Follow up to my previous question, after botched registration.

We have an Earth-like planet tidally locked to a star similar to Sirius A. The star does not belong to a binary system. The planet itself possesses an atmosphere, volcanic activity and large water bodies. As pointed out in this Physics SE, seasons would be the result of orbital eccentricity. A natural satellite orbits the planet.

The day side of the planet is inhospitable due to the heat from constant sunlight and most of it's water is likely underground. The twilight zone is also inhospitable due to strong winds created by the differences in temperature between the day and night sides. The night side is illuminated by the natural satellite, which has the same angular diameter as our Moon and an orbital period of about 30 hours.

Given that Sirius is 25.4 times more luminous than the Sun, would photosynthesis, and thus plant life, be viable in the night side of the planet? Would the plants need a different pigment, such as retinal rather than chlorophyl?

Thanks for JDługosz and Youstay Igo for their answers to the previous question.

  • $\begingroup$ You can contact Stack Exchange and ask them to merge your two accounts, to reduce the confusion here. Also, how far away is the planet from the star? $\endgroup$
    – HDE 226868
    Sep 12, 2015 at 21:25
  • $\begingroup$ Important note: just because the star is 25 times more luminescent than the sun doesn't mean that the moon would be that much brighter. The planet would have to be about 5 times as far away to keep from being roasted, so the apparent brightness would be about the same as on Earth. Sirius A would actually be quite obnoxious to orbit, because it would have peak emissions in the hard ultraviolet range (nearly X-rays) while the sun has peak emissions in the visible range. $\endgroup$
    – user11599
    Nov 10, 2015 at 8:24

2 Answers 2


Photosynthesis is definitely possible. As the star is 25 times brighter (and hotter) than our sun, the moon would be really really bright and the "night" side of the planet would be in effect as bright as a winter sunlit day on earth. Photosynthesis is ON!

The only issue I have with such ghastly planet is the raging storms! In reality for such a planet, the vortexes and storms would not perpetually blow on the poles, but in fact ALL the planet's surface would be ravaged by horrible dunes and cyclones, the force of which makes me shudder in horror with the very thought of them. Just find some way of getting rid of these vortexes and the rest is all cool.

Oh yes. I recall. You will also need to get some form of ozone layer and a strong magnetic field. The magnetic field would keep the planet safe from the outrageously powerful magnetic storms on the star and the ozone layer would protect it from the devastating gamma rays, x-rays and ultraviolet radiation which would otherwise make life (on either side of the planet) impossible.

Large water bodies are good. The more water you have on your planet, the lesser would be the temperature difference. I would suggest a planet nearly twice the size of earth with 80% of it's surface covered with deep oceans. You will still get really large continents for your civilizations.

I think if you introduce really really lofty mountain ranges on the horizon boundary between day and night sides, you can effectively break the wind power to less than half. Higher in the atmosphere, storms would be RAGING but near the surface, breezes would be mild and pleasant.

  • $\begingroup$ Thank you for this answer. It's great that you not only gave the answer to the question, but you also expanded on the subject. I didn't expect such a "moonday" to have luminance of about 2000 lux (for winter), I expected the value to be in the hundreds lux. Very informative. $\endgroup$
    – G-Temez
    Sep 11, 2015 at 19:37
  • $\begingroup$ It depends on you how much bright you want to make the night side. Generally speaking, an airless moon will be more shiny and reflect more of the sun's light, hence making the night side not only much more bright, but also slightly warmish. If you want to make the night side more dark (e.g. as dark as a full moon summer night on earth) then change the size and composition of the moon. Make the moon smaller so that it has lesser surface area for reflecting the star's light. Or give it an atmosphere so that it reflects less light than it receives from the star (that part of light is absorbed). $\endgroup$ Sep 11, 2015 at 22:41

It's unlikely that a tidally-locked planet would have a moon to begin with. In our solar system, most moons form outside of the geosynchronous orbit. The few moons that do form within that orbit are doomed to eventually spiral inward and collide with the planet, as is the case with Phobos, for example. So for a moon to form a stable orbit around a planet, it has to have an orbit that gives it a longer revolution period than the planet's rotation period. In the case of a tidally-locked planet, that would be beyond the planet's Hill Radius. So yes, in theory, any planet can have a natural satellite. However, tidally-locked planets lack the ability to keep one.


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