What would the sky color on terrestrial exoplanets be like?

Answer 1

nobody has found any proof there are trees. but if trees did exist on any other planets, then they would probably be; for example if the star the planet was orbiting was red, then the tree's leave's would probably be the colour of the sun, except at the opposite spectrum.

Answer 2

This is dependent on four factors:

1. the nature of the light source,
2. the composition of the atmosphere,
3. the thickness of the atmosphere,
4. the type of eye that is viewing the sky.

When we speak of planets our light source is usually a star. The light that a star emits follows a distribution curve that is close to something we call 'black-body' radiation curve. Without getting too descriptive the distribution is a skewed hump with a steep rise on the high energy side that has a definite termination energy, a mid-energy peak, and a low energy tail that just keeps getting smaller and smaller without ever falling completely to zero. This distribution determines how much light of a particular colour you get from your light source.

What determines what kind of light a star emits is (ultimately) is the mass of the star. A very small star will only manage to emit a small amount of very red light. Its light distribution will start in the red rise to a maximum amount in the Infra-red then fall off slowly into the microwave and taper-off towards nothing as it gets deeper and deeper into the longer radio frequencies. A slightly more massive star would have its light starting at a higher frequency, perhaps in the visible orange, would peak in the visible red, and then taper-off into the infra-red and beyond. This distribution is then shifted to the higher frequencies and is as well, greatly increased in amount.

A slightly more massive star yet would have its light start in the ultra-violet, peak in the visible yellow and taper-off into the lower frequencies and be greater in amount again. Such a star would be very much like our own sun. The light of the most massive of stars would begin in the x-ray region and peak just beyond the ultra-violet.

This is the light that is available to illuminate a sky. It delimits what colours are available to be seen. An observer on a planet illuminated by a small star would only be able to see the colours Red and lower as that is all that is available. If the observer were human (who's eye cannot see below deep red) they could only see what little red there is. As the mass of the star increases the range of colours available to be seen increases, as does the relative amount of these colours and their intensity as well. For a human observer this only concerns the visible light as that is the only colours the human eye can discern.

Next comes the composition of the atmosphere.

It is a fact of optics that light can only be reflected from objects that have a radius equal to the wave-length of the light. (We won't get into why here.) Nitrogen gas (N2) reflects all light starting at the low ultra-violet down to the higher blue - below this the wave-length of green, yellow, and all others is too long to be affected by such a small molecule. Oxygen gas is very similar in size to Nitrogen gas and only extends the range of blue slightly. And, as Oxygen gas is only 1/5 of the atmospheric composition the extra blue colour it adds to the sky is low.

[Now before we go further we should add that the Ozone layer in the upper atmosphere is opaque to Utra-violet light it shades out any copious violet from the sky. Therefore even though there is considerable water in the air as well as some methane, Helium and Neon that would reflect UV light there isn't much of it to be seen.]

Now there are some larger molecules in the atmosphere that would reflect some of the lower colours: CO2, Argon, Krypton, Xenon, and Radon; the concentrations of these gases are all very low and don't effect much.

There are also some larger particles in the atmosphere that are big enough to reflect red light: water droplets and dust particles - and in fact when the sun is below the horizon sunrises and sunsets can be quite red.

Lastly we should mention the thickness of the atmosphere. Regardless if a gas is transparent to a particular colour of light, it is never completely transparent and enough of it will shade out all light. So the thicker the atmosphere the less light there will be. And particular gases shade different colour by different amounts. it gets complicated.

So firstly you have to determine what kind of star you have and what kind of observer you have. A small star will only provide red light and below. And as a human observe perceptions stops at Deep Red - Red would be all that he would see about such a star.

Secondly you have to decide upon the composition of your atmosphere. An atmosphere of chlorine gas would be green. Green is the lowest frequency of light that chlorine gas can reflect and as green is more abundant than all other higher wave-lengths it dominates - but the real colour is a combination of the relative abundances of all the higher colours. A mixture of gases would give a more complex combination.

Thirdly the shade factor would greatly truncate the abundance of certain colours - as the Ozone layer does to all colour above UV.

An atmosphere of Hydrogen gas and Helium would appear Violet. (Jupiter, Saturn)

An atmosphere of Nitrogen and Oxygen would appear Azure. (Earth)

An atmosphere of Chlorine gas would be green.

An atmosphere of Carbon dioxide would appear Yellow. (Upper Venusian air)

An atmosphere of Xenon or Radon would appear Orange.

An atmosphere of dust particle and water droplets would be red.