How would gravity on another world influence human lifespan?

[DarthJess]

I am an author/novelist and I'm working on a huge science/fantasy project that's been in development for the past 5 years, and as a science fiction fan I've always hated when people get the science really wrong.

So my question is: how would the gravity of another planet influence human lifespan? Say humans were dropped on another world that was 1.37x more massive than Earth, and had been living there for the last 10,000 years. I'm assuming the first few generations would be affected more than later generations, but would that have much of an impact?

 

[Vanadium 50]

Nobody knows.

 

[phinds]

If you weigh 150lbs, how would you like to have an extra 50lb weight on you day in and day out all day every day? What effect do you think that would have on you?

 

[jrmichler]

You would need faster reflexes to stand up or walk. Think of the times that you stumbled and caught yourself, or dropped and caught something. In higher gravity, you would fall before you could catch yourself. And you would fall harder.

Mankind could evolve greater strength, but would they evolve better knee joints and feet? Obesity is a well known cause of arthritis, so increased gravity would have the same effect: https://www.hopkinsarthritis.org/pa...gement/role-of-body-weight-in-osteoarthritis/.

Try searching effect of zero gravity on human body, then assume the opposite would happen in increased gravity.

Not to mention the need for better supportive underwear for both men and women.

 

[Vanadium 50]

Try searching effect of zero gravity on human body, then assume the opposite would happen in increased gravity.

I don't think that works that way. Is drowning the opposite of dehydration?

 

[hutchphd]

1.37x more massive than Earth, and had been living there for the last 10,000 years.

This would not necessarilly change thevalue of g_local depending upon radius. In fact subtle secondary effects might be more interesting for geeks.

 

[Hornbein]

This would not necessarilly change thevalue of g_local depending upon radius. In fact subtle secondary effects might be more interesting for geeks.

Oh that's right. I didn't think of that. If the density remained the same then the surface gravity would only increase by 11%. The density would increase, boosting the gravity, but how much on a rocky planet?

Math: Let's say the density is unchanged. So the volume is proportional to the mass. The radius increases with the cube root of the volume. Gravity increases linearly with the mass but inversely to the square of the radius. What comes out is that gravity increases with the cube root of the increase in mass. 1.11 is the cube root of 1.37.

I know that in extreme cases radius decreases with increase in mass but I don't think that would happen on a rocky planet.

I'd guess if you want a 37% increase in gravity you need a 80% increase in mass. That's assuming a 4% increase in density.

--

It takes 0.45 seconds here on Earth for a body to free fall one meter. (I recently did that in a bicycle accident. This is long enough to be able to make a plan for impact.) On the 1.37 planet it would take 0.39 seconds. Not a big difference.

People would be shorter and stouter. No surprise there.

--

The increased internal pressure and limited increase in radius makes for increased surface heat. (Not completely certain of that.) The planet would be hotter. To cool down it could be further from the Sun. Then the seasons would be milder and the years longer. If you make the Sun dimmer to keep the distance from the Sun the same as Earth's then the Sun is less massive so the years are longer and seasons milder in this case as well. Or you could instead make the planet less radioactive to cool it down. About half of Earth's heat comes from radioactivity in the core. A decrease in that meansd fewer metals heavier than iron. Lead would be scarcer and gold even more of a rarity than it is now. So maybe lead would be the precious metal. How about : decrease heavy metals a lot, put the planet closer to the Sun. Shorter years, stronger seasons. Less heavy metals would mean fewer neutron star collisions in the past. A younger Universe would take care of that.

Zero radioactivity means distance to the Sun is 0.7 of what it is now to get the same heat. 220 day year. Almost the same as Venus.

Hmm, the increased radius also means collecting more solar energy. Using all the prior assumptions, that's about 50% more collected. Then decrease by the greater surface area radiating away the heat. I suppose they cancel out. Ho hum.

 

[Vanadium 50]

eople would be shorter and stouter. No surprise there.

That's the trope, yes. Any evidence that that's what happens?

 

[DaveC426913]

That's the trope, yes. Any evidence that that's what happens?

Well, natural selection is certainly a plausible mechanism, if the colonists don't take steps to avoid it.
Taller, spindlier colonists would injure themselves more often and more severely. Statistically, they would produce fewer offspring. This would bias the average height over time.

 

[Vanadium 50]

Any evidence that that's what happens?

Well, natural selection is certainly a plausible mechanism, if the colonists don't take steps to avoid it.
Taller, spindlier colonists would injure themselves more often and more severely. Statistically, they would produce fewer offspring. This would bias the average height over time.

So that would be a "no"?

 

[DaveC426913]

So that would be a "no"?

