<p>Not necessarily. There are lot of factors affecting the existence of a planet's atmosphere, the most predominant are: 1. Composition of the atmosphere. 2.Composition of the planet itself. 3. Star 4. Gravitational Acceleration</p>
<p>The composition of the atmosphere plays an important role in stability of the atmosphere of a planet. If the composition of atmosphere is similar to that of Mars or Pluto, the atmosphere is most likely to get eroded by solar winds. or may suffer a collapse respectively, Lets see what happened to Mars, if Mars replaced Pluto it will have a huge atmosphere compared to what it has now (solar winds from Sun severely affected the Mars' atmosphere.) But it always depends upon the type/size of the parent star and the distance from it (Pluto's perihelion?). but which most likely will not happen as you are talking about Super-Earth.</p>
<p>The Composition of planet itself can affect its atmosphere's stability, the important factor that comes in effect due to this is the gravity, because your super-Earth has to go under multiple challenges through its journey in its orbit, if the condition between your Planet and the parent star is not stable it may end up collapsing the atmosphere. it may exist if your super-Earth is similar to our Earth except its size and the star is as big as Sirius.</p>
<p>The Star. If your saying it is Orbiting a Neutron star or a Dwarf star (Proxima Centauri ?), Which will either erode or your atmosphere will collapse respectively, if the star is similar to that of Sun, the atmosphere will most likely to exist. The way the planet tidally locked to the star also affects the atmosphere's stability. Its not about distance between the star and the planet, The distance have a factor in maintaining the planet's atmosphere although but it can be as far as it could be or it can be as close as it can, the only thing is the atmosphere should be <em>stable</em> with respect to the factors mentioned. as long as you don't concern the state of the atmosphere.</p>
<p>Gravitational Acceleration, one factor which depends on the mass of the planet, also plays a role in atmosphere, greater the magnitude, greater the ability of the planet to retain its atmosphere</p>
<p>There a lot of other factors too, Everything affects Everything, But I mentioned some whose factor are not as small as air resistance.</p>

CC BY-SA 4.0
2021-12-20T15:41:12.803Z
Do you have any evidence to support this?
WarpPrime
CC BY-SA 4.0
2021-12-20T16:53:05.103Z
@fasterthanlight https://www.aanda.org/articles/aa/full_html/2020/06/aa37513-20/aa37513-20.html will be helpful although it does not fully explain it. Would also like to refer articles on Solar Winds' effect on Atmosphere (like https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space#:~:text=Solar%20wind%20and%20radiation%20are,results%20from%20NASA's%20MAVEN%20spacecraft.), range, impact, and tidal lock.
Kavin Ishwaran
CC BY-SA 4.0
2021-12-20T18:14:03.613Z
Concerning the super-Earth we can more exactly refer to Rowan-Robinson's planet from my previous question. At 4 Earth masses, it is likely a super-Earth (if it exists in the first place) and as far from the Sun as where my question about an atmosphere comes into play.
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-21T03:46:03.777Z
@Giovanni-ReinstateCeresPluto  Talking about Rowan Robinson Planet, it is still in the shelf and although its not high to explore through it, but no one brought it down yet. You have to include your reference in the question
Kavin Ishwaran
CC BY-SA 4.0
2021-12-21T07:14:07.170Z
The planet is just an example, it could be any super-Earth far enough from its parent star. Brown's and Batigyn's planet (~6 Earth masses), if it exists, may be a super-Earth as well.
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-21T07:25:03.937Z
Ok then, To be more specific there is no mass limitation for Atmosphere existence until you are not particular about distance. If your give an example of a star and the distance we can actually come to a conclusion
Kavin Ishwaran
CC BY-SA 4.0
2021-12-22T16:50:31.620Z
The other property you can add to the list of three is the magnitude of the gravitational acceleration for the planet, which depends on the mass of the planet. The greater the value of gravitational acceleration, the greater its ability to retain particular gases & thus retain an atmosphere.
Fred
CC BY-SA 4.0
2021-12-22T16:51:58.323Z
@Fred Oh yes, I will add it now
Kavin Ishwaran
CC BY-SA 4.0
2021-12-22T17:49:43.170Z
Gravitational acceleration / escape velocity are concerning the proximity to the star, but wouldn't a too far planet have its atmosphere frozen to the ground, that is the question? If Pluto's and Eris' atmospheres collapse (the moon Io's as well when eclipsed by Jupiter) wouldn't a very high gravity even _help_ them collapse? Or would a thick enough atmosphere always persist in gaseous form regardless of temperature?
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-20T15:31:29.303Z
2021-12-22T16:55:41.270Z
2021-12-22T16:55:41.270Z
Kavin Ishwaran
<p>The question as asked cannot be answered easily; there too many parameters which determine the thickness of a planet's atmosphere.</p>
<p>The main factors are the surface temperature and the planetary mass (which together define the escape velocity) and the atmospheric composition (which defines whether the gas molecules of the given temperature exceed or approach the escape velocity).</p>
<p>Luckily there have been studies on the matter like this one by Samuel Konatham and collegues (though their main focus is on atmospheres of potentially habitable planets): <a href="https://royalsocietypublishing.org/doi/10.1098/rspa.2020.0148" rel="nofollow noreferrer">https://royalsocietypublishing.org/doi/10.1098/rspa.2020.0148</a>
In essence: the terrestrial planets are usually capable to hold atmospheres, and so are the super earths. The Neptunian-style planets (in the habitable zone) often are not capable to hold atmospheres for long times due to their predominantly light-weight elemental composition of mainly hydrogen and helium. Further out: see our solar system - every planet has an atmosphere.</p>

