The Sunset on HD 209458 b


What does the sunset look like on HD 209458 b?

Amazingly, we know quite accurately.

This is because the colour of the sunset is exactly what is measured when collecting the transmission spectrum of the atmosphere of a transiting planet. The transmission spectrum of ‘209 was measured the STIS spectrograph on the Hubble Space Telescope. STIS covers visible wavelengths, and HD209458 is bright enough that the precision of the spectrum is sufficient for a precise translation into colours perceived by the human eye.

I have done this for ‘209, and here is the result:

Sunset on HD209458b (reconstructed from the HST/STIS transmission spectrum)

Compare with the sunset on HD 189733 b.

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.

One key difference with a sunset on Earth is that the “sun” is much larger from ‘209, because the planet is very close. As a result, there is no perspective from which the star would fit in only one layer of the atmosphere, as it does in an Earthly sunset. Instead of changing colour as it moves near the horizon, the host star spans all colours at once. In this plot the geometry is as would be observed from a space station orbiting about 10 000 km above the planet.


This post was inspired by the depiction of the sunset on “Osiris” by Alain Lecavelier des Etangs. See here.


Of course, as on Earth, the aspect of the sunset could vary from day to day and place to place. What we get from transmission spectroscopy is an average over the whole planet limb. Also, if the stuff responsible for the Rayleigh scattering is not uniformly distributed in height, the aspect could be slightly different – with some banding like a sunset with faint cirrus clouds near the horizon. On a more technical note, there are some aspects of the image that could be improved. I have tried to be precise in the transformation of the spectrum into colours perceived by the human eye, then in the conversion to computer-screen RGB colours. The diffraction effect (flattening near the horizon) is approximate, modeled on the flattening on the Sun from Earth. To estimate the luminosity and colour of the sky around the sunset, I used theoretical values for the proportion of scattered and absorbed light for hydrogen scattering and sodium abosorption.



About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.