Cloud signatures in transit spectra of exoplanet atmospheres

We present an analytical model for the transmission spectrum of a transiting exoplanet, extending previous models to incorporate condensation cloud base and top. We show that a cloud base (position where the cloud evaporates) can produce an observable inflection point in the spectrum. The wavelength and magnitude of the inflection can be used to break the degeneracy between the atmospheric pressure and the abundance of the cloud- forming material (for a given cloud particle size); moreover, we can derive the abundance of the cloud constituent by measuring the magnitude of the inflection drop. An observed inflection also provides a specific point on the atmospheric P-T profile, giving us a thermometer to directly validate or rule out postulated cloud species. We have applied the model to the transit spectrum of HD189733b, where we identify a possible inflection and examine its implications about the T-P profile of the atmosphere and the abundance of the candidate cloud material.