<p>When astronomers discuss the "habitable zone" around a star they're really concerned with someone likely to provide liquid water and the right amount of radiation for basic life (cells) as we understand them.  But this doesn't mean they're suitable for humans.</p>

<p>Maximum mass is not an issue, it's gravity that matters.</p>

<p>We need a few things from gravity to be human habitable.</p>

<p>Firstly we're "designed" (evolved) to operate in a one-gee environment.  As far as I know there have been no studies on long term high gee effects on humans (low gee, yes, but not high gee).  However, considering the "plumbing" and structure involved in a human, it seems unlikely to me we'd adapt long term to more than a small percentage larger than one-gee.  Much more on a sustained basis would seem likely to increase physical stress on the bones, muscles, tendons and so on and lead to more rapid development of related illnesses and disabilities.  The plumbing (fluids are being moved all over your body) can cope with quite high gees for short periods, but it's simply not designed to cope with sustained high gee.  The strain on the circulatory system strikes me as the greatest vulnerability.</p>

<p>Movement in high gee is also an issue.  I'd be surprised if people could adapt to long term requirements without serious health side effects.  We'd also be more vulnerable to injury from falls and slips.</p>

<p>Consider how long you could walk in 1.5g.  It would be the equivalent of walking while carrying something weighing half as much as you do yourself. If you were 100 kilos, how long could you walk while carrying 50 kilos ?   If you had to do that all day, everyday, how long before the strain caused serious injuries and other health problems ?  Not long, I think.</p>

<p>So just on this basis I'd suggest a maximum sustained gee of about 1.1 is ballpark.  Walking around with ten kilos when you are 100 kilos sounds like a lot all day everyday, but it's probably doable.</p>

<p>But gravity does more than provide a way to nail us to the ground.  It provides a way to nail the atmosphere to ground at an atmospheric pressure high enough for us to use, and not too high for us.</p>

<p>Now it would appear from human habitations in the deep sea, that we can sustain quite high atmospheric pressures, so that would not seem likely to be a problem.  What tends to hurt humans is shifting quickly from high to low pressures.  As I've argued that we can't sustain high gees for long and need something close to one gee for health reasons, the atmosphere doesn't seem to be a problem in that case.</p>

<p>So what does 1.1g mean in terms of planet size and density.</p>

<p>The gravitational pull is proportional to planet average density and radius.</p>

<p>So we need a planet that's around the 1.1g mark meaning :</p>

<p>$$1.1 \rho_{earth} r_{earth} \approx \rho_{new-earth} r_{new-earth}$$</p>

<p>where $\rho$ is density and $r$ is radius and this represents the limit.</p>

<p>So this may make it easier to find suitable planets.  We aren't restricted to specific mass ranges, but can look for planets that are less dense and larger or more dense and smaller.</p>

<p>But this does make me wonder if the notion of human colonization of planets ever made any sense.  Building our own large habitats in space is starting to make a lot more sense to me as it restricts us less.</p>

CC BY-SA 3.0
2017-05-30T08:56:53.613Z
Thank you for a wonderful answer. I also had the most reservation about the circulatory system, especially the cerebral circulation being the most sensitive and fragile to even the smallest changes in gravity.
Anatoly
CC BY-SA 3.0
2017-05-29T14:14:35.323Z
2017-05-29T14:14:35.323Z
StephenG - Help Ukraine
<p>What is the maximum planet mass for human habitation? How many Gs can humans withstand on a constant basis?</p>

CC BY-SA 3.0
2017-05-29T11:46:14.970Z
I'm voting to close this question as off-topic because it isn't about astronomy, but human physiology.
ProfRob
CC BY-SA 3.0
2017-05-29T13:26:07.077Z
The mass of a planet with gravity similar to Earth and a "habitable" surface is astronomy.   Uranus is 14 times the mass of the Earth and it has lower gravity, but it's obviously no good for human habitation (floating city perhaps?).    The first half of the question is astronomy.
userLTK
CC BY-SA 3.0
2017-05-29T14:55:22.640Z
Likely unknown. You have to find the maximum gravity at which people survive at least through puberty. AFAIK, no one has run a centrifuge that long.
Wayfaring Stranger
<exoplanet>
What is the maximum planet mass for human habitation?
Anatoly
# What is the maximum planet mass for human habitation?

