I'm designing an Earth-like planet orbiting around a K5V star in the habitable zone (the planet is at a distance of 0.41 AU).

The star has a mass of 0.70 M and a surface temperature of 4,526 Kelvin.

As for the planet, it has a diameter of 12,879 km, a surface gravity of 1.5 g (compared to Earth's 1 g) and its atmosphere is significally thicker than Earth's, but has a pretty similar composition of gasses.

Because of a number of collisions that took place during its formation, the planet is tidally locked to the star with an orbital period of 115 days, spinning about 14.59 kilometers per Earth hour at the equator.

Now, I wanted my planet to be habitable at the day side and not only at the twilight zone like most tidally locked planets in fiction. I've read that if a planet's atmosphere is thick enough, the wind currents can carry heat effectively from the day side to the night side allowing for liquid water to exist. The problem here is that I don't know if my planet will have sufficient wind currents due to Coriolis effect since it spins significantly slower than Earth. One thing I considered was the fact that since the planet is tidally locked, the atmosphere at the cooler night side would be a lot thicker than at the day side, and I thought that the difference in pressure would allow for strong winds to form and carry heat from one side to another.

So my final question is: Can my planet carry heat effectively from one side to another? If not, what do I have to change?

(Note: I don't fully know the mechanics of the atmosphere or wind currents. If there is something else I need to consider please let me know)

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    $\begingroup$ Considering that you have gone down to specifying the surface temperature of the star and the planet radius to the single digit, "significantly thicker" is not a quantity which can be used in estimates. $\endgroup$
    – L.Dutch
    Sep 22 at 7:17
  • $\begingroup$ L.Dutch My apologies. I'm a bit new at this so I don't know all the details that are needed to create a world. I thought of writing the atmosphere having 1.5 atm compared to Earth's 1 atm but again I don't know all the factors that need to be considered when designing an atmosphere. $\endgroup$ Sep 22 at 7:36
  • $\begingroup$ Does your planet have oceans? Those transport heat too; enough to prevent sea ice in some models. $\endgroup$
    – rek
    Sep 23 at 2:01

5 Answers 5


In short: I doubt there is a mechanism powerful enough to achieve what you are looking for.

The distance of your planet to the sun is less than half the distance between earth and sun, so the incoming radiation per area is more than four times as high (radiation from a point source decreases quadratically with distance). The temperatures on the day side will be murderous. There will be most likely some sort of winds that will bring some amount of heat to the other side, but the same effects apply here on earth and it does not change the overall distribution of climate zones very much. Ocean currents might also carry a lot of heat, but I think it is not strong enough either and your day side will probably not have any liquid water. It might help to place your planet much further away from the sun (more than 1 AU) to make the day side habitable but then the twilight zone and the night side will be extremely cold.

Venus might offer another solution: The days on Venus are extremely long, so it is similar to your planet, but temperatures don't change much during the day-night-cycle. This is due to an extreme greenhouse effect. A planet with a similar atmosphere as Venus that would be further away from the sun might have acceptable temperatures everywhere. However, you need a strong greenhouse gas (your high pressure might help here). The atmosphere of Venus consists mostly of carbon dioxide which is toxic to heterotrophic life (i. e.: all animals) in this concentrations. This applies of course only if your inhabitants have a similar metabolism than life here on earth.

M. A. Golding correctly pointed out that I did not consider the specifics of the star you described. I will not bore you with the details but just show a few assumptions and the result. I assume that the sun has the same density as ours and the surface temperature is the same everywhere. Applying Boltzmann's law and the distance of your planet, the center of the day side would receive 2300 W/m^2 which is double what the earths equator receives at maximum (not counting atmospheric absorption at both planets). There is some uncertainty to it, but it essentially means that the temperature of the center of the day side is around 400 K, not considering the greenhouse effect which might add another 30-50 K for an earth-like atmosphere. This is far from habitable, but potential inhabitants might not need to move to the twilight zone to find habitable conditions. Somewhere between the center and the twilight zone there might be a sweet spot. Decreasing the surface temperature of the sun will reduce the incoming radiation drastically and lead to habitable conditions even with a strong greenhouse effect as described with Venus.

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    $\begingroup$ You forget that the star was less luminous than the Sun. It is supposed to be a K5V star with about 0.17 the luminosity of the Sun. Perhaps you should recalculate the amound of radiation a planet would receive at 0.41 AU from such a star. $\endgroup$ Sep 22 at 16:42

If not, what do I have to change?

