain.

But somehow, somehow, one eager terraformer was not aware of any of this, and he decided to colonize this microbe-exclusive, no-oxygen super-Earth with plant, fungus and animal colonists from our Earth, colonists who had not evolved to live under either a crushing atmosphere or a crushing sea-level surface. We've tried and tried, even after we've turned blue, to reason with him, but he wouldn't listen. He insisted that he'd come up with a way to counter the gravity issue. Would such a solution exist without resorting to destructively shrinking the planet's size?

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    $\begingroup$ No it would not $\endgroup$
    – Slarty
    Jan 26, 2020 at 23:44
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    $\begingroup$ The equation for the surface gravity of a planet reduces to $\rho*R$. Increase the radius, reduce the density proportionally, and you can keep surface gravity the same. So, just don't have so much iron in the core. $\endgroup$
    – Spencer
    Jan 27, 2020 at 0:41
  • $\begingroup$ @Spencer Why are people having this issue with diameter? Isn't it better to go all the way than just stop halfway from the finish line? Also, how do I reduce density? $\endgroup$ Jan 27, 2020 at 0:48
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    $\begingroup$ It's unclear to me what you consider the "finish line". You have assumed the Super-Earth will have super-high gravity, but that isn't necessary. You can keep the diameter and have reasonable gravity; just reduce density. Do this by changing the planet's composition. Less metal, more light elements. $\endgroup$
    – Spencer
    Jan 27, 2020 at 0:52
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    $\begingroup$ @JohnWDailey The Earth's core is iron and nickel, which are both very dense, hence them sinking and other elements floating on top. If the ratio were tilted, you could add a large volume of lighter elements in return for removing a much smaller volume of those denser ones. The lightest elements will be the ones numbered lower on the periodic table, which is also convenient since the light elements H, O, N and C are the primary building blocks of life as we know it. $\endgroup$
    – StephenS
    Jan 27, 2020 at 5:26

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Depends, how high gravity do you mean? This study's summary https://arxiv.org/abs/1808.07417 for example suggests that 3-4 g might be doable for humans to train for. As for other life forms, they too should have no problem with that level of gravity. Of course, it would require adaptation, and prepare for human collonists to become fantasy dwarves rather quickly(in manner of few generations).

As far as I can tell, he needs no special solution, just normal things. There's no try, only do. Release the plant and fungal life, it'll find a way, release the animals(though animals that do well in extreme situations), and train your colonists in 2g in a spinning spacecraft, like they're hamsters.

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    $\begingroup$ The linked study appears to be focused on theoretical limits of human locomotion, not the "weakest links", which would be joints, spinal disks and cardiovascular system. Having said that, humans should be able to live long-term at 3-4g with the assistance of motorized platforms and water tanks. $\endgroup$
    – Alexander
    Jan 27, 2020 at 17:32
  • $\begingroup$ Reminds me of the Jinxians in Larry Niven's Known Space novels $\endgroup$
    – The Daleks
    Feb 29, 2020 at 6:41
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Spin it faster. Really fast.

CoRot-7b for example is average Super-Earth at about 1.6 Earth radius (10k) and 8 Earth masses. It would be about 3G at the surface according to a gravity calculator I found.

Rotation doesn't counter much gravity; the rotation the Earth counters less than a tenth of a percent of Earth's gravity at 24 hrs per rotation.

However according to a centrifugal force calculator I found, at 10k radius a rotational period of 1.2 hours would counter 2 gravities.

This might be able to be a bit slower because spinning that fast is going to cause the planet to expand by an unknown amount. The expansion would be greatest at the equator and nonexistent at the poles leading to an ovoid shape.

The effect would not be equal across the planet; the -2G spin effect is at the equator, at the poles the full planetary 3G would be felt.

This brings up interesting dynamics like a 45 minute day, the difficulty of flying over the poles, the effect on migratory birds, and I don't know what the effect on atmosphere density would be across latitudes.

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    $\begingroup$ The most funny effects would be from Coriolis force. This planet would have quite a strong weather! $\endgroup$
    – ksbes
    Jan 27, 2020 at 12:24
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    $\begingroup$ I think it might be funnier to see a 50lb north-bound goose slowly become a 150lb north-bound goose. At some point it's going to be very confused that it can't fly. $\endgroup$
    – Slam
    Feb 3, 2020 at 11:28
  • $\begingroup$ No, no, no! Just give it a incredibly strong magnetic field! I'm sure that won't have any negative consequences. ;P $\endgroup$
    – Muuski
    Feb 3, 2020 at 20:24
  • $\begingroup$ @Slam I never understood this obsession with "radius". Wouldn't you rather go all the way through ("diameter")? $\endgroup$ Feb 29, 2020 at 12:59
  • $\begingroup$ You can say 1.6x radius or 1.6 diameter, same thing. But the calculation for g-force on a spinning surface is based on distance to center not distance to the other side. $\endgroup$
    – Slam
    Mar 1, 2020 at 16:43
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Planets, including super earths, come in all sorts of sizes. There are literally trillions of planets and moons in our galaxy alone, and it’s easy to imagine a slightly larger or denser rocky planet with 1.1g or 1.2g on which humans could survive with little adjustment needed. With rigorous training, the average human can walk around on a planet with 3 to 4 times as much gravity as what we feel on Earth, but even elite athletes will struggle to take a few steps under 5g. For reference: https://www.discovermagazine.com/the-sciences/whats-the-maximum-gravity-we-could-survive

You mention a “crushing sea level surface,” but you’re forgetting about buoyancy, which effectively negates the force of gravity. If a world is rendered uninhabitable by its crushing gravity, floating in the sea or any other body of water would allow humans or native life forms to survive. It’s likely that on a super earth with limited terrestrial life, all of the large animals will be found in the sea. The only reason why features on the sea floor might be flatter is that they feel their own weight, as there is no upward force exerted by water beneath them.

Beyond that, have you considered exoskeletons? This technology already exists. Here, it allows humans to lift heavier loads, but it could also support their bones on a planet where they weigh twice as much. These also come in handy when spacefaring explorers’ muscles and bones have atrophied as a result of life in zero g.

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