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I am developing a fictional planetary system in which a large gas giant planet (slightly less than the mass of Saturn), has migrated into the habitable zone during the formational years of the system, and hosts habitable moons.

The star in question is a K0V Orange Dwarf, which is reasonably quiet (i.e. doesn't flare often or at all anymore)

In trying to determine what colour my gas giant should be, it became clear to me that photochemical reactions in the atmospheres of these planets are a major factor in determining what compounds are present, and thus their colouration.

Most importantly, in our own solar system, Jupiter and Saturn receive more UV radiation (which breaks down methane into other compounds), than Uranus and Neptune (which are able to retain methane, and are thus bluer.)

Since my fictional gas giant is orbiting its star very closely to be within its habitable zone, my initial thought was that the planet would not be able to retain methane, and would therefore lack blue colouration. However, I then remembered that K-type and M-type stars are cooler, and therefore emit less UV radiation in the first place (except for flares).

What I am trying to determine is this; Does a quiet K-type star emit a sufficiently low fraction of its output in the UV spectrum, that a Jovian/Saturnian type planet would be blue or blue-white even at a habitable distance?

System parameters:

Star (K0V)

  • 0.85 Sol masses
  • 0.75 Sol radii
  • 0.40 Sol Luminosity
  • ~5,250K surface temperature
  • Age: ~8 gya

Planet (Gas Giant)

  • 82 Earth masses
  • ~50,000km radius
  • Semi-Major Axis: 0.85 AU
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  • $\begingroup$ Sudarsky's gas giant classification might help. $\endgroup$ – TheDyingOfLight Jan 20 at 22:08
  • $\begingroup$ @TheDyingOfLight I had encountered Sudarsky's, yes. However, my understanding of it is that it applies only to a G-type star. $\endgroup$ – Arkenstein XII Jan 20 at 22:25
  • $\begingroup$ As far as I understand it its only temperature based. Sure, the colours might vary a bit depending on the illumination, but unless the star gives off a lot of UV it shouldn't mess up the chemistry of the atmosphere too much. So if you assume class 2 or 3, the equelibrium temperature should give you the semi major axis range. That said, I don't have a PhD in atmospheric chemistry. So take all of this with a grain of salt. $\endgroup$ – TheDyingOfLight Jan 21 at 20:40
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You have two questions to consider here: Can compounds required for blue atmospheres form in significant amounts on this planet, and are the temperatures right for them to condense and form clouds?

I talked about atmospheric composition and color in an answer to a related question. Essentially, the question of whether or not compounds like ammonia and methane (known as volatiles) - can exist in a giant planet's atmosphere depends on the orbit of the planet in relation to the star's frost line. The frost line is the point at which, in the protostellar nebula, these compounds could condense. This critical temperature is thought to be around 145 Kelvin. For a solar-type star, the frost line would have been around 2.7 - 2.8 AU. I suspect your star would have a frost line slightly lower than this, perhaps 2.5 AU. This would seem to indicate that your setup is impossible.

However, giant planets have been found quite far inside the frost line; notable are the gas of exoplanets known as hot Jupiters. These planets migrated inwards through interactions with the protoplanetary disk or planetesimals early in the system's history, allowing giant planets to orbit quite close to their parent stars. You can easily place your planet inside the frost line if you allow migration to occur.

Now we get to our second issue: condensation. In general, different gases condense and are dominant at different temperatures, and so the color of the atmosphere depends on the planet's temperature. The effective temperature of a planet scales as $T_{eff}\propto L_*^{1/4}$, where $L_*$ is the parent star's luminosity. Plugging in the numbers, this means that your planet should have an atmospheric temperature (neglecting greenhouse effects) suitable for water vapor but too hot for methane or ammonia. Water vapor clouds could lend a blue color to the atmosphere, but unfortunately it would not be aided by the presence of atmospheric methane or ammonia.

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  • $\begingroup$ Isn't the dominant color expected to be blue in the 300-900K range? The lower end could still support habitable moons, depending on details, especially if water clouds are allowed. $\endgroup$ – CAE Jones Jan 20 at 22:16
  • $\begingroup$ @CAEJones I'm curious as to where you're getting that figure; the slides I linked to indicate we'd see whiter and reader atmospheres at those temperatures, due to alkali metals. 100 to 250 K should yield teal-ish colors, thanks to ammonia and methane, which is why I'd consider that a good low-temperature zone of blue beyond the water vapor regime. Plus, Sudarsky et al. note that above ~350 K, temperatures are too high for water to condense, so the atmosphere would be clear or white, not blue. $\endgroup$ – HDE 226868 Jan 20 at 22:18
  • $\begingroup$ I see. So, methane would have to be in a condensed form to contribute to the planet's appearance? Being present as an atmospheric gas is not going to apply? $\endgroup$ – Arkenstein XII Jan 20 at 22:22
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    $\begingroup$ @ArkensteinXII Right, condensation and cloud formation matters. You have to both have the compound in abundance - which I argue is possible - and have it be at the right temperature to condense and in fact dominate over other compounds, which I believe is not possible in the current setup. $\endgroup$ – HDE 226868 Jan 20 at 22:25
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    $\begingroup$ @ArkensteinXII I'd assume a blue-white color, with blue coming from both Rayleigh scattering and clouds of water vapor. $\endgroup$ – HDE 226868 Jan 20 at 23:21

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