CC BY-SA 4.0
2021-12-22T17:53:18.410Z
Mercury and Ceres don't have any significant atmospheres unless you're having the same stance as Mr. Witthoft (see my talk with him in comments). I ain't sure what you mean by "Neptunian-style planets (ice giants?) often aren't capable to hold atmospheres" because gas giants are _only_ atmosphere except for a solid/liquid core.
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-23T07:20:17.633Z
There are gas giants very close to their star btw. https://en.wikipedia.org/wiki/Hot_Jupiter
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-22T17:27:41.447Z
2021-12-22T17:27:41.447Z
planetmaker
<p>Short Answer:</p>
<p>The question and answers seem to ignore one important factor in the ability of planets and other worlds to retain atmosphere.</p>
<p>Escape velocity.</p>
<p>The escape velocity of a world depends on the mass, radius, and average density of that.  At the present time a minimum mass of 0.12 Earth mass seems to be considered necessary, but not sufficient, for a planet to retain a significant atmosphere for a long time.</p>
<p>Long Answer.</p>
<p>Planets with higher surfaces gravities usually have higher escape velocities.</p>
<p>Planets with higher escape velocities usually have higher surface gavities.</p>
<p>But different formulas are used to calculate the surface gravity and the escape velocity of a world.   They do not change at the same rate as each other with different planetary masses and diameters.</p>
<p>If the question asks aboutjplanets retaining atmosphere in order to be habitable for life, it should be mentioned that there are different types of habitability.</p>
<p>For example, there is habitability for humans beings.  Planets habitable for human beings should be a subset of planets on which some organisms found on Earth could survive, which should be a subset of planets with liquid water where some lifeforms similar to some Earth lifeforms could survive, whichou could be a subset of planets where beings with hypothetical alien biochemestries might possibly survive if there actually can be life using those hypotheical biochemestries.</p>
<p>Here is a link to a scientific discussion of planetary habitability for humans, <em>Habitable Planets for Man</em> Stephen H. Dole, 1964:</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 13 to 19 Dole considers human requirements for atmosphere, including minimum needs for a few gases and maximum limits for them and for many other gases.</p>
<blockquote>
<p>To summarize then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7 milllimeersof mercury.  In addition, the partial pressure of the inert gases must be below certain specified limits and the other toxic gases must not be present in more than trace amounts.  Some nitrogen must be present so that nitrogen in combined form can find its way into plants.</p>
</blockquote>
<p>Since a human habitable planet must have liquid water, there must also be some water vapor in the atmosphere.</p>
<p>Dole presented a table giving the time for an atmosphere to escape with various ratios betgween the escape velocity of a planet and the root-mean-square velocity of air particles in the escape layer of the atmosphere, table 5 on page 35.</p>
<p>If the escape velocity is one or two times the root-mean-square velocity of the gases, the planet will lose them instantly.  If the escape velocity is three times, it can hold the gases for a few weeks.  If the escape velocity is four times, the planet can hold the gases for several thousand years.  If the escape velocity is five times, the planet can hold the gases for about 100 million years.  If the escape velocity is six times, the planet can hold the gases infinitely long.</p>
<p>Dole sums it up:</p>
<blockquote>
<p>If a planet is to be able to capture a gas, the planetary escape velocity must be three or four times the root-mean-square velocity; for a planet to retain a gas permenently, the escape velocity must be about five or six times the root-mean-squae velocity.</p>
</blockquote>
<p>On pages 54 to 58 Dole calculates the minimum mass necessary for a planet to retain an oxygen-rich atmosphere.  The temperature, and thus the velocity, of oxygen molecules and atoms in the exosphere or escape layer of the atmosphere is important to the planet's ability to retain oxygen.</p>
<p>On page 54 Dole considered it possible for a habitable planet with a habitable surface temperature range to have temperatures in its exosphere as low as 1000 degress K (276.85 C or 1340.33 F).</p>
<blockquote>
<p>However, if we take as a rough approximation that maximum exosphere temperatures as low as 1000 K are not incompatable with the required surface condiitons of a habitable planet, then the escape velocity of the smallest planet capable of retaining atomic oxygen may be as low as 6.25 kilometers per second (5 X 1.25).  Going back to figure 9, this may be seen to correspond to a planet with a mass of 0.195 Earth mass, a radius of 0.63 Earth radius, and a surface gravity of 0.49 <em>g.</em></p>
</blockquote>
