Starting in 2017, a worldbuilder by the name of Dylan Bajda conceived a new twig in the speculative evolution branch of the science fiction tree: The seedworld. The twig was called Serina: A Natural History of the World of Birds. Here, he explored the evolution of life on an Earthlike moon orbiting an unnamed gas giant. The twist is that the lifeforms of that moon are originally from Earth, with the "heroes" of the story being the domestic canary. Since then, a few other seedworlds have been conceived (like "Hamster's Paradise" on Tumblr), but going back to Serina itself, there are a couple of issues to contend with for anyone trying to copycat that bird's nest and plant Earth species of organisms on an Earth-sized moon orbiting a gas giant:

  1. Tidal locking, which means one side always faces the parent, so there are no day-night cycles, just one-half eternal light and one-half eternal darkness
  2. One year lasting days rather than months

Life on Earth had neither of those problems to contend with back home, so how will the seeded Earth species adjust to these radical differences?

  • 5
    $\begingroup$ Both consequences are... off. A moon tidally locked to a gas giant won't have regions of eternal light and enteral darkness, while a planet tidally locked to a star will. A "year" of the primary will also be a year as far as seasons go on the moon's hemispheres. $\endgroup$
    – ltmauve
    Oct 2, 2021 at 4:56
  • $\begingroup$ Also, a lot of tidally locked moons have an orbital period of only a few days or less. Do you want to specify a period of 30 (Luna) - 80 (Iapetus) days? $\endgroup$
    – ltmauve
    Oct 2, 2021 at 5:07
  • $\begingroup$ @ltmauve The basic issue of a moon-year still stands. Life on Earth has adjusted to living a year spanning 400-365 days. That makes the seasons just long enough for migrants to travel vast distances. $\endgroup$ Oct 2, 2021 at 5:15
  • $\begingroup$ what @ltmauve is saying is that a tidally locked moon will still have a day-night cycle, because its stuck to face the planet, not the central star. Our moon does. $\endgroup$
    – L.Dutch
    Oct 2, 2021 at 5:21
  • 3
    $\begingroup$ I’m voting to close this question because you're asking a question that is really about someone else's work and not really about your own. $\endgroup$
    – elemtilas
    Oct 2, 2021 at 20:02

4 Answers 4


One day lasting several earth-months

Earth-days have little meaning on this Earth-like planet/moon, because it does not rotate. In fact, it does rotate, once per orbit, but this rotation is as slow as the orbit itself because of the tidal lock. Sundawn to sunset takes half the duration of the orbit. One orbit around the gas giant could take several of our earth-months. A resulting "year-day" consists of a dark season, a morning season, a light season and an evening season.

A permanent high water tide on one side

The tidal lock dictates water high tide is place-bound and permanent. Water will spread less, you'll probably have one giant ocean where the high tide is (toward the gas-giant), permanently in one place, with a habitable land mass in it, or around it. The inhabitable region on this Earth-like planet/moon could be reduced significantly, because only coastal regions of the giant continent will interact with the large ocean, water inland will easily evaporate out during the long daylight periods. Chance is, a desert could form and only coastal regions would be inhabitable.

Extreme winter/night

On earth, we regard "seasons" as long periods having low and high average temperatures over the day. The difference between summer and winter is about 20-50 degrees Celcius, depending on the location on our planet.

For this Earth like planet/moon orbiting a gas giant, the temperature difference could become as large as 200-300 degrees Celcius. Seasons will be equal to day and night, winter darkness will be "eternal" and very cold.

To make this inhabitable, it requires proper design of atmospheric conditions ! you'll need a thick atmosphere with plenty of CO2 to keep heat in, little energy will reach the surface for very long periods. Heat conservation better be good.. else inhabitants will not survive.

Choose your gas giant distance to the star carefully. In the permanent light periods, the outside could be made agreeable, say -20 to 40. When winter sets in on the planet side with water, the ocean will freeze. Choose your water amount with care, else there will be no rain in the moderate seasons. Some large inland lakes would be nice..

Polar regions best suited for settlement

Settling in one place will become quite difficult and preferably in polar regions, that have eternal evening. Elsewhere, during winter night periods, human inhabitants should be safely indoor, or underground.

Canary will be a migratory bird

The opening question, "Will the seeded Earth species adjust to these radical differences?". Yes, with some adjustments and limitations. Seeds can germinate when temperature and moisture have a good balance. That moment could last several Earth-weeks, in any location in the coastal regions. A challenge will be, to protect life and nature on the surface, when winter comes.. plant life on the surface would be single-season, or require genetic modification, to be protected in place. Animals will either move, or live underground. Your canary will become a migratory bird, seeking shelter elsewhere when winter comes, like geese do. It may be on the move all seasons, around the coast.

