For story purposes, I am building a world where all life exists on the moon of a non-habitable planet (the moon is not tidally locked). How can I explain this in a convincing and mostly scientific way? If not, how can I handwave it?

For the moon to have an atmosphere and oceans it must be quite massive; this would require the planet to be even more so. The planet must be rocky so that the moon-dwellers can attempt to land things on it. How can I explain the lack of an atmosphere on the planet? Or, if this is not possible, how can I explain a poisonous or inhospitable atmosphere?

I am not addressing the issue of magnetic fields just yet, so any comments on this are off topic.

  • $\begingroup$ Can you add a tag to define which kind of answer do you expect? Science based or magic? $\endgroup$ – L.Dutch - Reinstate Monica Aug 18 '18 at 17:06
  • $\begingroup$ Our system of Earth and moon looks a lot like what you're describing. Can we swap the features of our planet (hot iron core, liquid water on surface, good rotisserie/rotational period, etc.) with the moon? How do we explain that a more massive object so close to our habitable-moon has less atmosphere? $\endgroup$ – Qfwfq Aug 18 '18 at 17:34
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    $\begingroup$ Personally, I love the Avatar-style moon of a gas-giant. The expeditions could be to other moons instead of the mother planet. Just food for thought. $\endgroup$ – n00dles Aug 19 '18 at 7:53
  • $\begingroup$ Your main problem may be the second leg of the expedition. You need a Saturn V to take off Earth, while a small lunar module can take off Luna. But if your moon is similar to Earth, you wiil need a Saturn V to take off it... and something much more monstruous to take off the planet. $\endgroup$ – Luís Henrique Aug 20 '18 at 15:05
  • $\begingroup$ @LuísHenrique interesting... thank you for that $\endgroup$ – Aric Aug 20 '18 at 15:09

What exactly is the problem? Titan in our own solar system is an example of a moon with atmosphere and (small) oceans. Admittedly, it's a little chilly for humans, but other life forms may not be so picky :-)

For a more human-friendly instance, consider a system where the moon is Earthlike, while the planet is a super-Earth: https://en.wikipedia.org/wiki/Super-Earth The higher gravity & denser atmosphere could cause a runaway greenhouse effect, so that the planet is more of a "super-Venus".

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    $\begingroup$ No--Titan can hold it's atmosphere because it's cold. It can't hold a terrestrial atmosphere. $\endgroup$ – Loren Pechtel Aug 19 '18 at 2:30
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    $\begingroup$ ALSO; Titan is a moon of Saturn, so no expeditions to the planet's surface are possible. OP speciffied a rocky planet. $\endgroup$ – n00dles Aug 19 '18 at 7:49
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    $\begingroup$ @Loren Pechtel: Did I say that it could? No, I just use Titan as a starting point: observing the limited set of planets available for close inspection, we see an example of a moon with atmosphere & oceans, so it is not impossible for such to exist. Then we see from exoplanet observations that super-Earths exist. Thus it doesn't seem unreasonable that a super-Earth (large but still rocky) might have an Earthlike moon. $\endgroup$ – jamesqf Aug 19 '18 at 17:02

It's a commonly held misconception, but you really don't need a lot of gravity to retain an appreciable atmosphere. Conversely, gravity alone is no guarantee that a body will have an appreciable atmosphere. There is a lower bound which is dependent on atmospheric composition and temperature, but it's not that high. Wikipedia has a handy chart of escape velocity versus temperature and what this means for the ability to retain various gases; it shows that by gravity alone, at a temperature of a warm 300 K, you'd need an escape velocity at the surface of about 5 km/s to retain an oxygen/nitrogen atmosphere.

jamesqf has already mentioned Titan which, as per Wikipedia, has a surface gravity of 1.352 m/s2 (less than that of Earth's moon) and an escape velocity of 2.639 km/s (about half of what you'd need to retain an oxygen/nitrogen atmosphere at 300 K as per Wikipedia's chart linked above), yet a surface atmospheric pressure of almost 147 kPa (over 1.4 times that of Earth). Now compare this to Mars (3.711 m/s2, 5.027 km/s, 0.4–0.87 kPa) or Venus (8.87 m/s2, 10.36 km/s, 9 200 kPa) and you can easily see that there is no simple, direct relationship between atmospheric density and surface gravity. Other factors dominate. For comparison, Earth has an atmospheric pressure of 101.325 kPa (international standard atmosphere), a surface gravity of 9.807 m/s2 and an escape velocity of 11.186 km/s.

