<p>One thing that tidal forces tend to do is the tidally lock a moon to the planet that it orbits or a planet to the star that it orbits.</p>
<p>If a planet is tidally locked to its star one side will have eternal day and the other will have eternal night.  And thus it is feared that the day side will get too hot for life and all the air and water will freeze solid on the night side, making the planet uninhabitable.</p>
<p>The habitable zone for planets around dim stars is so close to the star that a planet in the habitable zone will be rapidly tidally locked to the star.</p>
<p>There are two possible ways a world can be habitable orbiting such a low mass star.</p>
<p>One is if the circulation of air and water between the light and dark sides of a tidally locked planet is great enough equalize the temperatures and prevent the water and air from freezing out on the dark side.</p>
<p>The other is if the world is a large moon orbiting a giant planet in the habitable zone the moon will become tidally locked to the planet and thus will not be able to be tidally locked to the star.  Thus as the moon orbits around the planet it will have alternations of day and night on all parts of its surface and will not suffer from the temperature extremes resulting from being tidally locked to its star.</p>
<p>Thus tidal effects on a planet are an important factor to consider whether a planet in the habitable zone of the most common type of star, a class M main sequence star, can be habitable.</p>
<p>Stephen H. Dole, in <em>Habitable Planets for Man</em> (1964) discussed the effect of having a very large moon, or being a moon of giant planet, could have on the habitability of planets orbiting dim stars very closely.</p>
<p><a href="https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf" rel="nofollow noreferrer">https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf</a></p>
<p>On pages 74 to 75 Dole believed that could reduce the minimum mass needed for a star to have a habitable planet from 0.72 the mass of the Sun down to 0.35 the mass of the Sun, from about a K3V star to an M3V star.</p>
<p>Dole believed that a planet in the habitable zone of a star with less than 0.35 the mass of the Sun could not be habitable because it would be so close to its star that the stellar tidal forces would make the planet uninhabitable for humans.</p>
<blockquote>
<p>If we assume that the destructive tide level is 20 feet, then the new lower limit on stellar mass would be 0.35 solar mass.</p>
</blockquote>
<p>I am not certain whether Dole was referring to 20 foot mid ocean tides, 20 foot water tides at the seashore, or 20 foot tides in the ground.</p>
<p>As we know, most of the energy which warms the surface of the Earth is radiation from the Sun.  The Earth's surface only gets a tiny proportion of its heat from Sunlight reflected from the Moon, the decay of radioactive isotopes inside the Earth, and tidal heating of the interior of the Earth.</p>
<p>But other worlds might be different.  Those minor sources of surface heat on the Earth could be important sources of heat on other worlds, and might make worlds which don't get enough radiation from their stars warm enough for life.  In fact, tidal heating could be intense enough to heat up a world too much and make in uninhabitable.</p>
<p>If the surface of a world is too hot, the surface water will evaporate more rapidly than it condenses and rains back onto the surface.  Water vapor is a greenhouse gas.  If more and more water vapor accumulates in the atmosphere, it will trap more and more heat from the star, warming the surface and evaporating more and more water in a feedback loop.  Eventually all the water from the planet's surface will be water vapor in the atmosphere and the surface will be dry and lifeless.  Ultraviolet light from the star will break up water molecules and the hydrogen will swiftly escape into outer space leaving only the oxygen atoms behind.</p>
<p>And there are calculations of the maximum amount of heat per unit of surface area a world can have before suffering such a runaway greenhouse effect.</p>
<p>&quot;Exomoon habitability constrained by illumination and tidal heating&quot; 2013, by Rene Heller and Rory Barnes discusses the constraints of tidal heating on the potential habitability of hypothetical exomoons orbiting exoplanets.</p>
<p><a href="https://arxiv.org/pdf/1209.5323" rel="nofollow noreferrer">https://arxiv.org/pdf/1209.5323</a></p>
<p>In it they introduce the concept of the circumplanetary habitable edge, the minimum orbital distance from a giant planet for a moon to orbit to avoid a runaway greenhouse effect.</p>
<p>See also &quot;Magnetic shielding of Exomoons beyond the circumplanetary habitable edge&quot; by Rene Heller.</p>
<p><a href="https://arxiv.org/abs/1309.0811" rel="nofollow noreferrer">https://arxiv.org/abs/1309.0811</a></p>
<p>So the minimum amount of heat per unit area of surface that a world can have before suffering a runaway greenhouse effect is the maximum amount of heat be per unit area of surface that a habitable world can receive from all sources and remain habitable.  Assuming that a world's surface gets no heat from any other source, the maximum about of tidal heating the surface can receive is equal to the minimum amount of heat necessary &amp; sufficient to cause a runaway greenhouse effect.  If a world has other sources of heat, the minimum amount of tidal heating to cause a runaway greenhouse effect will be less.</p>
<p>And therefore the minimum amount of tidal stress necessary to cause tidal heating sufficient to cause a runaway greenhouse effect is the maximum amount of tidal stress a habitable world can possibly have.</p>
<p>In my humble opinion tidal stress can make an otherwise habitable world uninhabitable in several different ways.  And in my opinion tidal heating causing a runaway greenhouse effect would require the least amount of tidal stress to make a world uninhabitable.</p>
<p>Therefore, once the amount of tidal stress necessary to produce enough tidal heating to cause a runaway greenhouse effect on a specific world with specific conditions is calculated, there should be no need to calculate the greater amounts of tidal stress needed to make that specific world uninhabitable in other ways.</p>

