Why do gas giants have similarly coloured stripes?

Question

After looking at Jupiter and searching 'exoplanets gas giants' on google I found that many had stripes on them. I found that pretty peculiar. So why do they have stripes. I think it has something to do with different gas densities but I do not know for sure. If so, then why do some gas giants like Jupiter have stripes with the same colour in different latitudes. Lastly- why do said stripes seem to always be roughly the same colour- like how Jupiter is roughly beige overall.

Here are some pictures of gas giants- I don't think it would help much but why not?

Answer 1

To be crystal clear: The depictions of exoplanets in the image included in the question are all artists' impressions of what those exoplanets might look like. We simply do not know what any exoplanet looks like (and we won't be able to resolve surface features for the foreseeable future), and the best we can do is to provide educated guesses.

The depictions you've quoted are just that: educated guesses, based on the examples that we do have access. And, since Jupiter and Saturn both have striped patterns, it's a good educated guess that exoplanets with similar dynamics will present a similar appearance. But, again: the key words here are "guess" and "similar".

In Jupiter and Saturn, the banded patterns appear as a combination of convection in the upper layers of the planet, and the differential rotation of the gas (i.e. different parts of the cloud layer rotate at different rates). This separates the flow into circular convection cells (the bands and belts), which have different thermal characteristics (because of the convection) and thus different chemistries, which gives them different colours.

It's a reasonable guess that gas-giant exoplanets will have similar dynamics, in which case the banded pattern (which is part of the dynamics) will be present. But this is a guess $-$ we don't know, and won't know, until we look.

Answer 2

I’m going to give a rough answer as I’m still not an expert. Nevertheless, the reason that Gas Giants appear to have a particular color composition is due to their own chemical composition of gases in the planet.

Depending on the chemical composition of the gases and other compounds including ice-crystals the gases absorb different energies of photons(dictated by the specific energy level gaps in the electron arrangement of the atoms). Hence they absorb certain “colors”(red has lowest energy while blue/violet has highest energy) of light while reflecting others. Hence the about-same color composition from one planet to another depending on their roughly same composition of gases/other molecules throughout their atmospheres.

For example Jupiter’s Orangish/Red-Brown colors are mainly due to a compound called ammonium hydrogen sulfide. While the methane in Neptune’s upper atmosphere absorbs a lot of red-ish light so it reflects blue light, hence its blue color.

Now to address the final issue of the difference in intensity or slight change of the colors in the planets, that is due to the specific air currents and pressure differences changing the densities and compositions of the atmosphere(like giant wisps of clouds moving about) and hence these variations.

Answer 3

A lot of gasses are clear, colorless (or have a few sharp spectral features).. Most solids, however, unless they are very pure, absorb light (because of atomic proximity, spectrum lines are broader, and any impurity adds new lines).

So, like on Earth, we'd expect sunlight to penetrate deeply into atmosphere, but eventually run into something opaque and... stop. That makes heat at the bottom of the atmosphere, SO we expect the warmer lower-atmosphere gas to expand, and rise, and that means the atmosphere isn't just stratified, it's stirred.

The spin of the planet makes a coriolis effect, and that breaks the symmetry of the air movement, and the result is a series of bands of toroidal-flow encircling the planet, around the planetary spin axis. On Earth, the 'Hadley cells' are known from antiquity (because of the 'doldrums' between the cells, ocean navigation is sensitive to this airflow pattern).

So, it is predictable that atmospheric circulation will include belts around the spin axis; as for colors, those could be cloud effects if the circulating gas has any condensation of components, or could be a surface-temperature or distribution-of-dust effect secondary to the air circulation pattern.