<p>Dole considered such a small planet capable of retaining an oxygen atmopshere but not capable of producing one, and then when on to estimate the inimum mass for a planet that could produce an oxygen atmosphere.</p>
<p>0.63 of Earth's radius of 6,371 kilometers or 3,959 miles would 4,013.73 kilometers or 2,494.17 miles, and the diameter of that planet would be twice the radius, of course - 8,027.46 kilometers or 4,988.34 miles.</p>
<p>Since Dole wrote a lot has been discovered about the surface temperatures of various world's in our solar system, and about the temperatures in the exospheres or escape layers of the atmosphere's of those worlds which have atmospheres.</p>
<p>So it is possible that modern estimates might change Dole's view that an exopshere temperature of a habitable planet could be as low as 1000 degress K (276.85 C or 1340.33 F).</p>
<p>Figure 9 on page 31 gives the relationship between the mass of terrestrial type planets and their radius, surface gravity, and escape velocity.  Today we have more accurate information about the masses and radii of some solar system planets, and we have some information about the masses and radii of terrestrial type planets in other star systems.</p>
<p>So together, those two factors could make the minimum mass of a planet capable of retaining an oxygen rich atmosphere different from what Dole calculated.</p>
<p>Here is a link to a 2013 article discussing the potential habitability of hypothetical planetary mass exomoons of giant planets orbiting in the circumstellar habitable zones of other stars:</p>
<p><a href="https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf" rel="nofollow noreferrer">https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf</a></p>
<p>In this case habitabiity means teh more general case of habitablity for lifeforms which require liquid water and not the more specific cases of habitablity for humans.</p>
<p>The mass range of a hypothetical habitable exomoon is discussed on pages 3 &amp; 4.</p>
<blockquote>
<p>A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕,
Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger
2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and
to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al.
1996; Gurnett et al. 1996), suggesting that satellite masses &gt; 0.25M⊕ will be adequate for considerations of exomoon
habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal
Heller &amp; Barnes (2013) – Exomoon habitability constrained by illumination and tidal heating
3
4 Maintained by Robert Jacobson, <a href="http://ssd.jpl.nasa.gov" rel="nofollow noreferrer">http://ssd.jpl.nasa.gov</a>.
heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is
given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity
and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly
suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed
around 2M⊕ (Gaidos et al. 2010; Noack &amp; Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect
approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for
Exomoons with Kepler (HEK) project (Kipping et al. 2012).</p>
</blockquote>
<p>The sources for a minimum mass of 0.12 Earth mass for a planet or moon to retain a substantial atmosphere for long time are given as;</p>
<blockquote>
<p>Williams, D. M., Kasting, J. F., Wade, R. A. 1997, Nature, 385, 234</p>
<p>Kaltenegger, L. 2000, ESA Special Publication, 462, 199</p>
</blockquote>
<p>So someone can look up those articles to see the basis of their calculations.</p>
<p>So it appears that a minimum mass of 0.12 Earth mass is necessary for a planet to retain a dense atmosphere for a long time.</p>
<p>And it is also important for a planet to have a strong enough magnetosphere to defect most charged particles from the solar wind and prevent them from knocking off atmospheric particles.  Except for an exomoon orbiting within the magnetosphere of a giant planet, of course.</p>
<p>So having a high enough mass and escape velocity partially depends of the temperature and thus velocity of the air particles in the exosphere of the planet (r or moon).  The closer it is to the star, and the hotter the star is, the hotter the planet's exosphere will be.</p>
<p>The farther the planet is from the star, the cooler its exosphere will be, and the smaller the planet can be and retain a dense atmosphere.</p>
<p>But you don't want the planet to be so far away for the star and so cold that its atmosphere freezes out.</p>
<p>And a planet should also have a strong enough magnetosphere to divert most charged particles in the solar wind.  The closer the planet is to the star, and the more active the star is, the stronger the magnetosphere will have to be, and vice versa.</p>
<p>And from what I have read on the subject, it seems rather difficult to predict how strong the magnetosphere of a world will be.  I think as a general rule, The more massive a world is, and the faster it rotates, the more likely it will be to have a strong magnetosphere.</p>
<p>But as far as I know there is no formula to calculate how strong the magnetosphere of a world will be from its mass and its rotation rate.</p>