CC BY-SA 4.0
2024-10-19T20:35:21.223Z
Thanks, that was a really detailed answer! I actually read that first study already, albiet a while ago, and I was wondering whether the 10-parent-radii distance would still hold up for something far more dense, like a brown dwarf or a white dwarf. Probably not. I'll read that other one too.
Kazon
CC BY-SA 4.0
2024-10-23T19:28:38.613Z
"The habitable zone for planets around dim stars is so close that a planet in the habitable zone will be rapidly tidally locked to the star." The end state is a 1:1 spin-orbit resonance for a low e. If outer planets are sustaining e at a sufficiently high value, the planet will be captured into a higher spin-orbit state (like Mercury).  A close-in planet in a nonsynchronous spin state is experiencing much higher tidal heating. This may cause a change in rheology, which will drive the planet out of the higher spin state, and send it to synchronism (or pseudosynchronism, like TRAPPIST-1 b)
Michael_1812
CC BY-SA 4.0
2024-10-19T05:31:51.390Z
2024-10-19T05:31:51.390Z
M. A. Golding
<p>There is no clear limit on the effects on the planet - maybe high volcanism is fine for some types of life, or they could even exploit tidal stresses. One can argue that beyond the Roche limit there is no world, so this is a proper limit. But maybe there could be other environments than planetary or moon surfaces.</p>
<p>However, there is also hard limit: beyond a certain tidal acceleration molecular bonds are not poossible. When the acceleration is greater than <span class="math-container">$g &gt; E/dm$</span> (where <span class="math-container">$E$</span> is the bond energy, <span class="math-container">$m$</span> a typical atomic mass) chemical bonds are not possible. For carbon bonds, this is about <span class="math-container">$10^{16}$</span> Earth gravities.</p>

CC BY-SA 4.0
2024-10-19T05:34:38.313Z
I don't know why your answer got a downvote.  In any case I have found a method where a much lower level of tidal forces would make a world uninhabitable.
M. A. Golding
CC BY-SA 4.0
2024-10-18T19:38:44.540Z
2024-10-18T19:38:44.540Z
Anders Sandberg
<p>How much tidal stress can a world experience before becoming uninhabitable to life (as we know it)? I believe having oceans helps to reduce stress, as well as low eccentricity. I'm asking because I love the possibility of life on exomoons and around white dwarfs.
If this would belong better in another space (not really sure what they're called here), I apologise.</p>

CC BY-SA 4.0
2024-10-16T18:23:39.573Z
There isn't a way to answer this.  Firstly we aren't sure what the limits of "inhabitable" actually are.  What may be deadly for life on Earth might be quite comfortable for life that has evolved to deal with a planet under tidal stress.  Secondly, there are too many confounding variables,  you've mentioned two: oceans and eccentricity. I'm sure we could think of several more: rotation rate, axial tilt, mass, composition and diameter.  So there isn't a simple "a stress of 2.5 is the upper limit" or whatever.
James K
CC BY-SA 4.0
2024-10-16T19:47:04.550Z
Regarding "If this would belong better in another space" - perhaps the [WorldBuilding StackExchange](https://worldbuilding.stackexchange.com). But not here.
David Hammen
CC BY-SA 4.0
2024-10-17T00:38:15.870Z
I'd say this question can be considered on-topic here (though it's somewhere near the edge). Yet with the right kind of view or take on the question it fits. Habitability of planets is part of astronomy, and these kind of session are found in professional conferences also for years (e.g. just recently EPSC).
planetmaker
CC BY-SA 4.0
2024-10-17T00:43:19.337Z
Not too long ago planets were found in the habitable zone of white dwarfs (e.g. https://www.universetoday.com/154536/planet-found-in-the-habitable-zone-of-a-white-dwarf/ and reference therein) - and there is indication that they are not new planets, but did survive the AGB phase somehow
planetmaker
CC BY-SA 4.0
2024-10-17T06:00:54.697Z
@JamesK " we aren't sure what the limits of "inhabitable" are." == That's what he's asking. What are the limits of "inhabitable" for life as we know it, given extreme tidal activity . This question is legitimate and shouldn't be closed.
Miss Understands
CC BY-SA 4.0
2024-10-17T14:42:02.657Z
I'm having a hard time thinking of any amount of tidal force that would preclude life, unless it's actually hitting the roche limit and disintegrating your planet into space debris. A water world with a decent atmosphere and floating algal mats wouldn't care much of anything about tidal forces, it seems to me.
Darth Pseudonym
CC BY-SA 4.0
2024-10-18T14:14:59.380Z
This is too broad. "Life (as we know it)" is everything from bacteria that would love more thermal vents to much more picky humans.
Mithoron
CC BY-SA 4.0
2024-10-21T19:05:17.693Z
life can be an assembly of 5-50 molecules, and simple life can handle extremes, while complex life needs stability.
bandybabboon
<tidal-forces><life><astrobiology>
Maximum tidal stress for life?
Kazon
# Maximum tidal stress for life?

