Dusty tails of evaporating exoplanets. I. Constraints on the dust composition

Context. Recently, two exoplanet candidates have been discovered, KIC 12557548b and KOI-2700b, whose transit profiles show evidence of a comet-like tail of dust trailing the planet, thought to be fed by the evaporation of the planet’s surface.
Aims. We aim to put constraints on the composition of the dust ejected by these objects from the shape of their transit light curves.
Methods. We derive a semi-analytical expression for the attenuation of the dust cross-section in the tail, incorporating the sublimation of dust grains as well as their drift away from the planet. This expression shows that the length of the tail is highly sensitive to the sublimation properties of the dust material. We compute tail lengths for several possible dust compositions, and compare these to observational estimates of the tail lengths of KIC 12557548b and KOI-2700b, inferred from their light curves.

Results. The observed tail lengths are consistent with dust grains composed of corundum (Al2O3) or iron-rich silicate minerals (e.g., fayalite, Fe2SiO4). Pure iron and carbonaceous compositions are not favoured. In addition, we estimate dust mass loss rates of 1.7 ± 0.5 M⊕ Gyr-1 for KIC 12557548b, and > 0.007 M⊕ Gyr-1 (1σ lower limit) for KOI-2700b.