Most gases are clear. So why are there not any planets or stars that are more or less clear in color?
I suspect this question may be extremely stupid.
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$\begingroup$ It's good to feel uninhibited enough to be able to ask childish questions like this! $\endgroup$– Surgical CommanderCommented Feb 11, 2015 at 6:17
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$\begingroup$ Why is it childish though? It probably is but i'm too stupid at the moment to see why. $\endgroup$– math_loverCommented Feb 11, 2015 at 6:17
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1$\begingroup$ Hi Joshua. The question I've linked discusses the mechanisms that make things clear or not. If you're only asking about stars, and not planets, then it's because stars are made from plasma not neutral gas. Obviously planets are solid (mostly) so whether gases are clear or not is irrelevant. $\endgroup$– John RennieCommented Feb 11, 2015 at 7:27
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3$\begingroup$ It seems reasonable to ask why gas-giants like Jupiter and Saturn are relatively opaque. After all they are composed mostly of Hydrogen and Helium (even if most of it may be in a liquid state). $\endgroup$– RedGrittyBrickCommented Feb 11, 2015 at 9:20
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2$\begingroup$ It's a good question, and neither the question nor the correct answer have anything whatsoever to do with the linked "why is water clear?" question. The actual answer is that even things like water or hydrogen gas, which are clear in small quantities, are no longer clear if you scale them up to the size of a planet. I hope someone has the time to write up a good answer explaining this. $\endgroup$– N. VirgoCommented Feb 11, 2015 at 12:05
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2 Answers
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In short, you're making a mistake of scale. No gas is truly clear. Outside of Los Angeles, the atmosphere around us seems pretty clear if we look at a nearby wall or building. But try looking at a distant mountain, and you will notice it is blue tinted from Rayleigh scattering. To some degree every gas both absorbs and scatters light. If instead of looking at a mountain a few kilometers away, you were trying to look through a high density gas giant tens of thousands of kilometers in diameter, then you would find this effect over a thousand times stronger.
You might at first think that this means gas planets could be clear on the edges then where there is less gas to look through. For an interesting experimental limitation of this, search around for some pictures of the Earth from the ISS. You will see our own atmosphere is not clear when you look at it from the side, because one must look through such a long distance. To see the degree of transparency, look at the many photos from the ISS of the moon seen through the edge of the Earth's atmosphere. You will see it fade into a blue tint, and distort from refraction.
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You are probably thinking of gas giant planets like Jupiter, Saturn, Uranus or Neptune. The Jupiter is the most colorful one due to a larger number of elements it contains (phosphorus, sulfur and hydrocarbons that can form from methane in hot depths of the planet.
Neptune is much cleaner - it is composed of 80% hydrogen, 19% helium and 1% methane and its bluish color is attributed to weak absorption of red light by methane. This absorption is hardly noticeable in usual conditions but if the whole planet is made of gas, even the weakest absorption becomes apparent.
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$\begingroup$ This is kind of funny. I didn't realize that that is what gives gases their color. Now, what if we have a gas giant composed of almost 100% gas, and the gases in its composition have very weak absorption (can you suggest some gases like this), then if the planet is small enough, we can have it be clear, no? $\endgroup$ Commented Feb 11, 2015 at 14:02
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$\begingroup$ Because the internals of a gas planet are warm and turbulent, there will be strong variations of refractive index. Therefore, even a colorless planet will not be transparent in the same way a snowball is not transparent. $\endgroup$– gigacyanCommented Feb 11, 2015 at 19:09