CC BY-SA 4.0
2021-12-24T07:10:51.737Z
Your entire (very interesting) post seems like an answer to "Are planets _close_ to their star more likely or less likely to have an atmosphere?". My question asks the opposite, and the only statement in your post concerning this is "But you don't want the planet to be so far away [...] so that its atmosphere freezes out". Well, how far is too far for that?
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-23T21:12:07.993Z
2021-12-23T21:12:07.993Z
M. A. Golding
<p>Are non-gaseous planets (e.g. super-Earths) that are either rogue or very far from their parent star(s) less likely or more likely to have a significant atmosphere?</p>
<p>On one hand planets closer to their sun are more likely to be stripped of their atmosphere by solar winds et cetera, hence any planets that are too close to the sun won't have atmospheres, e.g. Mercury.</p>
<p>On the other hand planets too far from their sun will have their atmosphere collapse to the ground, as is the case with Eris and to some degree Pluto when they approach aphelion.</p>
<p>Is a very far distance from the Sun then favorable for a terrestrial planet to have an atmosphere or rather not? Is there something like an &quot;atmosphere zone&quot; around stars, a distance in which it's neither too hot for the atmosphere to get lost nor too cold for an atmosphere to collapse?</p>

CC BY-SA 4.0
2021-12-20T20:05:24.423Z
All solar planets outside Mercury have an atmosphere. And even Mercury has a thin atmosphere... if you want to call that. So ... what is "no atmosphere" in your definition? https://en.wikipedia.org/wiki/Atmosphere_of_Mercury
planetmaker
CC BY-SA 4.0
2021-12-21T07:09:07.590Z
@planetmaker On Earth I'd set the space boundary to above most of the ozone layer, to an altitude around 35 km (22 mi). The pressure there is around 0.08 psi, so that would be a universal atmosphere-space demarcation. It means Venus, the Earth, Mars and Titan have significant atmospheres.
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-21T10:07:42.173Z
@planetmaker 1000 Pa is 0.145 psi, the "sea level" average is 600 Pa (0.088 psi) and the Hellas Basin has as much as 0.2 psi. So Mars is well included. Several mountain peaks on Mars are in space.
Giovanni-Reinstate Ceres Pluto
<planetary-atmosphere><rogue-planet><trans-neptunian-object><atmospheric-escape>
Are planets far from their parent star more likely or less likely to have an atmosphere?
Giovanni-Reinstate Ceres Pluto
# Are planets far from their parent star more likely or less likely to have an atmosphere?