(Asked by: username_0 on 2024-10-16)

How much tidal stress can a world experience before becoming uninhabitable to life (as we know it)? I believe having oceans helps to reduce stress, as well as low eccentricity. I'm asking because I love the possibility of life on exomoons and around white dwarfs.
If this would belong better in another space (not really sure what they're called here), I apologise.

**Tags:** tidal-forces, life, astrobiology

> **Comments:**
> **username_1**: There isn't a way to answer this.  Firstly we aren't sure what the limits of "inhabitable" actually are.  What may be deadly for life on Earth might be quite comfortable for life that has evolved to deal with a planet under tidal stress.  Secondly, there are too many confounding variables,  you've mentioned two: oceans and eccentricity. I'm sure we could think of several more: rotation rate, axial tilt, mass, composition and diameter.  So there isn't a simple "a stress of 2.5 is the upper limit" or whatever.  
> **username_2**: Regarding "If this would belong better in another space" - perhaps the [WorldBuilding StackExchange](https://worldbuilding.stackexchange.com). But not here.  
> **username_3**: I'd say this question can be considered on-topic here (though it's somewhere near the edge). Yet with the right kind of view or take on the question it fits. Habitability of planets is part of astronomy, and these kind of session are found in professional conferences also for years (e.g. just recently EPSC).  
> **username_3**: Not too long ago planets were found in the habitable zone of white dwarfs (e.g. https://www.universetoday.com/154536/planet-found-in-the-habitable-zone-of-a-white-dwarf/ and reference therein) - and there is indication that they are not new planets, but did survive the AGB phase somehow  
> **username_4**: @username_1 " we aren't sure what the limits of "inhabitable" are." == That's what he's asking. What are the limits of "inhabitable" for life as we know it, given extreme tidal activity . This question is legitimate and shouldn't be closed.  
> **username_5**: I'm having a hard time thinking of any amount of tidal force that would preclude life, unless it's actually hitting the roche limit and disintegrating your planet into space debris. A water world with a decent atmosphere and floating algal mats wouldn't care much of anything about tidal forces, it seems to me.  
> **username_6**: This is too broad. "Life (as we know it)" is everything from bacteria that would love more thermal vents to much more picky humans.  
> **username_7**: life can be an assembly of 5-50 molecules, and simple life can handle extremes, while complex life needs stability.  

---

## 2 answers

### Answer by username_8 on 2024-10-19. Score: 4

One thing that tidal forces tend to do is the tidally lock a moon to the planet that it orbits or a planet to the star that it orbits.

If a planet is tidally locked to its star one side will have eternal day and the other will have eternal night. And thus it is feared that the day side will get too hot for life and all the air and water will freeze solid on the night side, making the planet uninhabitable.

The habitable zone for planets around dim stars is so close to the star that a planet in the habitable zone will be rapidly tidally locked to the star.

There are two possible ways a world can be habitable orbiting such a low mass star.

One is if the circulation of air and water between the light and dark sides of a tidally locked planet is great enough equalize the temperatures and prevent the water and air from freezing out on the dark side.

The other is if the world is a large moon orbiting a giant planet in the habitable zone the moon will become tidally locked to the planet and thus will not be able to be tidally locked to the star. Thus as the moon orbits around the planet it will have alternations of day and night on all parts of its surface and will not suffer from the temperature extremes resulting from being tidally locked to its star.

Thus tidal effects on a planet are an important factor to consider whether a planet in the habitable zone of the most common type of star, a class M main sequence star, can be habitable.

Stephen H. Dole, in *Habitable Planets for Man* (1964) discussed the effect of having a very large moon, or being a moon of giant planet, could have on the habitability of planets orbiting dim stars very closely.

<https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf>

On pages 74 to 75 Dole believed that could reduce the minimum mass needed for a star to have a habitable planet from 0.72 the mass of the Sun down to 0.35 the mass of the Sun, from about a K3V star to an M3V star.

