I have a planet orbiting at 0.2067 AU around a M-class red dwarf of mass and luminosity 0.4037 Msol and 0.0286 Lsol, respectively. The planet has an atmosphere of 93% CO2 and 3 atm. What I wonder is, how visible would other stars be during daytime and dawn/dusk?


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    $\begingroup$ 0.03 times the luminosity of the sun is only 5 exposure steps. The planet is 5 times closer to its star that Earth is to our sun, so that this makes up for 2.5 exposure steps. Overall, daylight is darker than on Earth by about 2.5 exposure steps, which is ... no big deal. (For comparison, a night illuminated by a bright full moon is about $\rightarrow$17$\leftarrow$ exposure steps darker than daylight at noon.) The only thing human eyes will notice is that sunlight at noon in mid-summer is not as blindingly bright as on Earth. No stars. $\endgroup$
    – AlexP
    Oct 31, 2023 at 19:40
  • $\begingroup$ @AlexP I can get a sense of what you mean by "exposure step". What and where should I research such? Is exposure step a photography term? $\endgroup$
    – Ylahris
    Oct 31, 2023 at 20:33
  • $\begingroup$ @Ylahris see exposure value. 5 stops would be equivalent to a 32-fold difference in light levels. $\endgroup$ Oct 31, 2023 at 21:17
  • $\begingroup$ @Ylahris: Eyes are photo cameras. A two-and-a-half-step difference in exposure value is quite mild. Increase sensitivity by one step (the eyes do this automatically), open the diagraphram (= the pupil of the eye) by one step (again, automatical), increase exposure time a little. As a numeric example: outdoors, not in the shade, in summer at noon the illuminance is around 100,000 lux, and human eyes find the light too bright and too harsh; typical indoors artificial lighting is between 200 (living room) and 750 lux (work areas such as a shop or a desk), 7 to 9 EV steps below summer noon. $\endgroup$
    – AlexP
    Oct 31, 2023 at 22:02
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    $\begingroup$ On how bright the sun would appear in an easy to understand way. It would be approximately 66.939 to 72.843% the brightness of day on Earth. (I used two different methods and got two different results, second one is more reliable). So, it would be dimmer, but would ultimately not be too notable. Stars would still not be visible as it is still a Apparent Magnitude of -26.396, which is still rather bright. Not sure how to calculate for the change in brightness due to the atmosphere, which would need to be taken into account with such alien atmospheres as 92%Co2 and 3 atm. $\endgroup$ Dec 4, 2023 at 18:55

1 Answer 1


how visible would other stars be during daytime and dawn/dusk?

Your planet is very likely to be tidally locked to its parent star... it is unlikely that a world that close to a star could still be rotating. The tidal locking timescale approximation listed on that page is proportional to the 6th power of orbital radius and inversely proportional to the square of the stellar mass so the fact that your sun is two fifths of the mass of Sol doesn't make up for your planet being 5 times closer to it. The Hill radius of your world will also be problematically low for the existence of a Moon-like moon, which might help maintain a day-night cycle.

AlexP already covered visibility of stars during the "day", to wit: not at all... the sun and the sky would be too bright, and the human eye's dynamic range too low to be able to see the comparatively dim light of other stars. Obviously, the non-sun -facing side of a tidally locked world will be nice and dark, though. In between the two extremes the sky will be dim enough that a human eye could pick out stars in the sky, just as it can in terrestrial twilight. Computing the exact time the stars would come out is clearly possible, but it is sufficiently difficult that I'll simply say that twilight will occur at some point, and better accuracy is left as an exercise for the reader.

(unless you're writing a simulator, I'd just gloss over the details because they just allow you to be wrong about stuff. if you have a rotating planet that close to a star, you've already handwaved things enough for you to handwave in the times of twilight, too)


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