(Asked by: username_0 on 2021-12-20)

Are non-gaseous planets (e.g. super-Earths) that are either rogue or very far from their parent star(s) less likely or more likely to have a significant atmosphere?

On one hand planets closer to their sun are more likely to be stripped of their atmosphere by solar winds et cetera, hence any planets that are too close to the sun won't have atmospheres, e.g. Mercury.

On the other hand planets too far from their sun will have their atmosphere collapse to the ground, as is the case with Eris and to some degree Pluto when they approach aphelion.

Is a very far distance from the Sun then favorable for a terrestrial planet to have an atmosphere or rather not? Is there something like an "atmosphere zone" around stars, a distance in which it's neither too hot for the atmosphere to get lost nor too cold for an atmosphere to collapse?

**Tags:** planetary-atmosphere, rogue-planet, trans-neptunian-object, atmospheric-escape
> **Comments:**
> **username_1**: All solar planets outside Mercury have an atmosphere. And even Mercury has a thin atmosphere... if you want to call that. So ... what is "no atmosphere" in your definition? https://en.wikipedia.org/wiki/Atmosphere_of_Mercury  
> **username_0**: @planetmaker On Earth I'd set the space boundary to above most of the ozone layer, to an altitude around 35 km (22 mi). The pressure there is around 0.08 psi, so that would be a universal atmosphere-space demarcation. It means Venus, the Earth, Mars and Titan have significant atmospheres.  
> **username_0**: @planetmaker 1000 Pa is 0.145 psi, the "sea level" average is 600 Pa (0.088 psi) and the Hellas Basin has as much as 0.2 psi. So Mars is well included. Several mountain peaks on Mars are in space.  

---

## 3 answers

### Answer by username_2 on 2021-12-20. Score: 2

Not necessarily. There are lot of factors affecting the existence of a planet's atmosphere, the most predominant are: 1. Composition of the atmosphere. 2.Composition of the planet itself. 3. Star 4. Gravitational Acceleration

The composition of the atmosphere plays an important role in stability of the atmosphere of a planet. If the composition of atmosphere is similar to that of Mars or Pluto, the atmosphere is most likely to get eroded by solar winds. or may suffer a collapse respectively, Lets see what happened to Mars, if Mars replaced Pluto it will have a huge atmosphere compared to what it has now (solar winds from Sun severely affected the Mars' atmosphere.) But it always depends upon the type/size of the parent star and the distance from it (Pluto's perihelion?). but which most likely will not happen as you are talking about Super-Earth.

