WASP-80b is one of the most important planets found by WASP-South. It is hot Jupiter, in a 3-day orbit, but its host star is unusually small and dim, being an K7/M1 star with a mass of only 0.59 that of the Sun. Only one other such system is known, namely Kepler-45, but that is faint at V = 16.9 and so hard to study. WASP-80 is much brighter, at V = 11.9.

A new paper by Mancini etal reports observing a transit of WASP-80b in four colours simultaneously, using the GROND instrument on the 2.2-m/ESO telescope at La Silla.

The transit depth is distinctly different in the different colours. Why might this be? The planet, of course, does not have a hard edge, it has an atmosphere, and a gaseous atmosphere can be more transparent to some colours of light and less transparent to others. Thus the planet will appear to be a slightly different size in different colours, and hence the transit can have a slightly different depth.

This is not easy to interpret, however, since the host star is also gaseous, with no sharp edge. Owing to limb-darkening it also appears to be a different size in different colours, which can complicate the interpretation. Studies such as this are in their infancy, but should allow us to prove the temperatures and compositions of atmospheres of exoplanets. This is one of the main aims of the forthcoming JWST mission and proposed exoplanet missions such as ESA’s EChO.