Ah, atmospheric optics is fun. Different worlds are going to have quite diferent colors: http://www.astrobio.net/index.php?option=com_retrospection&task=detail&i... http://pubs.giss.nasa.gov/docs/2007/2007_Kiang_etal_1.pdf http://pubs.giss.nasa.gov/docs/2007/2007_Kiang_etal_2.pdf Some good discussions on alien skies: http://www.bautforum.com/archive/index.php/t-60031.html http://www.shatters.net/forum/viewtopic.php?t=11138 We had some good ones on the 2320AD forum. Some notes: Cooler stars are less luminous but will have terrestrials orbiting closer. A planet in the lifezone will be sqrt(L) AU away and the luminosity in watts/m^2 will be roughly constant (L/4pi(sqrt(L))^2), about 1300 W/m^2 outside the atmosphere. The size of the sun will be different though. Assuming the star radius to scale as R ~ T^(-2)L^0.5 the angular diameter is going to scale like ~Rstar/distplanet = T^(-2)L^0.5/L^0.5 = T^(-2) - hot stars will be pinpricks of searing light, cool stars will cover more of the sky. This affects horizon light and colour perspective a lot. It turns out that the brightness of the horizon is set by how much of sky sphere is covered by the sun. So on planets with hot suns the horizon will be much darker and the colour perspective will be weak - distant mountains will not look as bluish as they do here. On a M star planet there will be more of a mistiness in the air, and the colour perspective will be more extreme (and likely a bit tinted; "look at yonder turquoise mountains!"). If the peak of the spectrum is at longer wavelengths than for the sun, there will be less Rayleigh scattering of the sunlight and the sky will look darker at the same light intensity from the sun. However, particle scattering may matter more. Ozone scattering is apparently responsible for keeping the sky blue at sunset and especially twilight, otherwise it would be a greyish-green blue at sunset and yellowish at twilight (this is from E.O. Hurlbut's paper in 1952). Planets with UV-rich stars might hence have much bluer twilights than planets with stabler stars (or less oxygen). Another messy factor: the scale height of the atmosphere. Pressure decreases exponentially with height, the scale height (a 37% decrease, ~8 km on Earth) is H=kT/Mg, where T is the temperature, M is the mean molecular mass and g is gravity. On a light planet the scale height will be larger, the atmosphere will be "higher", clouds will grow higher (but more slowly) - and light will have to pass through more air. But since low gravity worlds the total pressure is lower, so the amount of air scales just with temperature and M, not g! This means an equal amout of Rayleigh scattering, but more chances from diffuse scattering from dust and water, reddening the sun a bit and making the sky a more milky blue. On a heavy world M is going to be bigger in addition to a heavy g, so the scale height is going to be short. Clouds will be low, convection strong (fires can turn into nasty firestorms due to the chimney effect). The difference between zenith and horizon will larger, and there will be less scattering - a sharper, more dark blue sky with a low but intense horizon light. Flares might be important for habitability of worlds around M-class star http://www.centauri-dreams.org/?p=993 Halos and parhelia on alien worlds might also be quite different: http://www.atoptics.co.uk/halo/oworld.htm If you want the full story of atmospheric optics: http://homepages.wmich.edu/~korista/atmospheric_optics.pdf