I am writing a story that involves a planet that has three times Earth's mass. The composition of the planet is rocky and it has an atmosphere that is either breathable or breathable after an economical processing method. The planet's size is what an astronomer would expect from a planet with 3x Earth's mass.

I wonder if the atmosphere would retain enough hydrogen to make a difference in the everyday chemistry of the air and water.

But here's the main question: If a band of 100 humans, all of whom are healthy 20 year-olds, were placed on this 3g planet, how would the gravity affect their health? After a week? After five years?

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    $\begingroup$ Are you able to move three times your own body weight? They won't last much time. Also, it means a lot of cardiovascular effort, the heart won't be very healthy. $\endgroup$
    – Ender Look
    Dec 24, 2017 at 4:43
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    $\begingroup$ A planet 3x Earth's mass does not (necessarily) have 3x Earth's gravity. Here is a post explaining how to calculate surface gravity. Please clarify if you want the planet to be 3x Earth's mass or gravity. If you want it 3x Earth's mass, you have to give a radius in order to determine the surface gravity. $\endgroup$
    – kingledion
    Dec 24, 2017 at 4:59
  • $\begingroup$ 3G's means gravitational pull. While the mass/gravity distinction is important, I'm guessing the mass part was misspoken. $\endgroup$
    – Stephan
    Dec 24, 2017 at 7:20
  • $\begingroup$ Why go to such a planet anyway? It's surface is flat and featureless, and i guess if it's not very dry, then it is completely covered in one ocean. $\endgroup$
    – Karl
    Dec 24, 2017 at 10:38
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    $\begingroup$ I have a feeling this may be a duplicate of at least some of those, but it isn't a complete match so I'm not going to do a close vote. $\endgroup$
    – Tim B
    Dec 25, 2017 at 12:09

2 Answers 2


Firstly, we must begin by finding out the surface gravity of such a planet as surface gravity is not dependent only on mass but distance from the center, hence the planet's radius, because $$ g=G\frac{M}{R^2} $$

$g$ = surface, $G$ = Gravitational Constant($\approx 6.67408 × 10^{-11}\,m^3\,kg^{-1}\,s^{-2}$), $M$ = Mass of object, $R$ = Radius of object

Utilizing NASA's Exoplanet Archive and filtering for planets with the mass of 2.5-3.5 Earth masses (no planet will have exactly 3 Earth masses), we get 20 planets. That is a low figure. So we add more data from exoplanets.org to get to a grand total of 289 planets (9 excluded as they were discovered by microlensing and hence, we don't know their radii).

The average mass is $2.996925319\,M_\oplus$(Earth masses) (basically 3), and the average radii is $1.395256824\,R_\oplus$(Earth radius) OR $1.71541\times10^{25}\,kg$ and $8457.431881\,km$. This gives us a final surface gravity of $$ (6.67408 × 10^{-11}\,m^3\,kg^{-1}\,s^{-2})\times\frac{1.71541\times10^{25}\,kg}{(8457431.881\,m)^2}=16.00542\,m\,s^{-2} $$ which is around $1.62956\,g$ (Halo rings from Halo have a measured acceleration of $1.55\,g$).

And frankly, as discussions of hypergravity go, this is pretty low. Comparable to reentry acceleration of 1.8 g. This planet would, on a list, come between Jupiter's $24.92\,m\,s^{-2}$ and Neptune's $11.15\,m\,s^{-2}$.

The effects on humans in the long term will be more than noticeable, but not anything critically dangerous or too awesome of a spectacle. The average of 69.2 kg on earth would now weigh 101.03 kg, so not very comfortable, but survivable for a healthy 20-year-old adult. Falling would be a huge problem now as you will fall harder, but the bones and muscles and/or fat that would normally be pushing the weighing scales further would not be there to reduce the impact force. Hunting would much more difficult so would avoiding being hunted (assuming there is local fauna and flora) due to the increased stress on the heart while running. Sitting down would also be very tiring and would be forgotten in favor of lying down as then there is no vertical displacement of blood required. People will want to lose weight and go down to much more tolerable 75-80 kg range, but there actual mass would be between 46.8 and 50, meaning they would need to have a lot of there body to be pure muscle. Fasting, due to lack of food or just exhaustion, would be difficult as there would be little to no fat in the body.

