Colour of the sunset on WASP-12 b


A few weeks ago at the Heidelberg meeting, David Sing presented the spectrum of WASP-12b between 300 nm and 600 nm (near-UV to red). This is the first results of our large programme on the Hubble Space Telescope to observe the transmission spectrum of a sample of hot Jupiters at visible wavelengths.

At present the overall transmission spectrum in the visible is available for only two other planets: HD 189733 b, and HD 209458 b. In two previous posts, I used the resulting transmission spectra in the visible to reconstruct the aspect of the ‘sunset’ on these two planets (here and here). It turns out that the transmission spectrum in the visible measures exactly how the star would look to us through the planetary atmosphere –a sunset (or sunrise).

I couldn’t resist doing the same for WASP-12 b, even though the current data is still preliminary and incomplete (the portion redwards of 600 nm will be observed in a few months).

This is the result:

WASP 12 sunset

Compared to ‘189 and ‘209, the spectrum of WASP-12 b is almost flat over the visible wavelength range. As a result, the sunset has little colour. There is a greenish tinge in the image, but that is within the uncertainties about the present observations.

Maybe the most striking aspect of the sunset on WASP-12 b is not its colour, but how insanely large the parent star looks from the planet. The sun of WASP-12 b occupies a fraction of the sky five time larger than that of ‘209 – and more than three thousand times that of Earth!

Sunsets on WASP-12 b, HD 189733 b, HD 209458 b, and Earth, with the “suns” to scale.



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.