Determining the interior composition of small exoplanets is challenging but rewarding. I will introduce a new class of exoplanets, recently discovered with Kepler, that may provide a novel approach to this puzzle. These planets orbit so close to their host star that they are heated up to the point of evaporation. They spew out large amounts of dust grains, which end up in a comet-like tail trailing the planet. The shape of this dust tail depends on the material properties of the dust, and when it transits the host star, it produces a peculiar light curve. I will show how the detailed shape of this transit light curve can be used to probe the composition of the dust, and hence that of the evaporating exoplanet.