  • $\begingroup$ "My" canary? Where did you get that? $\endgroup$ Oct 2, 2021 at 21:12
  • $\begingroup$ @JohnWDailey You stated " The twist is that the lifeforms of that moon are originally from Earth, with the "heroes" of the story being the domestic canary" (maybe I did not understand the sentence, English is not my native language) $\endgroup$
    – Goodies
    Oct 2, 2021 at 21:17

Short Answer:

You are wrong about a moon tidally locked to its planet having eternal day on one side.

If a planet is tidally locked to its star, one side of the planet will always face the star and have eternal day and great heat, while the other side will always face away from the star and have ternal night an d grat cold.

Some older people might remember when astronomers believed that was the case with Mercury and possibly Venus.

If a moon is tidally locked to its planet, one side of the moon will always face the planet, and one side of the moon will always face away from the planet. And of course both sides of the planet will have alternating day and night, just like Earth does, because each side of the moon will alterntely face toward the star and away from it.

That is the case with all moons in our solar system which are close enough to their planets.

LOng Answer:

The Odds that a hypothetical habitable exomoon orbiting a giant exopolanet will have a day length similar enough to Earth's day length for Earth life to flourish there are fairly good.

The sidereal day of a planet or moon is the time period during which it turns 360 degrees relative to the distanct stars. For a tidally locked moon the sidereal day will equal it's orbital period around the planet.

The synodic day of a planet or moon is the time period during which it turns 360 degrees relative to the direction to the star that it orbits, the source of its light and heat. Because the planet or moon will be orbiting the star and changing the direction to the star while it is rotating, the synodic day will be different from the sidereal day.

Someone who wants to have a good understanding of the climate and weather on a fictional planet or moon will want to calculate the sideral and synodic days accurately.

How long sidereal and synodic days can a giant exomoon habitable for humans and other Earth lifeforms with similar requirements have?

Note that there are somewhat different lower and upper limits for the lengths of sidereal days and synodic days for habitable worlds.

In Habitable Planets for Man, 1964, Stephen H. Dole calculated and/or guessed the properties of planets which would be habitable for humans.


Dole found a minimum length for the sideral day of a habitable world and a maximum length of a synodic day of a habitable world.

The faster a planet rotates, the more oblate it will get, as the rock at its equator is pushed farther and farther from the center of the planet. If a planet rotates too fast it will become unstable and matter will be lost from its equator.

If a planet has too long a synodic day, the daylight period swill get too hot and the nightime periods will get too cold. Plants might die from lack of sunlight during the long dark nights.

On page 60 Dole wrote:

Just what extremes of rotation rate are compatalbe with habitability is difficult to say. Those extremes, however, might be estimated at, say, 96 hours (4 Earth days) per revolution at the lower end of the scale and 2 to 3 hours per revolution at the upper end, or at angular velocities where the shape becoemes unstable because of the high angular rotation rate.

So there are limits for the sideral day and the synodic day.

In the case of Serina, an Earth sized exomoon orbiting a giant planet in another star system, there are other limits to the length of a moon's sidereal day which would be equal to its orbital period around the planet.

Here is a link to a scientific discussion of the potential habitability of hypothetical exomoons orbiting giant exoplanets:


On page 20 they write:

The synchronized rotation periods of putative Earthmass exomoons around giant planets could be in the same range as the orbital periods of the Galilean moons around Jupiter (1.7–16.7 d) and as Titan’s orbital period around Saturn (&16 d) (NASA/JPL planetary satellite ephemerides)4

So they believe that a hypothetical habitable exommon of a giant exoplanet might have an orbital peirod and sidereal day of about 1.7 to 16.7 Earth days long.

They also write:

The longest possible length of a satellite’s day compatible with Hill stability has been shown to be about P)p/9, P)p being the planet’s orbital period about the star (Kipping, 2009a).

So if the orbital period of the planet around the star is not at least 9 times as long as the orbital period of the moon around the planet, the moon's orbit will not be stable in the long term.

In another paper, it is show that giant planets have a habitable edge, a mimimum distance from the planet that a potentially habitable moon has to orbit to avoid being overheated by its planet and suffering a runawy greenhouse effect (like the moon Io and the planet Venus suffered). That minimum distance is 5 times the radius of the planet.

Exomoons the size of Mars probably wouldn't generate their own magnetic fields to protect their atmospheres from being stripped away by the stellar wind of their stars. Thus they would have to orbit within 20 radii of the planet to be protected by the giant planet's magnetic field.



