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Let's say we have a Neptune like planet orbiting around a k5v orange dwarf star. The planet is about as massive as Uranus but its atmosphere contains a large amount of methane. Based on what I have read from other sources, methane is what gives planets like Uranus and Neptune its blueish colors. Distance from the star also plays an important role, since the more sunlight methane recieves the more orange in hue it becomes, which explains why Uranus is more greenish than Neptune. Now, my planet orbits a star with a luminosity of 0.17 and a temperature of 4,440 Kelvin. Based on this diagram:

enter image description here

I assume that most of the star's light shines somewhere around the yellow area of the spectrum (I am not sure, please correct me if I'm wrong) and so the radiance of the blue color would be much smaller than the Sun.

Amyways, to sum it all up, assuming the planet is about 26 AU away, what color would it appear?

(Note: the image used doesn't belong to me)

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  • $\begingroup$ Where is the observer located, that would say which color is the planet? With this small flux from the star I doubt that an observer at 1 AU from it would actually detect your planet with a telescope, let alone determine its color. $\endgroup$
    – Vesper
    Sep 15 at 12:35
  • $\begingroup$ Vesper I just imagined someone in a spacecraft that travels around the system. They can go as far or as close to any planet they want $\endgroup$ Sep 15 at 12:52
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    $\begingroup$ So the observer can be anywhere. Okay, this also allows for optics of any quality and exposition of any length to determine the planet's color. Then it comes to whether orange component from the star reflected by the planet's methane would dominate over blue. Gonna search a bit for methane's reflection spectre and intensity. $\endgroup$
    – Vesper
    Sep 15 at 13:03

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In fact what makes the Uranus color a bit less blue than it's sister Neptune is the presence of haze (way thicker on Uranus upper atmosphere). Without this haze, both planets would have the same color.

Now the question is : why does Uranus has a larger layer of haze? Well a paper from Irving & al is making the assumption that the turbulent atmosphere from Neptune is more efficient at getting rid of methane snow (codensation). Which does in fact not help your question but it was to demonstrate that the distance does not impact the color difference between the two sisters.

To give a bit of a response: it definitely depends on the methane depletion. Few studies from Molaverdikhani & al are investigating the effect of irradiation and clouds on methane depletion which could be useful for you. The fact is the methane depletion is related to the flux the planet receive, but also the Carbon to Oxygen ratio. It doesn't seem to be fully understood the effect of the radiation on the exact behaviour of the CH4 in the atmosphere but from what I learnt, methane seems to go deeper in the atmosphere with less radiation from the stars which could impact the absorbtion spectrum and so give a less blueish teint... (for a Neptune like)

I know I don't fully respond to the question but plenty of phenomenom in the Neptune atmosphere are not understood yet and the impact of solar radiation on the composition is one of them...

To add a bit more information, the fact your star is definitely less luminous and has a bigger part of its spectrum in the orange would make your Neptune darker possibly. What I would suggest is create a darker Neptune, blue and a bit green-ish. Which could be explained with the H2 and other volatil component being higher in altitude than the CH4... Hope that help.

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It would look a lot like Neptune

You're treating the planet like its reflectivity changes with the spectrum of light thrown at it. That's not how reflectivity works. An atmosphere that reflects blue will always reflect blue.

An argument can be made that should the star have less blue to reflect then other elements in the atmosphere that reflect other colors might stand out more. But remember, we're talking about an atmosphere like Neptune's, and whatever elements may be in its atmosphere, the greatest reflected wavelengths are the blues.

So, without going through all the gory details about the percentages of various gases and dust particles, their reflective spectroscopy, and the spectral distribution of the star—I'm pretty confident that your planet will look blue.

Even the shift in luminosity isn't an issue. Less light won't mean less blue. It only means your cameras need better light compensation.

This Stack has seen a LOT of questions about what color things will be under stars of different colors. One of the most useful presented a color chart of what an atmosphere of different pressures would look like under different star colors. One of the most remarkable aspects of the chart is how much it takes to wash out the blue.

The truth is, star color has much, much, much less to do with what color an object presents to an observer (with the exception of a mirror) than does the spectral reflectivity of the object. To wit, if an object only reflects the 460 nm wavelength of light (blue), then it doesn't matter what the source light's spectral composition is. You'll only see the 460 nm wavelength of light because that's all that's being reflected.

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