Asked by: Alex Pacynko, Bristol
The colour of the sky on an exoplanet depends on many things: the pressure, density and chemical composition of its atmosphere, the presence or absence of dust particles, vapour and clouds, the spectrum of the planet’s parent star, as well as the size, composition, colour and even biology of the planet itself.
On Earth, the sky is predominantly blue but becomes orange or red near the setting or rising Sun. On Mars, the opposite is true. These differences are mainly due to which compounds or gases are scattering and absorbing the sunlight. Scattering is the predominant factor in most atmospheres and since molecules scatter short wavelengths best and longer wavelengths the least well, this often results in blue skies. But large amounts of dust will lighten and sometimes redden sky colours. Mars’ atmosphere appears red because of the presence of iron oxide-rich dust particles. High-pressure atmospheres would be much lighter than lower pressure ones and could appear completely white or yellow.
Given the number of factors involved it isn’t unreasonable to suppose that exoplanet skies could be any colour at all – from blue or cyan, through green and yellow to red, orange and purple – even brown and white are possible.