The Composition of planet itself can affect its atmosphere's stability, the important factor that comes in effect due to this is the gravity, because your super-Earth has to go under multiple challenges through its journey in its orbit, if the condition between your Planet and the parent star is not stable it may end up collapsing the atmosphere. it may exist if your super-Earth is similar to our Earth except its size and the star is as big as Sirius.

The Star. If your saying it is Orbiting a Neutron star or a Dwarf star (Proxima Centauri ?), Which will either erode or your atmosphere will collapse respectively, if the star is similar to that of Sun, the atmosphere will most likely to exist. The way the planet tidally locked to the star also affects the atmosphere's stability. Its not about distance between the star and the planet, The distance have a factor in maintaining the planet's atmosphere although but it can be as far as it could be or it can be as close as it can, the only thing is the atmosphere should be *stable* with respect to the factors mentioned. as long as you don't concern the state of the atmosphere.

Gravitational Acceleration, one factor which depends on the mass of the planet, also plays a role in atmosphere, greater the magnitude, greater the ability of the planet to retain its atmosphere

There a lot of other factors too, Everything affects Everything, But I mentioned some whose factor are not as small as air resistance.

> **Comments:**
> **username_3**: Do you have any evidence to support this?  
> **username_2**: @fasterthanlight https://www.aanda.org/articles/aa/full_html/2020/06/aa37513-20/aa37513-20.html will be helpful although it does not fully explain it. Would also like to refer articles on Solar Winds' effect on Atmosphere (like https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space#:~:text=Solar%20wind%20and%20radiation%20are,results%20from%20NASA's%20MAVEN%20spacecraft.), range, impact, and tidal lock.  
> **username_0**: Concerning the super-Earth we can more exactly refer to Rowan-Robinson's planet from my previous question. At 4 Earth masses, it is likely a super-Earth (if it exists in the first place) and as far from the Sun as where my question about an atmosphere comes into play.  
> **username_2**: @Giovanni-ReinstateCeresPluto  Talking about Rowan Robinson Planet, it is still in the shelf and although its not high to explore through it, but no one brought it down yet. You have to include your reference in the question  
> **username_0**: The planet is just an example, it could be any super-Earth far enough from its parent star. Brown's and Batigyn's planet (~6 Earth masses), if it exists, may be a super-Earth as well.  
> **username_2**: Ok then, To be more specific there is no mass limitation for Atmosphere existence until you are not particular about distance. If your give an example of a star and the distance we can actually come to a conclusion  
> **username_4**: The other property you can add to the list of three is the magnitude of the gravitational acceleration for the planet, which depends on the mass of the planet. The greater the value of gravitational acceleration, the greater its ability to retain particular gases & thus retain an atmosphere.  
> **username_2**: @Fred Oh yes, I will add it now  
> **username_0**: Gravitational acceleration / escape velocity are concerning the proximity to the star, but wouldn't a too far planet have its atmosphere frozen to the ground, that is the question? If Pluto's and Eris' atmospheres collapse (the moon Io's as well when eclipsed by Jupiter) wouldn't a very high gravity even _help_ them collapse? Or would a thick enough atmosphere always persist in gaseous form regardless of temperature?  

---

### Answer by username_1 on 2021-12-22. Score: 1

The question as asked cannot be answered easily; there too many parameters which determine the thickness of a planet's atmosphere.

The main factors are the surface temperature and the planetary mass (which together define the escape velocity) and the atmospheric composition (which defines whether the gas molecules of the given temperature exceed or approach the escape velocity).

Luckily there have been studies on the matter like this one by Samuel Konatham and collegues (though their main focus is on atmospheres of potentially habitable planets): <https://royalsocietypublishing.org/doi/10.1098/rspa.2020.0148>
In essence: the terrestrial planets are usually capable to hold atmospheres, and so are the super earths. The Neptunian-style planets (in the habitable zone) often are not capable to hold atmospheres for long times due to their predominantly light-weight elemental composition of mainly hydrogen and helium. Further out: see our solar system - every planet has an atmosphere.

