There are seven planets, and each one has a different coloured sky. One has red, one has orange, all the way up to indigo and violet. Of course, they don't have a sky this colour all the time, but preferably for most of the 'daytime'.

However, those aren't the only limitations. Humans must be able to survive in each of the seven without special equipment, and it must be possible for people from any one planet to be able to survive (maybe with a little special training) on any of the other planets.

These must be 'humans', with at least roughly identical anatomy to us on Earth, though the 'blue' planet doesn't have to be Earth.

The tilt, composition of the bulk of the planet, colour and kind of star, can be different if necessary, and these planets do not have to be in the same solar system.

What is the best way to achieve this?

  • $\begingroup$ Sunglasses and maybe consider getting a prism... $\endgroup$
    – user6760
    Oct 28, 2017 at 13:36

2 Answers 2


Won't happen with gases. Not if you want Terran type life in the open. The blue of our sky is primarily Raleigh scattering. https://en.wikipedia.org/wiki/Rayleigh_scattering

This is going to be the bluish end of whatever light the atmosphere is transparent to.

Most gases aren't coloured. Exceptions: * Chlorine -- yellow-green * Bromine -- brown * Iodine -- purple * Nitrogen dioxide (NO2) -- redish brown.

But all of these are toxic.

Table with more colours: https://en.wikipedia.org/wiki/Color_of_chemicals

A better way to colour sky may be with dust. Mars's skies are pink due to superfine dust floating in the air.

Most of our earth dust is white or grey, so you get dull colourless skies.

Smoke is fine enough that it shows some Raleigh scattering -- hence bluish colour when lit from the front. Mountain haze (mostly organics and water) is bluish too. Lit from behind the colours are redish. The blue got scattered, and red is what's left. The more particles there are, the dimmer the light, which you perceive as being more saturated.

Gold, if fine enough (A few atoms thick) will be green in transmission, and gold in reflection. So around the sun in the sky it would be green. Away from the sun it would be gold. Raleigh scattering would add a blue component at right angles to the sun. The world would overall have a greenish look to it from the filtering. You need some interesting geology to create this.

In general any colour of dust should be usable this way:

If the particle size is large enough that it is opaque, then reflection is the only activity. The colour of the dust dominates, but is mixed with raleigh scattering.

If the particle size is small enough to be transparent, then you two colours -- the bulk colour as back scatter, and the sunlight minus the bulk colour as forward scatter, again with Raleigh scattering off the gases.

If the particle size is small compared to light, you get Raleigh scattering off of the particles too.

In addition to this, particle shape has an effect. Ice in Earth's atmosphere produces a bunch of effects from refraction, reflection and diffraction. Start here: https://en.wikipedia.org/wiki/Halo_(optical_phenomenon) With coloured crystals you could tint the resulting phenomena. Note that the angles mentioned in the article are specific to water ice. You may need to get more detail off the physics forums.

You're going to need to do a bunch more research, but you have to potential to put on quite a show in your story.

  • $\begingroup$ Remember, angle is also a factor. The sky is red at sunset/sunrise, so if the sun moved around the horizon instead of vertically through it, in a certain area, the sky could be considered to be red. $\endgroup$
    – Piomicron
    Oct 28, 2017 at 15:49

Aurora's Are The Key

Originally I was going to suggest there being two atmospheres, one with poisonous color giving chemicals, the other being the "safe" atmosphere, but now I have an even better idea...

All the planets could have the same atmosphere but really strong protective magnetic fields. The key is in the Sun. Each planet would be constantly hit with rays of the sun to form a consistent Aurora while certain conditions might change the color for example one planet will have a harsh sun so the sky will be red, another planet will be mountainous and the altitude will be really high and the sky will be green

More information here: http://www.webexhibits.org/causesofcolor/4D.html

  • 2
    $\begingroup$ Can you provide any detail as to what is required for each aurora? $\endgroup$
    – Piomicron
    Oct 28, 2017 at 17:08
  • $\begingroup$ Green and Greenish yellow are at high altitudes (120-180 km) Blue and violet at lower altitudes (below 120 usually), Red at even lower altitudes (90-100km) but also when the sun is “stormy”. The amount of chemicals in the atmosphere also affect it but the article explains it much better $\endgroup$
    – Ethan
    Oct 29, 2017 at 22:45
  • $\begingroup$ Okay, but for the sky to be that colour at all times...? $\endgroup$
    – Piomicron
    Oct 30, 2017 at 13:21
  • $\begingroup$ Basically it would be the same, the Aurora would be going constantly but the magnetic field in the center of the earth will keep the rays from killing people. People would be able to survive on the planet but it might be trouble entering the planet. Maybe it could even be a day/night thing to keep things balnced $\endgroup$
    – Ethan
    Oct 31, 2017 at 16:42

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