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Please note: I am asking about planet orbiting a star, not a moon orbiting a planet.

So I’ve been trying to get this planet down for a long while now but there are still so many pieces of the puzzle that I have yet to figure out; What types of climates and biomes would exist on this planet and how can I determine where these biomes would exist.

The planet is about 0.85x the mass of Earth, 1.15x the size, and about 1.75x the atmospheric density (likely subject to change). The planet is mostly ocean on the light side, with most of the continents around the twilight zone extending into the dark, frozen ice sheets.

I have tried researching how to determine the biomes and climates of planets but as expected it was all for Earth-like planets where the equatorial line as the main tropic zones, and extending laterally. I would assume a tidally locked planet would have its biomes change in a ring formation but I can’t be sure on the specifics like what biomes could possibly exist.

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  • $\begingroup$ I assume you suppose the "sunny side" ocean is ice free and perhaps warmish at the surface. Can you be explicit or provide more details in this matter? $\endgroup$ – Adrian Colomitchi Mar 10 at 8:19
  • $\begingroup$ What do you mean by "atmospheric density"? Is the "density of air in weight/volume or air" or "the atmospheric pressure" at at sea level? $\endgroup$ – Adrian Colomitchi Mar 10 at 8:22
  • $\begingroup$ The water would certainly be significantly warmer and ice free. Atmospheric density I meant as thickness of the atmosphere, how much of an atmosphere there is between the surface and space. $\endgroup$ – SentiCarter Mar 10 at 8:39
  • $\begingroup$ How far/close is your planet to its star? $\endgroup$ – TobyB Mar 10 at 9:14
  • $\begingroup$ We don't have much to work with here... On an ocean world the depth of the water will be the most important factor when thinking about what grows where. What else is there? Wind and waves? You won't get much of that on a planet as you described. You could try to calculate how the radiation per m^2 decreases the further you go from the mid, but there isn't much to get out of such calculations if you don't already have specifics about your alien plants/animals/whatever. good luck. $\endgroup$ – justthisonequestion Mar 10 at 10:07
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The more I think, the more I'm having this hunch that a tidally-locked planet with an ice free ocean on the sunny side is impossible. I know that is not what you asked, but the question's Comment section is not enough to detail my questions.


It's unlikely that you can have the tidal-lock on your ocean-only low-density water on the sunny side - doesn't make sense to have the lowest altitude of the lighter side of the planet attracted more by the star. At best, you can handwave a heavy asteroid or a former moon that had a "low speed collision" with the planet so that it sorta-sunk-in-the-mantle-but-didn't-reach-the-core, so that the planet's mass-center is askew, but this is likely to require a good bulge of rock straight in the middle of your ocean.

Then, a tidally locked planet is unlikely to have an active core - if it had, then the viscous friction between a slow rotating tidal-locked crust and a fast rotating core will not last long enough for life evolution. With a dead core, a magnetosphere will be weak (or non-existent). Long term, the atmosphere long term will be gone blown away by the solar wind; except that I now realize that's not the worse thing to happen to the atmosphere...

... the "dark side" will gobble your atmosphere fast - at the temperatures of cosmic space, it will act as a cold-trap for the gasses in the atmosphere. So, what gas is likely to freeze-solid on the dark-side first? I bet will be the water vapors. Now, either:

  • your ocean is small - then it will end with no-sunny-ocean-all-dark-side-ice fast; or

  • massive ocean - but then it will drive enough mass on the dark side, which will become heavier than the sunny side. A dead core makes the things even harder - there's no chance to distribute the weight by isostasy. Heavy-side farther than the lite-side is not a stable configuration, a tidally-locked body will minimize the potential energy turning their heavy side towards the attractor.

So... before getting to the "how's life on that planet", my question is "how do you explain the planet in the first place"?