(Asked by: username_0 on 2017-05-29)

What is the maximum planet mass for human habitation? How many Gs can humans withstand on a constant basis?

**Tags:** exoplanet

> **Comments:**
> **username_1**: I'm voting to close this question as off-topic because it isn't about astronomy, but human physiology.  
> **username_2**: The mass of a planet with gravity similar to Earth and a "habitable" surface is astronomy.   Uranus is 14 times the mass of the Earth and it has lower gravity, but it's obviously no good for human habitation (floating city perhaps?).    The first half of the question is astronomy.  
> **username_3**: Likely unknown. You have to find the maximum gravity at which people survive at least through puberty. AFAIK, no one has run a centrifuge that long.  

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## 1 answers

### Answer by username_4 on 2017-05-29. Score: 4 | Accepted answer

When astronomers discuss the "habitable zone" around a star they're really concerned with someone likely to provide liquid water and the right amount of radiation for basic life (cells) as we understand them. But this doesn't mean they're suitable for humans.

Maximum mass is not an issue, it's gravity that matters.

We need a few things from gravity to be human habitable.

Firstly we're "designed" (evolved) to operate in a one-gee environment. As far as I know there have been no studies on long term high gee effects on humans (low gee, yes, but not high gee). However, considering the "plumbing" and structure involved in a human, it seems unlikely to me we'd adapt long term to more than a small percentage larger than one-gee. Much more on a sustained basis would seem likely to increase physical stress on the bones, muscles, tendons and so on and lead to more rapid development of related illnesses and disabilities. The plumbing (fluids are being moved all over your body) can cope with quite high gees for short periods, but it's simply not designed to cope with sustained high gee. The strain on the circulatory system strikes me as the greatest vulnerability.

Movement in high gee is also an issue. I'd be surprised if people could adapt to long term requirements without serious health side effects. We'd also be more vulnerable to injury from falls and slips.

Consider how long you could walk in 1.5g. It would be the equivalent of walking while carrying something weighing half as much as you do yourself. If you were 100 kilos, how long could you walk while carrying 50 kilos ? If you had to do that all day, everyday, how long before the strain caused serious injuries and other health problems ? Not long, I think.

So just on this basis I'd suggest a maximum sustained gee of about 1.1 is ballpark. Walking around with ten kilos when you are 100 kilos sounds like a lot all day everyday, but it's probably doable.

But gravity does more than provide a way to nail us to the ground. It provides a way to nail the atmosphere to ground at an atmospheric pressure high enough for us to use, and not too high for us.

Now it would appear from human habitations in the deep sea, that we can sustain quite high atmospheric pressures, so that would not seem likely to be a problem. What tends to hurt humans is shifting quickly from high to low pressures. As I've argued that we can't sustain high gees for long and need something close to one gee for health reasons, the atmosphere doesn't seem to be a problem in that case.

So what does 1.1g mean in terms of planet size and density.

The gravitational pull is proportional to planet average density and radius.

So we need a planet that's around the 1.1g mark meaning :

$$1.1 \rho\_{earth} r\_{earth} \approx \rho\_{new-earth} r\_{new-earth}$$

where $\rho$ is density and $r$ is radius and this represents the limit.

So this may make it easier to find suitable planets. We aren't restricted to specific mass ranges, but can look for planets that are less dense and larger or more dense and smaller.

But this does make me wonder if the notion of human colonization of planets ever made any sense. Building our own large habitats in space is starting to make a lot more sense to me as it restricts us less.

> **Comments:**
> **username_0**: Thank you for a wonderful answer. I also had the most reservation about the circulatory system, especially the cerebral circulation being the most sensitive and fragile to even the smallest changes in gravity.  

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