From the source:

Unlike its sister planet HD ’189, the planet HD ’209 (“Osiris”) has a sunset that looks truly alien. The star is white outside the atmosphere, since its temperature is close to that of the Sun. It then acquires a bluish tinge at it sinks deeper, because the absorption by the broad wings of the neutral sodium lines (the spectral lines responsible for the gloomy orange of sodium street lighting) remove the red and orange from the star light. Deeper down, Rayleigh scattering by the molecules in the atmosphere starts scattering the blue part of the spectrum as well, so that the only frequencies that are able to squeeze past are green, then murky brown. Outside the star’s disc, the atmosphere has a faint glow in its upper parts, due to re-emission in the sodium line, then it become bluer because of the Rayleigh scattering.

Credit: Prof. Frédéric Pont