We have evidence that natural selection operates in this manner. A trait that is injurious to a population can tend to weed out that trait, other things being equal.

Can you propose a way of acquiring more direct evidence from an event that hasn't happened yet?

How much evidence do we need for a science fiction story?

 

[hutchphd]

Would they most importantly just be smaller (Galileo and his scale issues)?

 

[hutchphd]

This is long enough to be able to make a plan for impact

I like that feeling of slow motion: what usually goes through my head is

1. thank god for my helmut
2. Tuck and roll: lead with your butt
3. you are far too old for this
4. don't try to catch yourself: hands in prayerful repose
5. relax

Hope you bounced well.

 

[Vanadium 50]

Can you propose a way of acquiring more direct evidence from an event that hasn't happened yet?

Not my problem. It was your claim. If you have evidence for it, it's your job to produce it. If you don't, then its uninformed opinion, nothing more, nothing less.

natural selection operates in this manner

On Earth, most fatalities from falls occur long after childbearing years. Natural selection does not care if 70 year olds do or do not become 80 year olds.

 

[Tazerfish]

The increased internal pressure and limited increase in radius makes for increased surface heat. (Not completely certain of that.) The planet would be hotter.

I thought that stuff was important too, but in my environmental physics lectures they taught that surface temperature is basically just a function of solar radiation absorption and terrestrial infrared emission.
Shortwave in, Longwave out. Over long timescales they need to balance.
You get a sort of dynamic equilibrium with a stable temperature.

How that planet formed or what the pressure is doesn't matter at all, if I understand that correctly. It's just a matter of radiation balance.
Weird, right?

PS: Vanadium, I get the sentiment, but the tone is a little dismissive/harsh.

 

[DaveC426913]

Not my problem. It was your claim. If you have evidence for it, it's your job to produce it. If you don't, then its uninformed opinion, nothing more, nothing less.

False dichotomy. There is a lot of space between 'uninformed opinion' and 'have produced evidence here'.

I am comfortable with that middle ground. Especially considering this is about speculative fiction of the future.

Note the use of the word "would" in the title. This implies the OP is open to extrapolation.

Finally, moving forward, I'll ask you to have a little more decorum than to accuse a fellow member of having an uninformed opinion. It's rude and uncalled for.

 

[Hornbein]

I thought that stuff was important too, but in my environmental physics lectures they taught that surface temperature is basically just a function of solar radiation absorption and terrestrial infrared emission.
Shortwave in, Longwave out. Over long timescales they need to balance.
You get a sort of dynamic equilibrium with a stable temperature.

How that planet formed or what the pressure is doesn't matter at all, if I understand that correctly. It's just a matter of radiation balance.
Weird, right?

PS: Vanadium, I get the sentiment, but the tone is a little dismissive/harsh.

Back in the 19th century Lord Kelvin did the calculation and found that the temperature of the Earth should be about -120 centigrade. They didn't know about radioactive fission so it was a mystery.

Heat from when the heavenly body was formed is also important. Neutron stars usually have many millions of degrees of heat even though they often have no other significant source of energy. Heavenly bodies are hot when first formed due to contraction due to gravity i.e. pressure.

The magnetic fields of the Earth are due to a geodynamo powered by the cooling of the Earth.

 

[Hornbein]

I like that feeling of slow motion: what usually goes through my head is

1. thank god for my helmut
2. Tuck and roll: lead with your butt
3. you are far too old for this
4. don't try to catch yourself: hands in prayerful repose
5. relax

Hope you bounced well.

Almost no one in Tokyo wears a helmet. It's anamolous for such a cautious nation. There is so little traffic that biking isn't as dangerous, I guess. I had only minimal contact with my chin so in this case it wouldn't have made a difference.

Too low for tuck and roll. I would have landed on my face if I had tried that. Butt an impossibility. I don't see how that could happen. I was horizontal face down, going full speed (20mph) and flew about ten feet.

I'm 69 years old.

I saw I was headed for soft earth so opted to land on my chest. I didn't want to scrape off the skin of my palms and stuff the wound with dirt. Spread my arms out to the side so it was like flying. Maybe it was flying. It didn't hurt at all for two days so I thought I'd gotten off scot free but then my ribs did for the next six weeks.

Concrete would have been bad.

 

[hutchphd]

My two potentially nasty crashes have involved inattention to road hazards sufficient to send me over the bars. I use toe clips (the old fashioned strap kind) and so I am already in forward somersault mode when my slow-motion brain kicks in as I separate from the bicycle. It is one more reason I like toe clips. And the helmut allows me to not worry about unsightly forehead road rash as I plot my landing strategy. I have the luxury of riding mostly dedicated paved trails (many defunct railroad beds here in Mid-America: I live a block from the Pennsy Trail which largely follows the 1811? route of the Old National Road )
Please wear a helmut!