Addressing the (good) point made in comments that the low temperature is what allows Titan-as-we-know-it to hold on to so much atmosphere, that may very well be a part of the explanation for how Titan can hold on to its atmosphere, but it can't be the entire explanation for what is required for such a body to hold on to an appreciable atmosphere, or Venus would realistically have much less atmosphere than it does in real life. Note that I'm discussing using Titan for inspiration, not as the full answer.

Once you have a rocky body that is able to retain an appreciable atmosphere and has moderate gravity, the ability to support even Earth-like life pretty much comes down to just atmospheric composition and temperature. Titan is, again, slightly on the chilly side, but I don't see any reason why a Titan-like rocky body couldn't exist closer to its host star with an atmospheric composition more similar to that of Earth, which would result in a warmer climate.

Try putting a Titan-like, slightly more massive and possibly smaller (both of which will increase surface gravity), rocky moon in an orbit around real-life Mars or Venus, both of which would support landings (the only thing making Venus particularly difficult to land on is its noxious atmospheric composition); add some oxygen and carbon dioxide to its atmosphere (which is almost entirely nitrogen, at over 95% by volume; to match Earth, you want that down to about 80% by volume); and you'll probably be pretty close to what you need. As a bonus, adding the oxygen to Titan's atmosphere will also almost certainly get you a little bit of water vapor when the oxygen reacts with the hydrogen and methane in Titan's current atmosphere, which should itself help warm it up a touch — water vapor is one of the most potent greenhouse gases around. Titan has a few percent methane and around 0.2% H2, which, when it reacts with oxygen, should give you somewhere on the order of a percent or two water vapor by volume, which is comparable to that of Earth. All you'd need is to add some 25% oxygen to the existing atmosphere and let chemistry and (Earth-like) life do the rest.

You can also give the moon an active, liquid core and thus a magnetic field, which will help it retain its atmosphere over astronomical time scales. Orbiting a massive planet should help here by introducing tidal effects.

And if you're willing to stretch the definition of "moon" a little, also compare the masses of Pluto and Charon; the latter is so massive that the two can more accurately be described as a double-planet system than as a planet and a moon. (Technically, the barycenter of the Pluto–Charon system lies outside of Pluto, meaning that the two orbit a common point between them, rather than Charon simply orbiting Pluto.)

There's no real reason why such a moon can't orbit an uninhabitable planet. While a bit of a stretch for your specific question, consider that Europa orbits Jupiter; while Europa is at least semi-potentially inhabitable by hardy life, Jupiter is squarely uninhabitable.

Hence, really, no significant handwaving required.

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    $\begingroup$ One of those other major factors is temperature, though. A major reason why a world as light as Titan can hold onto so much atmosphere over astronomical timescales is because it's so bloody cold out by Saturn. You bring Titan in around Mars or Venus, and the atmosphere escapes to space on those timescales, and much of the surface also starts sublimating, because ice is a mineral out Saturn way, and a volatile substance in past the Belt. $\endgroup$ – notovny Aug 18 '18 at 20:02
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    $\begingroup$ Yes. Also, Titan consists mostly of water ice. If you bring it in orbit around Venus, not only will the nitrogen in the atmosphere expand so much to be blown away by the solar wind, also the entire planed will melt and eventually evaporate. $\endgroup$ – leftaroundabout Aug 18 '18 at 23:19
  • $\begingroup$ Above are the paragraphs that are missing after Other factors dominate. Heat and composition. Anything else? $\endgroup$ – Mazura Aug 19 '18 at 15:46
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    $\begingroup$ @leftaroundabout: Again, there's not (at least from the data we have) a simple relationship. Venus has lower gravity than Earth, and is closer to the sun so should be more affected by solar wind, yet it still has an atmosphere much denser than Earth's. $\endgroup$ – jamesqf Aug 19 '18 at 17:06
  • $\begingroup$ @mazura Molecular mass.The higher the Molecular mass of the gas in question, the slower it's molecules are moving on average at the particular temperature, and the less likely it is that a particular molecules of the gas that winds up on the upper fringe if the atmosphere gets sped up enough to leave. en.wikipedia.org/wiki/Atmospheric_escape?wprov=sfla1 $\endgroup$ – notovny Aug 19 '18 at 18:26

Both bodies in the past were capable of hosting life. Then one day, for a lucky coincidence, the system was invested by the gamma ray burst emitted from a close supernova, and the main planet was in the line of sight from the moon to the supernova.