You will need some sort of artificial heat transfer mechanism. Based on a quick reading of Hu & Yang, I think that the dark-side will be frozen with the day-side baked. They're modeling Gliese 581g, a world about the size you want, although their planet is orbiting closer to an M3V star, the amount of solar radiation received (estimated to be about $866 W m^2$) is reasonably close to what your world would receive. For comparison, the Earth receives about $342 W m^2$

I suspect that a superconducting band could be embedded in the ocean on the day-side and it would need to stretch around the dark-side in order to dump heat in order to melt the oceans and likely frozen atmosphere. The issue will be to balance between too hot and too cold.

The SF novel Empress of Eternity had a giant, continent sized, canal (in a far distant future Earth). I think this would give an idea of the size, engineering complexity needed and amount of idiots who are going to try to blow it up. One of the Ringworld novels mentioned widespread usage of room-temperature superconductors that were destroyed by the Puppeteers.

Hu, Y., & Yang, J. (2013, December 30). Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars. Proceedings of the National Academy of Sciences, 111(2), 629–634. https://doi.org/10.1073/pnas.1315215111

Modesitt, L. E. (2010, November 9). Empress of Eternity (1st ed.). Tor Books.


The atmosphere is a vastly complex 3D system with more variables than you can shake a stick at and no easy way to calculate all the details. Even professional atmospheric models would struggle as the proposed planet is so different and there would be no way to verify the result. The best that can be hoped is to give some general pointers and observations.

If this world has a remotely similar amount of water to Earth then either there will ultimately be a runaway greenhouse effect and all of the water will end up as steam in the atmosphere or water will be able to exist on the hot side because you can’t have one hemisphere with water and one without if the oceans are miles deep.

Oceans can absorb vast amounts of energy but currents tend to be slow, atmospheres can’t absorb nearly as much, but move much faster. However the key on this world will be latent heat and cloud. Water will absorb a vast amount of heat when it evaporates (and release it when it condenses) and the hot humid air will rise. This happens on Earth in the tropics the air cools as it rises and spreads out at the tropopause creating vast amounts of cloud and rain higher up in the atmosphere.

With such a dense atmosphere and so much energy I imagine this world would have “tropics on stilts” with vast amounts of cloud piled up for tens of km, fog mist and rain and lots of tropical storms all the time. Clouds have both cooling and heating effects but have a net cooling effect. Sufficient cloud might form to simply reflect enough of the incoming solar energy back into space (or it might not in which case the oceans will eventually boil).


Not gonna happen, and I'll give you a solid real-world example of why: Venus.

Venus isn't quite tidally locked. It actually rotates in the opposite direction to every other body in the solar system. We used to think that this was because of some interaction with another large body, but recently it's been calculated that the winds themselves are making it turn backwards.

When we think of an atmosphere of a tidelocked planet, we think that the atmosphere would spray out in all directions from the spot that points at the star, then return to that spot from the upper atmosphere. That would be a really complicated system, though, and it wouldn't be stable. Any small perterbation would give it a directional bias.

The planet's path around the star gives it a directional bias, as it plows through the solar wind. Once that takes hold, all of the energy winds up pushing the winds in one direction. With Venus, this has resulted in a single planetary hurricane. The atmosphere on Venus travels around the planet once every four days. It's constantly scouring the surface, to the point that it has completely countered the planet's rotation, and gone a bit further, pushing against the limits of how fast it can spin the planet, backwards, against the forces of tidal energy loss.

This is the phenomena that you're up against if you want to build that world.


It's my understanding planets like this are called "eyeball" planets because they resemble them. Mostly for the probability of a permanent hurricane resting on the equator.

No hard Science here, however... Given the right geology/typology above and below the ocean, could there not be several ocean currents leading into and out of the hot side to the cold?

"Super Gulf Stream" type currents. moving 50 million cubic metres per second at better than 8+ KMPH doesn't sound unreasonable compared to our own Gulf Stream on earth. Taking into account the right geology. One or more currents could maybe move enough heat away from the day side to the night.

The right set of circumstances seems no more unlikely to me than how unlikely that our Moon is exactly the right size and distance from earth to create our perfect solar eclipse. By the way, it's not mentioned in the OP, but does this planet have a moon? Is it tadal locked? is it in a permanent eclypse? what would this do to the planets heating profile?

  • $\begingroup$ Gillgamesh Thank you for your answer. This definetely helps clear things up a bit. As for your question regarding a moon, the answer is... Yeah, kinda? It has a large asteroid the size and mass of M. Everest orbiting around it, but I doubt if this will have any significant effect on the planet itself $\endgroup$ Sep 23 at 20:09

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