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Transit spectroscopy is a general term for the use of an extra-solar planet's transit to carry out transmission spectroscopy on the planet's atmosphere. It is generally differential spectroscopy, using the differences between the spectrum during versus outside the transit to isolate the characteristics due to the planet and its atmosphere. The planet's atmosphere transmits some wavelengths and absorbs others, the reductions depending upon the constitution of the atmosphere, so comparisons in the spectral energy distribution (SED) during transits versus otherwise yields information of interest.
Transit spectroscopy of host stars' Lyman alpha emission lines have been used to detect and study hydrogen exospheres of exoplanets and their atmospheric escape.
Occultation spectroscopy is a similar term for the occultation (i.e., secondary eclipse) of a planet and analysis of the difference as well as the light curves at different wavelengths at the beginning and end of the eclipse reveal information about EMR emitted (the planet emission) and reflected (due to albedo) by the planet.