> **Comments:**
> **username_0**: Mercury and Ceres don't have any significant atmospheres unless you're having the same stance as Mr. Witthoft (see my talk with him in comments). I ain't sure what you mean by "Neptunian-style planets (ice giants?) often aren't capable to hold atmospheres" because gas giants are _only_ atmosphere except for a solid/liquid core.  
> **username_0**: There are gas giants very close to their star btw. https://en.wikipedia.org/wiki/Hot_Jupiter  

---

### Answer by username_5 on 2021-12-23. Score: 0

Short Answer:

The question and answers seem to ignore one important factor in the ability of planets and other worlds to retain atmosphere.

Escape velocity.

The escape velocity of a world depends on the mass, radius, and average density of that. At the present time a minimum mass of 0.12 Earth mass seems to be considered necessary, but not sufficient, for a planet to retain a significant atmosphere for a long time.

Long Answer.

Planets with higher surfaces gravities usually have higher escape velocities.

Planets with higher escape velocities usually have higher surface gavities.

But different formulas are used to calculate the surface gravity and the escape velocity of a world. They do not change at the same rate as each other with different planetary masses and diameters.

If the question asks aboutjplanets retaining atmosphere in order to be habitable for life, it should be mentioned that there are different types of habitability.

For example, there is habitability for humans beings. Planets habitable for human beings should be a subset of planets on which some organisms found on Earth could survive, which should be a subset of planets with liquid water where some lifeforms similar to some Earth lifeforms could survive, whichou could be a subset of planets where beings with hypothetical alien biochemestries might possibly survive if there actually can be life using those hypotheical biochemestries.

Here is a link to a scientific discussion of planetary habitability for humans, *Habitable Planets for Man* Stephen H. Dole, 1964:

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

On pages 13 to 19 Dole considers human requirements for atmosphere, including minimum needs for a few gases and maximum limits for them and for many other gases.

> To summarize then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7 milllimeersof mercury. In addition, the partial pressure of the inert gases must be below certain specified limits and the other toxic gases must not be present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.

Since a human habitable planet must have liquid water, there must also be some water vapor in the atmosphere.

Dole presented a table giving the time for an atmosphere to escape with various ratios betgween the escape velocity of a planet and the root-mean-square velocity of air particles in the escape layer of the atmosphere, table 5 on page 35.

If the escape velocity is one or two times the root-mean-square velocity of the gases, the planet will lose them instantly. If the escape velocity is three times, it can hold the gases for a few weeks. If the escape velocity is four times, the planet can hold the gases for several thousand years. If the escape velocity is five times, the planet can hold the gases for about 100 million years. If the escape velocity is six times, the planet can hold the gases infinitely long.

Dole sums it up:

> If a planet is to be able to capture a gas, the planetary escape velocity must be three or four times the root-mean-square velocity; for a planet to retain a gas permenently, the escape velocity must be about five or six times the root-mean-squae velocity.

On pages 54 to 58 Dole calculates the minimum mass necessary for a planet to retain an oxygen-rich atmosphere. The temperature, and thus the velocity, of oxygen molecules and atoms in the exosphere or escape layer of the atmosphere is important to the planet's ability to retain oxygen.

On page 54 Dole considered it possible for a habitable planet with a habitable surface temperature range to have temperatures in its exosphere as low as 1000 degress K (276.85 C or 1340.33 F).

> However, if we take as a rough approximation that maximum exosphere temperatures as low as 1000 K are not incompatable with the required surface condiitons of a habitable planet, then the escape velocity of the smallest planet capable of retaining atomic oxygen may be as low as 6.25 kilometers per second (5 X 1.25). Going back to figure 9, this may be seen to correspond to a planet with a mass of 0.195 Earth mass, a radius of 0.63 Earth radius, and a surface gravity of 0.49 *g.*

Dole considered such a small planet capable of retaining an oxygen atmopshere but not capable of producing one, and then when on to estimate the inimum mass for a planet that could produce an oxygen atmosphere.