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    $\begingroup$ I would not worry about the 'cold trap' effect. Looking at some numerical simulations of a tidally locked planet with an assumed earth like atmosphere, you get temperature gradients that are nowhere excessive to freeze out Nitrogen or Oxygen. Source: eso.org/public/unitedkingdom/videos/eso1629g $\endgroup$ – Whitecold Mar 10 at 14:23
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    $\begingroup$ Oh, wow, very interesting. Some reading to do for me, thanks. $\endgroup$ – Adrian Colomitchi Mar 10 at 14:35
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    $\begingroup$ You only end up with all the ice locked on the dark side fast if the atmosphere isn't thick enough to distribute heat to the dark side to melt that ice... and "thick enough" isn't actually all that thick. $\endgroup$ – Logan R. Kearsley Mar 10 at 20:55
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Need to make some assumptions here:

  1. This planet with given data has 63% of Earth gravity and a density less than teluric planets of the Solar System, comparable with the Galilean moons Io and Europa. A 75% more density atmosphere in a planet with 32% more surface is 2,31x more massive, giving with weak gravity aceleration only 12% more pressure.

  2. The day hemisphere is water covered, no harsh desert, means planet has a good distance of their star, and its also a stable star, not a explosive red dwarf, otherwise all atmosphere would blowed away. The atmosphere absorb and keep heat very well, in the planet far of expected goldlike zone, like 2 AU or more to one Sun-like star. Probably the planet has a thick ozone layer and the ionosphere give some magnetic shield similar to Venus induced magnetosphere. This distance from the star also make high layers not so hot to escape from the weak gravity of planet (escape velocity is 9612m/s, 86% of Earth).

  3. The lack of iron or other heavy elements in the core explains the low density and in same way is coherent with formation in cold regions. Mineral diversity could came after the planet formed with later asteroid bombardment.

  4. To lock one face of the planet in the far orbit will demand a lot of time, enough to the heat of the core of planet gone. Tectonic activity will be low, or even none. Winds in this time will erode all major geological features and even the meteor impacts will be few noted. With low atmospheric escape and dominant winds the temperature in the planet is most uniform than expected in a tidal locked planet, without major extremes (demand calculate with orbit, star type and total of greenhouse gases), I would bet in something like Earth. Clouds are dominants all time and since high air density, winds are slow in surface levels, probably like one windy day.

  5. The geoidal format of planet will make the highlands stay around equator and waters will concentrate in the poles (as Adrian explained). Since the planet is covered in water in the sunny side and low density, its probably is a ocean planet with some fews continental lands, similar with Venus terraformation maps suggests.

  6. If the eccentricity of orbit is high, even the star zenith pole could be habitable in the winter and their antipode in the summer.

  7. I like the questions of Adrian but a planet will evolve life a long before to will tidal locked. The greenhouse effect of atmosphere will keep the temperatures uniform even before planet stuck in this position, making the ice masses no so high like he estimate. Major problem is explain a planet far of star tidal locked in the star lifetime, being this star not a red dwarf. I would bet in a giant impact in later formation slowing greatly the motion or even the accretion disk around this star in early days was slow. Also the low density of planet will make the planet be more waterish than a shallow giant ocean and if with a big eccentricty this ocean can dry partially in summer.

In resume, looks like a cold Venus, in a far position.


Assumptions done, continents will have a smooth shape, gentle hills, with islands and some long peninsulas in equatorial zone. The habitable zone is large. No desert or high montain climates, except maybe for the polar region in the center of night hemisphere, where an ice sheet like that one in Artartica will happen. Atlantic side of North America seems the most similar of that your world would looks like about climate.

Life would happen before planet tidal locked and probably happen countless of mass extinction events, maybe a high speciation. Plants, and fungus will grow to resist constant winds, and use it more than animals to polinization and spread seeds. Fauna will have a great trouble to keep that species evolve in land without a circadian cycle but not impossible to happen, turning into complete diurnal or complete nocturnal. With small distance to water in any land point, no geological challenges and a prosperours environment to evolve in water (also no great deep in oceans) the fauna will be diverse in amphibious. Hard to imagine it, but they would evolve to heat-blood to explore twillight and night side, under and above the waters. In tropical zones where some islands can evolve tall trees maybe even some flying animals.

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    $\begingroup$ Sorry for the lack of explanation in many parts, I never realized what was exactly needed. The star type has been changing often with mostly stars smaller than our own to make tidal locking easier. Red Dwarves I have been told can support life with these conditions under very specific or even lucky circumstances although I may likely go a little bigger to support a more stable star. I want enough scientific accuracy to make this possible, bur it doesn’t need to be perfect. $\endgroup$ – SentiCarter Mar 10 at 17:09

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