If a hot Jupiter has a magnetic field of a few Gauss it would be surrounded by a magnetosphere that would carve out a hole in the stellar wind of the host star. Since the planet orbits rapidly, this would lead to a “bow shock” where the magnetosphere ploughs through the stellar wind.

In a new paper, Richard Alexander, of the University of Leicester, and co-authors, report computer simulations of this effect for several hot Jupiters, including WASP-12b and WASP-18b.

In the colour-coded figure (see scale on the right) the blue and red show the density of the stellar wind. A low-density (black) magnetosphere surrounds each planet (white dots).

Since these planets orbit edge on to us, the bow shock would absorb ultra-violet light from the star, and so produce a characteristic light-curve with a broad dip preceding the transit.

This magnetospheric bow-shock is a possible alternative explanation for the UV absorption observed in WASP-12, which has previously been attributed to material being lost from the planet owing to Roche-lobe overflow. Alexander et al suggest that WASP-18 is a critical test of these models, since the much higher gravity of the massive planet WASP-18b means that there should not be any Roche-lobe overflow.