 

[Tazerfish]

Back in the 19th century Lord Kelvin did the calculation and found that the temperature of the Earth should be about -120 centigrade. They didn't know about radioactive fission so it was a mystery.

Heat from when the heavenly body was formed is also important. Neutron stars usually have many millions of degrees of heat even though they often have no other significant source of energy. Heavenly bodies are hot when first formed due to contraction due to gravity i.e. pressure.

The magnetic fields of the Earth are due to a geodynamo powered by the cooling of the Earth.

Yes, the release of gravitational binding energy and radioactive decay is very important for the internal temperature of the earth and other planets.
Moreover, you rightly point out that by virtue of the magnetic field these internal processes affect the surface life.

However, the story of lord Kelvin's calculation is not quite how I remember it.
Simple calculations using the Stephan Boltzmann law yield roughly -15C, not -120C. Additionally, the remaining 30C can be explained by the natural greenhouse effect. Shortwave radiation has no trouble getting in, Longwave radiation is impeded on its way out.
This calculation is also *extremely* oversimplified, not taking into account the temperature profile of the earth or atmosphere, albedo or emissivity.


If you're still on the fence, perhaps I can convince you by referring to sources.
Eh, well. I hope Wikipedia will do as well ^^
This is from the current version of the Wikipedia article on the energy budget of the earth (1.Aug 2024):

Earth's internal heat sources and other minor effects​

edit
See also: Earth's internal heat budget and Anthropogenic heat
The geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW)[12] and split approximately equally between radiogenic heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m2 and represents only ***0.027% of Earth's total energy budget at the surface***, being dwarfed by the 173000 TW of incoming solar radiation.[13]

Or you can compare the roughly 340W/m^2 shortwave radiation with this map of geothermal flux measured in *milliwatt* per square meters: https://en.m.wikipedia.org/wiki/Earth's_internal_heat_budget

PS: Sorry for detailing this, it was only tangentially related to life on more massive planets.

 

[hutchphd]

This calculation is also *extremely* oversimplified, not taking into account the temperature profile of the earth or atmosphere, albedo or emissivity.

The amazing thing to me is how close this simple model comes to the actual temperature. I've worked it through several times. As I recall the surface emissivity is mostly unimportant but the atmosphere profile is not. Carl Sagan gave us all a lesson in the possible consequences in his early (PhD) work on Venus and subsequent clarion calls about greenhouse effects (I think orchestrated ignorance started in the early 1970s).

 

[Tazerfish]

I had some thoughts on whether or not natural selection/evolution would happen, leading to smaller stouter humans.

We could try looking at earth.
Did favourable traits get selected for recently?

Over relatively recent timespans (10k years), the most striking perhaps is the development of lactose tolerance.
https://www.smithsonianmag.com/scie...e-europeans-ability-to-digest-milk-180980483/
So we have at least one example of human evolution in recent times.

Skin colour:
The evolution of white skin is also a relatively recent event, happening on similar timescales to the 10k years of the OP.
Because humans still needed vitamin D, despite the much weaker UVB radiation, lighter skin which doesn't absorb as much of the UVB has been selected for.
https://www.science.org/content/article/how-europeans-evolved-white-skin


But people were starving and dying of disease...
Admittedly, humanity is doing its best at fighting back the selection pressures. Whether you have multiple sclerosis or get infected with COVID—modern medicine, better nutrition and social safety nets mean that what could have killed you centuries ago will be much more manageable today.
As a result, the tree of genetic diversity grows. But we've stopped nature from pruning the branches.



I found another example of recent evolution: size differences in response to temperature.
Bergmann's rule says that colder climates produce larger animals.
Mammals have to constantly expend energy so as to not freeze to death. Because mass grows faster with size than surface area, there is an advantage to being large when it's cold.
That's a common explanation as to why mammoths were significantly larger than elephants and why penguins living in the depths of Antarctica are much larger than those living on the tip of South America.

This study, https://link.springer.com/article/10.1007/s11692-023-09616-1? , looked at sizes of the Genus Homo and found a link between their sizes during ice ages and interglacials.

By much the same logic, we'd assume that people living in the frigid north (or Highlands) are taller and heavier than their equatorial counterparts.
That is despite the evolutionarily short timescales on which humanity has settled the world; Europe was only settled by humans in the last 50k years or so.