Of course even larger exomoons the size of Earth could generate their own magnetic fields to protect their atmospheres. But they would have to rotate fast enough to generate strong magnetic fields, and if they were tidally locked they would have to orbit close enough to have a orbital period and thus rotation period which was fast enough.

So there is probably a zone of 5 planetary radii to 20 planetary radii around a giant planet where a large enough and otherwise suitable moon could be habitable.

Similations showed that the only orbits around Neptunne sized and Saturn sized planets which were close enough to become protected by their panet's magnetic fields would inside the habitable edge and thus a runway greenhouse effect would make thos emoons uninhabitable.

But jupiter sized planets would have a zone of 5 to 20 planetary radii where it would be possible for habitable moons to orbit.

Of course planets can get a lot more massive than jupiter. The dividing line between the most massive planets and hte least massive brown dwarfs is about 13 times the mass of jupiter. And a writer might want to put a habitable world in orbit around a brown dwarf. The dividing line between the most massive brown dwarfs and the least massive stars is about 75 to 80 times the mass of jupiter. So we could set the upper mass limit for a brown dwarf to have potentially habitable moons (or planets or whatever they should be called) obiting it at about 70 Jupiter masses to be safe.

So what would the radii of objects in those mass ranges be? I don't know how to calculate them.

But I have read that planets and brown dwarfs more massive than Jupiter can only have a slightly larger radius than Jupiter, and then their increased mass and gravity makes them more and more dense and compressed instead of larger. I have read that in fact there is only about a 15 percent variation in the radii of objects more massive than Jupiter, in the mass range from more massive planets to brown dwarfs to the least massive stars.

So I would guess that all planets and brown dwarfs more massive that Jupiter have radii between 0.8 and 1.2 Jupiter radii.

The equatorial radius of Jupiter is about 71,492 kilometers or 44,423 miles.

So the inner habitable edge around any planet or brown dwarf more massive that Jupiter should be somewhere betweeen 285,968 kilometers and 428,952 kilometers. Though (it might be different around a brown dwarf). That gives an orbital circumference between 1,796,789.935924 and 2,695,184.903885 kilometers.

The outer habitability edge at twenty radii around any planet or brown dwarf more massive that Jupiter should be somewhere betweeen 1,143,872 kilometers and 1,715,808 kilometers. (Though it might be different around a brown dwarf). That gives an orbital circumerence between 7,187,159.743694 and 10,780,739.615541 kilomters.

So using the formula to calculate the orbital speed at a specific distance from a body with a specified mass, rough lower and longer limits for the obital periods, and thus sidereal days, of tidally locked moons in the habitable zones of giant planets and brown dwarfs can be calculated.

If the planet has one jupiter mass, a moon orbiting at 5 Jupiter radii or 357,460 kilometers will have an orbital period and day of 1.37807 Earth days or 33.0736 hours. A moon orbiting at 20 Jupiter radii or 1,429,840 kilometers would have an orbital period and day of 11.0245 Earth days or 264.589 hours.

If the planet or brown dwarf has 13 Jupiter masses or 0.004163 solar mass :

A moon orbiting at 285,968 kilometers would have a period of 0.273882 days or 6.57316 hours.

A moon orbiting at 428,952 kilometers would have a period of 0.503153 days or 12.0757 hours.

A moon orbiting at 1,143,872 kilometers would have a period of 2.19105 days or 52.5853 hours.

A moon orbiting at 1,715,808 kilometers would have a period of 4.02522 days or 96.6056 hours.

If a brown dwarf has 70 Jupiter masses or 0.066857 solar mass:

A world orbiting at 285,968 kilometers would have a period of 0.118040 days or 2.83296 hours.

A world orbiting at 428,952 kilometers would have a period of 0.216853 days or 5.20447 hours.

A world orbiting at 1,143,872 kilometers would have a period of 0.944319 days or 22.6636 hours.

A world orbiting at 1,715,808 kilometers would have a period of 1.73482 days or 41.6358 hours.

So a moon orbiting a giant planet with a mass between one Jupiter Mass and 13 Jupiter masses at a distance of 5 to 20 radii should have an orbital period and sidereal day between 0.273882 days or 6.57316 hours. and 11.0245 Earth days or 264.589 hours.

So a world moon orbiting a brown dwarf with a mass between 13 Jupiter masses and 70 Jupiter masses at a distance of 5 to 20 radii should have an orbital period and sidereal day between 0.118040 days or 2.83296 hours. and 4.02522 days or 96.6056 hours.

So most of the possible orbital eperiods between 5 and 20 radii of the parent body around Jupiter or larger palnets or around brown dwarfs should be within the possible range for ahabitalbe world suggested by Dole of 2 or 3 hours at the shortest to 96 hours (four eArth days) at the longest. And every jupiter mass or more massive palnet or brown dwarf should have an irbit where a moon would have a sidreal day of I Earth day.

