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?
- 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