0.63 of Earth's radius of 6,371 kilometers or 3,959 miles would 4,013.73 kilometers or 2,494.17 miles, and the diameter of that planet would be twice the radius, of course - 8,027.46 kilometers or 4,988.34 miles.

Since Dole wrote a lot has been discovered about the surface temperatures of various world's in our solar system, and about the temperatures in the exospheres or escape layers of the atmosphere's of those worlds which have atmospheres.

So it is possible that modern estimates might change Dole's view that an exopshere temperature of a habitable planet could be as low as 1000 degress K (276.85 C or 1340.33 F).

Figure 9 on page 31 gives the relationship between the mass of terrestrial type planets and their radius, surface gravity, and escape velocity. Today we have more accurate information about the masses and radii of some solar system planets, and we have some information about the masses and radii of terrestrial type planets in other star systems.

So together, those two factors could make the minimum mass of a planet capable of retaining an oxygen rich atmosphere different from what Dole calculated.

Here is a link to a 2013 article discussing the potential habitability of hypothetical planetary mass exomoons of giant planets orbiting in the circumstellar habitable zones of other stars:

<https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf>

In this case habitabiity means teh more general case of habitablity for lifeforms which require liquid water and not the more specific cases of habitablity for humans.

The mass range of a hypothetical habitable exomoon is discussed on pages 3 & 4.

> A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕,
> Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger
> 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and
> to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al.
> 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon
> habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal
> Heller & Barnes (2013) – Exomoon habitability constrained by illumination and tidal heating
> 3
> 4 Maintained by Robert Jacobson, <http://ssd.jpl.nasa.gov>.
> heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is
> given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity
> and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly
> suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed
> around 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect
> approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for
> Exomoons with Kepler (HEK) project (Kipping et al. 2012).

The sources for a minimum mass of 0.12 Earth mass for a planet or moon to retain a substantial atmosphere for long time are given as;

> Williams, D. M., Kasting, J. F., Wade, R. A. 1997, Nature, 385, 234
>
> Kaltenegger, L. 2000, ESA Special Publication, 462, 199

So someone can look up those articles to see the basis of their calculations.

So it appears that a minimum mass of 0.12 Earth mass is necessary for a planet to retain a dense atmosphere for a long time.

And it is also important for a planet to have a strong enough magnetosphere to defect most charged particles from the solar wind and prevent them from knocking off atmospheric particles. Except for an exomoon orbiting within the magnetosphere of a giant planet, of course.

So having a high enough mass and escape velocity partially depends of the temperature and thus velocity of the air particles in the exosphere of the planet (r or moon). The closer it is to the star, and the hotter the star is, the hotter the planet's exosphere will be.

The farther the planet is from the star, the cooler its exosphere will be, and the smaller the planet can be and retain a dense atmosphere.

But you don't want the planet to be so far away for the star and so cold that its atmosphere freezes out.

And a planet should also have a strong enough magnetosphere to divert most charged particles in the solar wind. The closer the planet is to the star, and the more active the star is, the stronger the magnetosphere will have to be, and vice versa.

And from what I have read on the subject, it seems rather difficult to predict how strong the magnetosphere of a world will be. I think as a general rule, The more massive a world is, and the faster it rotates, the more likely it will be to have a strong magnetosphere.

But as far as I know there is no formula to calculate how strong the magnetosphere of a world will be from its mass and its rotation rate.

> **Comments:**
> **username_0**: Your entire (very interesting) post seems like an answer to "Are planets _close_ to their star more likely or less likely to have an atmosphere?". My question asks the opposite, and the only statement in your post concerning this is "But you don't want the planet to be so far away [...] so that its atmosphere freezes out". Well, how far is too far for that?  

---