I think this checks out, though, as always, there are a ton of confounders and complications.
For example, since we use clothes now, the evolutionary costs of being small have been circumvented (in part). Loosing heat too fast has been out- engineered.


Okay. So selection pressures have been solved technologically. People don't really reproduce or die differently because they have this or that trait—assuming that's true, then there still is sexual selection.

Maybe smaller people move more easily on that planet, causing health and beauty ideals shift and so the "unattractive" tall people have less offspring.
Now, this is just a highly speculative example and I make no claims that it would turn out to be true.

But features like the colourful feathers of many birds are not a product of any better adaptation to the environment, it's just that the opposite sex finds them attractive. So if you want to speculate about how the population changes, think about their cultures, their beauty ideals and who gets a lot of children and who gets few.

 

[Tazerfish]

The amazing thing to me is how close this simple model comes to the actual temperature. I've worked it through several times. As I recall the surface emissivity is mostly unimportant but the atmosphere profile is not. Carl Sagan gave us all a lesson in the possible consequences in his early (PhD) work on Venus and subsequent clarion calls about greenhouse effects (I think orchestrated ignorance started in the early 1970s).

I think part of the reason is the fourth power (T^4) in the stefan-boltzmann law. 10% higher temperatures would mean 1.1^4 = 1.46 times more thermal emissions.

You'd have to get the radiation balance *really* wrong in order for the temperature to be vastly different.
Anyways, we were talking about higher gravity planets!

 

[Tazerfish]

So my question is: how would the gravity of another planet influence human lifespan?

God. I think we all forgot the specifics of the question.
It's about human lifespans!
Human lifespans!

Maybe that was just me.

We can first look at all-cause mortality.
3% or so die from accidents, so even if that increases quite a bit, it wont really make a dent. Unless you increase gravity to 2x, I'd just neglect it.

The interesting bit would be heart disease.
A third of all people die from it.
This is the only other cause of death that I expect to change with gravity.

I had this whole post written up how we can assume that heart disease will rise with gravity.
After all, the blood pressure in your feet increases with gravity. Just think of what happens to fighter jet pilots at 2, 5 or even 10g. It's clearly demanding to keep blood pumping against gravity.
Specifically, when bending down your head's blood pressure rises more; your feet generally have higher blood pressure. Clearly, that must be a risk factor!
In order to follow this lead, I looked for studies on heart disease and height, because being taller is *kind of* like living in higher gravity in terms of blood pressure.
But the correlation is actually the other way around. Tall people have _less_ heart disease :/
I don't think that link is causal, but it shoots my gravity, height and heart disease theory down pretty conclusively.

Then I went on a short dive into how giraffes deal with high blood pressure—a problem that humans would have too if we lived in a much higher gravity world.
(Because giraffes are very tall, you know ^^)

We've not really made much progress on this question.
But *if* having large blood pressure differences between your head and toes *were* a big problem, then we *would* see a taller people have a lot more heart disease.
The fact, that we don't, implies that it's not a very big deal.
We also don't see giraffes and Elephants die from heart failure all the time, although they arguably are adapted to their environment, so that's not super solid evidence.

It's also important to keep in mind that aging is very nonlinear. As a simple model, all cause mortality doubles roughly every 8 years. If you've got a 1% chance to die at 52, then you'll have a 2% chance to die at 60.
Even if we **doubled** the death rate due to heart disease (at all ages), all cause mortality would only increase to 1.3, costing somewhere on the order of 5 years of life expectancy. It's just really improbable to die young.


WHAT DOES IT ALL MEAN?
My best guess?
Life expectancy would basically be the same.
So long as there is no massive difference in gravity, I wouldn't expect any significant change in life-expectancy.

Any big advancement in medicine, difference in nutrition, obesity, smoking, drinking ...
All of that is going to be more important than 10% more gravity.

 

[Hornbein]

My two potentially nasty crashes have involved inattention to road hazards sufficient to send me over the bars. I use toe clips (the old fashioned strap kind) and so I am already in forward somersault mode when my slow-motion brain kicks in as I separate from the bicycle. It is one more reason I like toe clips. And the helmut allows me to not worry about unsightly forehead road rash as I plot my landing strategy. I have the luxury of riding mostly dedicated paved trails (many defunct railroad beds here in Mid-America: I live a block from the Pennsy Trail which largely follows the 1811? route of the Old National Road )
Please wear a helmut!

