The blue color of Earth’s sky is not an inherent property of the atmosphere. It results from the complex interplay between our planet’s atmosphere and the light from our Sun, a yellow-white G-type star. If Earth orbited a “red sun,” the physics governing the sky’s appearance would remain the same, but the visual experience would be dramatically altered. This hypothetical change requires examining how light interacts with air and how a cooler star’s light differs from our own.
How Earth’s Atmosphere Scatters Light
The current blue color of Earth’s sky is produced by Rayleigh scattering, which describes how electromagnetic radiation is scattered by particles much smaller than its wavelength. When sunlight enters the atmosphere, these tiny air molecules absorb the light and then re-radiate it in all directions.
This scattering is highly dependent on the wavelength of the light. Shorter wavelengths, such as violet and blue, are scattered far more intensely than longer wavelengths, such as orange and red. Blue light is scattered approximately 5.5 times more effectively than red light, causing it to diffuse across the entire dome of the sky.
The light that travels directly from the Sun appears yellow-white because much of the blue light has been removed by scattering. This preferential scattering is why the sky appears blue away from the solar disk. The red hue seen at sunrise or sunset occurs because the sunlight must travel through a much longer column of atmosphere, scattering away nearly all the blue light and leaving only the reds and oranges to reach our eyes.
Defining the Spectrum of a Red Sun
A “red sun” refers to a star with a much lower surface temperature than our Sun, such as an M-dwarf. Our G-type Sun peaks in the yellow-green spectrum at 5,800 Kelvin, while a red sun ranges from 2,300 to 4,000 Kelvin.
Wien’s displacement law dictates that a cooler stellar surface shifts the peak energy output toward longer wavelengths, concentrating light in the red and infrared regions. Consequently, a red sun emits significantly less high-energy, short-wavelength light, such as blue and violet photons. The flux ratio of red to blue light from a typical M-dwarf can be a factor of six times greater than the same ratio from our G-type Sun.
Predicting the New Sky Color
With a red sun, the light entering Earth’s atmosphere would be overwhelmingly composed of longer, less-scattered red and orange wavelengths. Because Rayleigh scattering is so inefficient for these longer wavelengths, there would be very little light available to be scattered across the sky dome.
The sky would appear much darker than it does now, likely a deep, muted shade of red or brown, depending on the star’s spectral type and orbital distance. Although the small amount of short-wavelength light emitted would still be scattered most efficiently, the overall effect would be minimal due to the low quantity of those photons.
The scattered light that did occur would likely be a yellowish-red or a deep salmon pink. This color results from the minimal blue light scattering mixed with Mie scattering, which involves larger dust particles and aerosols. The star itself would appear as a prominent, deep red or orange disk against a dark, twilight-like backdrop.