Dole believed that a planet in the habitable zone of a star with less than 0.35 the mass of the Sun could not be habitable because it would be so close to its star that the stellar tidal forces would make the planet uninhabitable for humans.

> If we assume that the destructive tide level is 20 feet, then the new lower limit on stellar mass would be 0.35 solar mass.

I am not certain whether Dole was referring to 20 foot mid ocean tides, 20 foot water tides at the seashore, or 20 foot tides in the ground.

As we know, most of the energy which warms the surface of the Earth is radiation from the Sun. The Earth's surface only gets a tiny proportion of its heat from Sunlight reflected from the Moon, the decay of radioactive isotopes inside the Earth, and tidal heating of the interior of the Earth.

But other worlds might be different. Those minor sources of surface heat on the Earth could be important sources of heat on other worlds, and might make worlds which don't get enough radiation from their stars warm enough for life. In fact, tidal heating could be intense enough to heat up a world too much and make in uninhabitable.

If the surface of a world is too hot, the surface water will evaporate more rapidly than it condenses and rains back onto the surface. Water vapor is a greenhouse gas. If more and more water vapor accumulates in the atmosphere, it will trap more and more heat from the star, warming the surface and evaporating more and more water in a feedback loop. Eventually all the water from the planet's surface will be water vapor in the atmosphere and the surface will be dry and lifeless. Ultraviolet light from the star will break up water molecules and the hydrogen will swiftly escape into outer space leaving only the oxygen atoms behind.

And there are calculations of the maximum amount of heat per unit of surface area a world can have before suffering such a runaway greenhouse effect.

"Exomoon habitability constrained by illumination and tidal heating" 2013, by Rene Heller and Rory Barnes discusses the constraints of tidal heating on the potential habitability of hypothetical exomoons orbiting exoplanets.

<https://arxiv.org/pdf/1209.5323>

In it they introduce the concept of the circumplanetary habitable edge, the minimum orbital distance from a giant planet for a moon to orbit to avoid a runaway greenhouse effect.

See also "Magnetic shielding of Exomoons beyond the circumplanetary habitable edge" by Rene Heller.

<https://arxiv.org/abs/1309.0811>

So the minimum amount of heat per unit area of surface that a world can have before suffering a runaway greenhouse effect is the maximum amount of heat be per unit area of surface that a habitable world can receive from all sources and remain habitable. Assuming that a world's surface gets no heat from any other source, the maximum about of tidal heating the surface can receive is equal to the minimum amount of heat necessary & sufficient to cause a runaway greenhouse effect. If a world has other sources of heat, the minimum amount of tidal heating to cause a runaway greenhouse effect will be less.

And therefore the minimum amount of tidal stress necessary to cause tidal heating sufficient to cause a runaway greenhouse effect is the maximum amount of tidal stress a habitable world can possibly have.

In my humble opinion tidal stress can make an otherwise habitable world uninhabitable in several different ways. And in my opinion tidal heating causing a runaway greenhouse effect would require the least amount of tidal stress to make a world uninhabitable.

Therefore, once the amount of tidal stress necessary to produce enough tidal heating to cause a runaway greenhouse effect on a specific world with specific conditions is calculated, there should be no need to calculate the greater amounts of tidal stress needed to make that specific world uninhabitable in other ways.

> **Comments:**
> **username_0**: Thanks, that was a really detailed answer! I actually read that first study already, albiet a while ago, and I was wondering whether the 10-parent-radii distance would still hold up for something far more dense, like a brown dwarf or a white dwarf. Probably not. I'll read that other one too.  
> **username_9**: "The habitable zone for planets around dim stars is so close that a planet in the habitable zone will be rapidly tidally locked to the star." The end state is a 1:1 spin-orbit resonance for a low e. If outer planets are sustaining e at a sufficiently high value, the planet will be captured into a higher spin-orbit state (like Mercury).  A close-in planet in a nonsynchronous spin state is experiencing much higher tidal heating. This may cause a change in rheology, which will drive the planet out of the higher spin state, and send it to synchronism (or pseudosynchronism, like TRAPPIST-1 b)  

---

### Answer by username_10 on 2024-10-18. Score: 1

There is no clear limit on the effects on the planet - maybe high volcanism is fine for some types of life, or they could even exploit tidal stresses. One can argue that beyond the Roche limit there is no world, so this is a proper limit. But maybe there could be other environments than planetary or moon surfaces.

However, there is also hard limit: beyond a certain tidal acceleration molecular bonds are not poossible. When the acceleration is greater than $g > E/dm$ (where $E$ is the bond energy, $m$ a typical atomic mass) chemical bonds are not possible. For carbon bonds, this is about $10^{16}$ Earth gravities.

> **Comments:**
> **username_8**: I don't know why your answer got a downvote.  In any case I have found a method where a much lower level of tidal forces would make a world uninhabitable.  

---