Though there is some hope as long as there are blood-thirsty man eaters out there, as the new citizens just lie down continuously for a day or two with minimal movement, acclimating to the new environment as the heart and skeletal muscles increase in size and strength. No place for flabs here. Except when they are starving and only flabs survive.

Long term survival:-

Reproduction wouldn't be impossible, and pregnancy wouldn't not kill (atleast in mice). But the babies would be lighter, and much more suited to the environment they were born into. The original survivors would also be much stronger and comfortable than before, but leagues behind their children and grandchildren. This is all assuming the original survivors didn't die out, had a guide book of some kind, built some sort of shelter (which may crush them in an earthquake) and had a food source with a decent availability.

Sooo, yeah.

  1. Have the sudden stress and environment kill some 40% of the initial 100, maybe feed the corpses to the survivors to sustain them.
  2. Get used to the new environment.
  3. Get food.
  4. Get defenses.
  5. Get shelter.
  6. Get it on.

Then humanity will do what it does, conquering lands and messing up habitats.

In between, you may accidentally end up ripping off some survival show.

  • $\begingroup$ Thank you for a remarkably good answer! I will try to correct my mistakes so that I can come up with a planet with 3x Earth's gravity. How would our hypothetical people do then? $\endgroup$ Dec 24, 2017 at 19:04
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    $\begingroup$ @JamesGrossmann Dead. No other details required. Just dead. $\endgroup$ Dec 24, 2017 at 19:05
  • $\begingroup$ @JamesGrossmann You need to adjust the planet's mass and its radius, so the surface gravity will be 3 g. Refer to the question cited by kingdelion in his comment for starters. $\endgroup$
    – a4android
    Dec 25, 2017 at 8:40

You mention three times Earth's gravity, so I'll go by that. Feel free to adjust the below as needed!

Let's say your average character masses 100 kg. (That's not too outlandish, and it's an easy number to work with.) Since Earth's gravity is just a shade under 10 m/s2, that works out to a force (or weight -- note the difference!) of about 100 kgf, 1,000 N or 1 kN.

On a planet with 3 G surface gravity, the same mass (100 kg) will exert three times the force for about 3 kN, approximately equivalent to massing 300 kg on Earth.

How to get from 100 kg to 300 kg on Earth, without altering someone's physiology? Easy, add some weights! Put your characters on Earth, each carrying a well-balanced backpack containing about 200 kg of Stuff. Exactly what you put in the backpack is relatively unimportant. Now, have them go about their daily routine. Better yet, assume this planet is uncultivated, so put them in the middle of a wilderness area.

  • Can they build a shelter? (Remember that materials will also feel heavier, and probably be denser in response to the much higher gravity making it even worse!)
  • Forage?
  • Hunt?
  • Defend themselves against local wildlife? Local intelligent life?
  • Get out of any troublesome situation they find themselves in, such as treacherous ground?

Consider that in the military, carrying half your body weight for extended periods of time still isn't exactly trivial. That would correspond to a gravity of 1.5 G.

It's hard to put an exact figure on how all of this would impact their lives after a particular amount of time, but I think it's safe to say that they would be going through hell. I wouldn't give them long to survive in such an environment, as they would likely be having difficulties even doing something as simple as gathering and drinking water.

And that's without even considering the effects of such gravity on such simple things as, say, their ability to breathe...

  • $\begingroup$ This really doesn't take into consideration the additional gravity on the internal organs. Sure, the muscles might be able to adapt, but the organs? I doubt it. $\endgroup$ Dec 25, 2017 at 19:27
  • $\begingroup$ @JustSnilloc I think the final two paragraphs, and especially the final one paragraph, covers that. It's just a whole lot harder to quantify. $\endgroup$
    – user
    Dec 25, 2017 at 21:22

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