So the design of the planetary system in the previous question goes on, following some of the precious suggestions and reference links, I somehow made the model work. I've been digging for similar questions but the answers differ from each other quite a bit.

And here's the question, for a tidally locked planet orbiting a red dwarf (assuming M2V class) at such distance, what would the sky and sun look like?

1) I assume the sky to be a bit greenish-cyan? Since a red dwarf shines mainly towards the red-IR side of the spectrum, and hardly any blue lights, green would be the colour most scattered and thus the sky would look more green than blue. Also note that the atmosphere got to be thick enough for the circulation of the temperature difference, and may block a good fraction of the light.

2) The planet is at around ~0.15AU(?) but the sun has 44% solar radius - I assume with human eye it would look quite a few times larger than we see the sun now, with a dim red colour? Would everything on the day hemisphere look red as well?

3) Also wondering if there is a possibility to eliminate the colour difference between this planet and earth. Since the sun looks a lot dimmer with light toward the red spectrum, everything would look a lot darker and blue/purple would be extinct. Will it be realistic to have colours look exactly (or at least close) to what earth is now without a lot of technology interference? (which is, with a natural cause, somehow the colours are corrected to earth-like)

Guess these questions are pretty much on the same topic so this won't be considered too broad like last question. Thank you for reading and any help would be appreciated.


1 Answer 1


2) Since the angular size linearly depends on the star radius, but (for distance>> radius) inverse linearly on the distance, the size (in degrees) is $0.44/0.15 \approx 2.9$ the angular size of the Sun.

3) Temperature of a typical red dwarf with $0.44R_☉$ is $3400 K$, according to Wikipedia. A typical incandescent light bulb has a temperature somewhere between $2000 K$ and $3000 K$, so assuming blackbody radiation, generally the light will look a bit whiter than a typical lightbulb-lit scenery (minus the scattered light). Considering fantastic chromatic adaptation of human vision, it would probably not seem really different from the landscape of our Earth.

  • $\begingroup$ So when the sunlight makes the colour spectrum shifted toward red humans can still see them as much like we are seeing on earth, even if they've never seen what it'd look like on earth? Because in a place with coloured lights, like in a red-lit room (well you don't see this much but let's assume" I pretty much mostly see red, none or very little adaptation is seen. For brightness I can see how it's happening everyday, but I'm quite unsure about colours. $\endgroup$
    – Yinnes
    May 25, 2016 at 16:17
  • $\begingroup$ @Yinnes But the spectrum won't be red-only - it will be just shifted towards red (infrared, actually). You can do a simple test: take a room well lit with incandescent bulbs (granted, this experiment would be easier to do some years ago), look at the surroundings - would they look out of colo(u)r? Compare with the look when the same room is sunlit. $\endgroup$ May 27, 2016 at 8:46

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