As long as we're at this digression:

I was on a paved trail and had just reached the bottom of a downslope. A natural spring meant permanent water on the path. This allowed algea to grow. When I hit the algea the front wheel lost all traction. The bike went off to the left, I flew off to the right, avoiding the pavement. The bike has no top tube so I launched clean off. I had an injured left big toe at the time so I was very glad that wasn't impacted at all. I got the toe injury by running up an escalator while wearing sandals. The escalator was unique in that had a landing : a flat spot in the middle that still had the moving bed. The tip of a sandal caught on the escalator. This bent down the sandal, removing all protection, and slammed my toe down. The ribs of the escalator scraped off the thick skin, so I had two parallel gouges each the width of an escalator rib. It bled but not enough to overflow the sandal so I didn't get bloody footprints all over the Tokyo Metro. That would have resulted in severe social censure.

A gouge is worse than a cut because the skin has to grow back and fill the gap. This took three weeks. Most of the toe skin detached so it turned white and looked awful until it reattached. I was careful to not let it get infected and avoided situations where someone might step on it (ooch, just the thought...). Loading it with dirt or soaking it in ditch water would have been bad.

 

[DaveC426913]

God. I think we all forgot the specifics of the question.

I didn't... I was sittin' back, watchin' y'all wander all over the countryside...

Any big advancement in medicine, difference in nutrition, obesity, smoking, drinking ...
All of that is gonna be more important than 10% more gravity.

It doesn't even have to be an advancement. Judicious and diligent application of existing technology (in your future world) would surely be top-priority. You can't have your ciitizens - as a population - headed for an early and unpleasant grave. Daily regimens - health checks and exerecise - would be mandatory.

Lots of mandatory exercise, probably bone supplements, back and limb braces, exo-skeletons (they already exist today in prototype form, and we haven't even put a human on Mars).


Maybe take some inspiration from the routines of the ISS and other space stations of yore.

The practicality and extent of this is up to you the author, if you want a well-funded mission, or if you want something like in 'Firefly', where they had the tech to reach out into "the 'verse" but then the human economy collpased, leaving the colonies stranded, with failing tech and inadequate infrasctructure.

 

[Hornbein]

Yes, the release of gravitational binding energy and radioactive decay is very important for the internal temperature of the earth and other planets.
Moreover, you rightly point out that by virtue of the magnetic field these internal processes affect the surface life.

However, the story of lord Kelvin's calculation is not quite how I remember it.
Simple calculations using the Stephan Boltzmann law yield roughly -15C, not -120C. Additionally, the remaining 30C can be explained by the natural greenhouse effect. Shortwave radiation has no trouble getting in, Longwave radiation is impeded on its way out.
This calculation is also *extremely* oversimplified, not taking into account the temperature profile of the earth or atmosphere, albedo or emissivity.


If you're still on the fence, perhaps I can convince you by referring to sources.
Eh, well. I hope Wikipedia will do as well ^^
This is from the current version of the Wikipedia article on the energy budget of the earth (1.Aug 2024):

Earth's internal heat sources and other minor effects​

edit
See also: Earth's internal heat budget and Anthropogenic heat
The geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW)[12] and split approximately equally between radiogenic heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m2 and represents only ***0.027% of Earth's total energy budget at the surface***, being dwarfed by the 173000 TW of incoming solar radiation.[13]

Or you can compare the roughly 340W/m^2 shortwave radiation with this map of geothermal flux measured in *milliwatt* per square meters: https://en.m.wikipedia.org/wiki/Earth's_internal_heat_budget

PS: Sorry for detailing this, it was only tangentially related to life on more massive planets.

Huh. I was working off of old memory -- 30 years ago? -- so I guess I misremembered or the info is now outdated. I also recall that the greenhouse effect was discovered in order to explain the high temperatures of Venus, but maybe that's wrong too.

 

[Tazerfish]

What do you think?
Let's use the "wisdom of crowds" ;)

I expect a 10% increase in gravity, corresponding to 1.37 times the mass of earth, to have a negligible effect on life expectancy.

For a 37% increase in gravity, I'd expect more accidents, joint problems and heart disease which in turn drives up mortality.
While this is pure speculation, I'd guess that the effect is comparable to being obese on earth, costing somewhere around 5 years of life expectancy.

Does that seem reasonable?
Which camp are you in—more or less effect?

 

[DaveC426913]

I forget. Did you account for the fact that a more massive planet, given the same density, has a larger radius? Therefore surface gravity scales approximately linearly with radius, not geometrically.

 

[hutchphd]

This is just a wild-ass guess guesstimate so what difference does the predicate assumption make? Incidentally astronaut Jim Irwin had notable arythmias while on the lunar surface, eventually perishing from heart attack in 1991 after a bicycle ride (no he did not fall off). Thats the only data I know of such as it is. There is of course the experience of Superman....I don't know what gravity was on Krypton. Enough silliness. Too much perhaps.