Scientists seem to be vacillating as to whether a Red Dwarf planet could support life or not. Imagine a G-Type star much like our Sun. Imagine an orbit that would make an Earth-Sized planet warmer than Mars, but noticeably colder than Earth—Very Marginally habitable.

Now put a Red Dwarf in that orbit and put our Earth-Like Planet in a tidally-locked orbit around the Red-Sun. One side has perpetual Red day. The orbit around the Red Sun should be short—eight to ten days or so. Yellow days will be four or five days long and then yellow night will be four or five days long.

If our planet is tilted and the orbit around the yellow Sun is eccentric, then we'd have "Meta-Seasons"—lets ignore that for now. Some models suggest that there should be enough atmospheric heat exchanged in a simple Red Sun locked-planet to noticeably warm the dark side.

I'd like to imagine a world where dark side nights are cold, but not as cold as Antarctica whereas the bright side doesn't get drastically hotter during yellow day — but the photosynthesis increases a great deal during yellow day.

I want reindeer herding nomads on the dark side, hereditary aristocratic plantation owners close to the center (Yeah all models don't feature a perpetual cyclone at the high-noon" position on bright side.) and hillbilly highlander types out towards the edges where the Red Sun hangs low in the sky.

Does anyone see any flaws in the scheme that I haven't seen?

  • $\begingroup$ if you could specify some masses and distances we could try to see if such a system is stable or not. I am afraid it isn't. Not on the time span needed for life to develop. $\endgroup$
    – L.Dutch
    Aug 28, 2017 at 5:14
  • $\begingroup$ is the Earth-Sized planet a perfect greenhouse with a built-in dynamo? depends on your astronomical distance sunbathing in ionizing rays would sterilize virtually all multi cellular organism except if you count "water-bear" and roaches... which will eventually succumb to arthritis :) $\endgroup$
    – user6760
    Aug 28, 2017 at 5:23
  • 2
    $\begingroup$ @user6760 Arthritis? In tardigrades and cockroaches? Please explain how or provide access to sources that explain how this could occur. Sounds very curious. $\endgroup$
    – a4android
    Aug 28, 2017 at 5:39

3 Answers 3


Some basic points first:

  • How far should the planet be from the Red Sun? For an M6 red dwarf with 10% of our Sun's mass, an orbit at 0.05 AU gives you a period of 12.9 days, which is a bit more than what you wanted, but this is necessary for the following.
  • How much irradiation will the planet get from the Red Sun? A typical M6 dwarf is only 0.09% as luminous as the Sun. Since your planet will be much closer, that will compensate: $0.05^{-2} \times 0.0009 = 0.36$, which means the Red Sun will supply 36% of our Sun's power on Earth.
  • How much irradiation will the planet get from the Yellow Sun? Let's split the difference between Earth and Mars and say your Red Sun (with its planet) orbits at 1.2 AU from the primary. Therefore the irradiation at that distance will be $1.2^{-2} = 1.44$ times less, or about 69% of the Sun's on Earth. That is, the solar irradiation your planet will receive (on all wavelengths) will be about the same as Earth's on the Red-Sun hemisphere, and a third less on the opposite one.

A lot of the light coming from the Red Sun will be in the red and infrared part of the spectrum. Chlorophyll is rather good at catching red photons, surprisingly, but it will probably not be enough. The experts say that under two different suns it's likely plants will evolve to use one type of light or the other. But with a third of their usable energy coming from the Red Sun, I'd guess plants on the Red-Sun hemisphere will not miss that; they might conceivably use something like chlorophyll during the Yellow day and a different pigment at night.


Let's give a look at life on our planet.

All plants have developed the ability to use solar light to produce sugars for their metabolism, and the rest of the food chain is based on this. There are few exceptions, where the main energy source is something else (black smokers in the ocean depth, etc.), but the common lesson is that life can thrive if it can rely on a single energy supply. We don't have bacteria which can live both in a black smoker and in sunlit environment. Only the upper food chain can be less choosy.

Now, in your world there are two potential sources: the yellow star and the red dwarf.

Based on what I stated above, I would say that life would adapt to only one of the two.

My bet would go mostly on the yellow star, as it can be more energetic.

  • $\begingroup$ Moonlight is sunlight, so plants are be perfectly able to use it; the problem is that moonlight is very very much weaker than sunlight -- about one million times weaker, or 20 exposure steps; this means that whatever photosynthesis may take place at night is totally negligible. $\endgroup$
    – AlexP
    Aug 28, 2017 at 11:05
  • $\begingroup$ @AlexP, I am no plant physiologist, but I suspect that since the process is optimized for daylight, having photosynthesis during night can waste rather then producing resources. Anyway, I'll take that line off. $\endgroup$
    – L.Dutch
    Aug 28, 2017 at 11:45
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    $\begingroup$ @AlexP It turns out you are both wrong, as any farmer can tell you. There's a definite (large) difference if you plant some species (mainly fast-growing, like salad) at first quart or at last quart. In the first case there's much faster growth. I suspect it's because the full moonlight keeps photosynthesis going, so it will ramp up quicker at dawn. $\endgroup$
    – ZioByte
    Aug 28, 2017 at 12:54

Red dwarf stars always pose an additional problem you hadn't considered. They tend to be very, very active. The reason for that is they are, essentially, entirely convective. And it's precisely the convection that creates the magnetic field that, in turn, is responsible for the cyclic activity.

Even though we are talking about a much smaller and much colder star than our Sun, it would be dramatically more active. So the tidally locked planet would be constantly bathed in starwind and charged particles.

However, what seems to be a problem can work out as the solution. Instead of photosynthesis the organisms could had evolved in a way that they get their energy from the solar storms. According to this article on wikipedia, the solar wind from our sun consists basically of electrons, protons and alpha particles. On Earth, our magnetic field conducts these particles to the poles, where they form the aurorae, however in a planet where the stellar wind is much stronger, these particles could easily reach the surface. There's a region known as the South Atlantic Anomaly where this happens on Earth, and which is linked to a much higher skin cancer and satellite damage rates.

Photosynthesis is, in short, a chemical reaction that needs photons. A similar reaction, but involving electrons with energies compatible to the stellar wind isn't so far.


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