While the main planet was stripped from most of its atmosphere, losing the ability of supporting life, the moon was protected by the main body and didn't suffer any damage.

  • $\begingroup$ "the main planet was stripped from most of its atmosphere, losing the ability of supporting life" The answers to my old question What would the Earth eventually look like, if it was somehow stripped of its atmosphere today? indicate that even in such a situation, it's possible that an atmosphere could be retained or reformed fairly quickly. So existing life (excepting hardy life such as bacteria and tardigrades) might have a hard time making it through the interim period, but it's possible that such a world would come out of it with life. $\endgroup$ – a CVn Aug 18 '18 at 18:55
  • $\begingroup$ @MichaelKjörling, you have a point. But since the OP is not giving any time scale, the exploration of the main planet may be happening shortly after the gamma shower, when the atmosphere and life have not yet fully been restored. $\endgroup$ – L.Dutch - Reinstate Monica Aug 18 '18 at 18:58
  • $\begingroup$ I like this idea, perhaps the people could land on the planet to find fossil evidence? That would be cool! $\endgroup$ – Aric Aug 18 '18 at 23:31
  • $\begingroup$ Awesome answer, I think. I read this and it describes a noxious atmosphere resulting from a nearby supernova. $\endgroup$ – n00dles Aug 19 '18 at 8:15

Isaac Asimov's oeuvre contains an example of this involving our very own planet and satellite. Humans terraformed the Moon, and then later a nuclear war left Earth uninhabitable. If high radiation levels on the planet are a problem, you can make this have happened in the very distant past, such that they've now dropped to habitable levels.


If you want to have the moon and the planet be explorable by your race instead of having a moon and planet may I recommend a binary planet? https://en.wikipedia.org/wiki/Double_planet

Effectively this is a system where two planets of similar mass orbit around their combined center of gravity. This way you can have your planet and "moon" be close to the same size so you don't need to have your inhabitants worry about radically different gravities. Since your moon is allowed to be quite massive under this model you can easily have it be the size of earth.

  • $\begingroup$ This is such a cool idea! $\endgroup$ – Aric Aug 21 '18 at 7:40

The easiest way to meet the requirements you've stated is for both the habitable body that your characters are starting from and the uninhabitable body they're travelling to, to both be satellites of a gas giant.

There are all kinds of ways to make this work, including scenarios where the second moon used to be habitable and no longer is. Right off the top of my head, a coronal mass ejection would be the simplest explanation. If you hypothesize a planet like Jupiter with two habitable moons, one of the two could easily be shielded entirely by the bulk of the gas giant itself, while the other could be completely stripped of its atmosphere.


One way to do this would be to have the planet and moon be near the inner (hot!) edge of the system's habitable zone, such that the primary's thicker atmosphere was enough to tip it into a runaway greenhouse effect, whereas the secondary, with its thinner atmosphere, stayed habitable; think of a bigger, wetter Mars orbiting a super-Venus. This probably works best if the secondary is fairly dry compared to Earth (to weaken the positive temperature feedback from water evaporation and lessen the chance of the secondary undergoing a runaway greenhouse); it doesn't have to be bone-dry, but you'll maybe want to think of having small, confined seas rather than world-spanning oceans.


The planet took a major hit in the last billion years. Any life it had was wiped out.

  • $\begingroup$ The question is about the moon, not the planet. $\endgroup$ – Vincent Aug 19 '18 at 3:39
  • $\begingroup$ @Vincent You've got an Earthlike moon around a super-Earth. The super-Earth should be inhabited except it got hit by a rock big enough to be a biological reset button. $\endgroup$ – Loren Pechtel Aug 19 '18 at 5:29
  • $\begingroup$ By going this route, you'd have to explain how that major impact could be major enough to wipe out every last shred of life on the planet, including hardy bacteria and life deep under water, and ideally how it would prevent life from re-forming. Last I looked, the best hypothesis was that if the building blocks and environment are there, you'd have a hard time not eventually ending up with some kind of life, even if only bacteria. See also the point on astrobiology in Radar evidence of subglacial liquid water on Mars, R. Orosei et al. $\endgroup$ – a CVn Aug 19 '18 at 10:05
  • $\begingroup$ @MichaelKjörling In time life would evolve again. It just hasn't had time. $\endgroup$ – Loren Pechtel Aug 19 '18 at 22:57
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    $\begingroup$ Life, uh, finds a way $\endgroup$ – Aric Aug 20 '18 at 7:48

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