But it is the synodic day of the moon which is important. The synodic day should always be a bit longer than the sidereal day of the moon. That ill not be a big problem if hte difference in length between the sidereal day and the synodic day is short enough.

The bigger the difference between the orbital period of the moon around its planet and the orbital period of the planet around its star is, the shorter the synodic day will be. So the longer the planet's orbital period is compared to the moon's orbital period, the easier it will be to for the moon to have an orbital period and sidereal day of 0.75, or 0.81, or 0.92, etc. of an Earth day, and have a synodic day exactly one Earth day long.

And the closer the synodic day of the moon is to the synodic day of Earth, the less trouble plants and animals trasported from Earth will have adjusting and adapting to life on the moon.


Well, one of the first things I can think of is that sleep schedules are going to have to adjust. An organism with a circadian cycle not attached to light levels will have an advantage, as it can quickly fall asleep during the long day and quickly wake up and operate cleanly during the long night. Though I suspect a lot of animals will develop hibernation abilities, as the long days and nights will produce more extreme temperature variations. Thus, many animals will hibernate through the cold nights. Look at how animals adapt to deserts, where a lack of water means extreme temperature variations.

Plants will also need to have their own adaptations. Because of the long day in which to photosynthesize and the long night in which plants must rely on stored energy to keep their metabolism going, plants will have more storage for energy than on Earth. This will be a potential food source for animals, so there will be an evolutionary arms race over those, just like over leaves.

Nocturnal animals often have less visual acuity in favor of hearing or smell, so it's likely that these will develop in non-hibernating species. Human trichromatic vision is useless at night without artificial lights, so it is less useful to a species active at night.


Tidal locking, which means one side always faces the parent, so there are no day-night cycles, just one-half eternal light and one-half eternal darkness

Whilst this can be true for a planet orbiting a star, there's no long-term stable way of having a moon exhibit these characteristics. Tidal locking of moons around gas giants happens quickly and early in their evolution, and will result in one hemisphere always facing in towards the parent gas giant and one hemisphere always facing out from it.

I'll assume that the moons are orbiting in more or less the same orbital plane as their parent. Bear in mind that some situations with highly inclined moon orbits (such as Triton) will have different day-night cycles, but still won't behave as you've described.

These two hemispheres, inward-facing and outward-facing will be very different: the side facing the gas giant never really has a proper night, but instead has a long twilight that is probably much brighter than even the brightest moonlit night on Earth. It will have an eclipse period every "day", the length of which will depend on its orbit which you haven't shared with us. The outer facing side will have a proper night, and a long one at that. It will also never experience any eclipses.

One year lasting days rather than months

Terminology note: there's still a "year" that will affect the moon, and that year is associated with the orbit of its parent planet around the primary star. This means there can still be seasonal changes that take long periods of evolve. "Year" then is a poor term for its own orbit. If you don't like the term "day" for its orbital period, consider "month", given its origins in our own language from the period of the Moon.

Exactly what the effects of a long night will be on the moon will depend on whether it is the inward-side or the outward-side, and the nature of the atmosphere. A dense, thick atmosphere can help insulate the world from heatloss during the long dark periods. The atmosphere might undergo super-rotation as it does on Titan to help even out temperatures.

In any case, nights will be long and likely cold, so diurnal species will need to be able to enter a torpor-like state to preserve energy... a kind of short periodic semi-hibernation. Noctural species may also do the same, though they'd be more at risk of overheating and dehydration rather than freezing.

The ability to build, find or excavate shelter seems likely to be important, and that isn't something that I'd necessarily call a simple straightfoward "adaptation". Species less suited to this may simply be unable to thrive in the new environment.

Diet might be the real problem. I suspect that adapting small mammals to the environment might be relatively straightforward, but adapting the plants and invertebrates that they feed upon is likely to be much harder. All plants will likely need to be frost-hardy, and fruit, if it exists at all, seems likely to be quite different from anything found on Earth if it is intended to resist freezing overnight. Flowers and pollinators could still exist, at least, and nuts and seeds should function well enough. Many kinds of insect that rely on non-freezing above-ground temperatures are likely to never adapt, though things that live primarily underground and only surface briefly to mate and die will be fine.

Perhaps you should ask about plants first, and then invertebrates, and then it will be possible to give a better answer about vertebrates. In any case, I suspect that small rodent-like things, and small mustelids and felines that prey on them will be the winners in this situation, regardless of the other variables, because most of the desirable traits are already there.


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