The Sunset on HD 189733 b


What does the sunset look like on HD 189733 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. We have measured the transmission spectrum of ‘189 with the STIS spectrograph on the Hubble Space Telescope. STIS covers visible wavelengths, and HD 189733 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 ‘189, and here is the result:

Sunset on HD 189733 b (reconstructed from the HST/STIS transmission spectrum)


Compare with the sunset of HD 209458 b.

Basically, the sunset looks like a glorious sunset on Earth, on a very clear day with some dust in the air. This is because in both cases, Rayleigh scattering is the dominant mechanism. On Earth the scattering is caused by molecules and air-borne dust in the air. On ‘189 the Rayleigh scattering is thought to be caused by silicate dust. One key difference with a sunset on Earth is that the “sun” appears much larger from ‘189, 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 of ‘189 spans all colours at once, from its original orange (HD 189733 is a star much cooler than the Sun, it is orange rather than white) to very dark red in the thick layers of the lower atmosphere. In this plot the geometry is as would be observed from a space station orbiting about 10 000 km above ‘189. The star is 25 times larger in the sky than the Sun from Earth.

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, modelled on the flattening on the Sun from Earth. To estimate the colour and luminosity of the sky I have assumed that the haze in the atmosphere of ‘189 is made up of highly reflective dust grains.




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.