Nightside pollution of exoplanet transit depths

Out of the known transiting extrasolar planets, the majority are gas giants orbiting their host star at close proximity. Both theoretical and observational studies support the hypothesis that such bodies emit significant amounts of flux relative to the host star, increasing towards infrared wavelengths. For the dayside of the exoplanet, this phenomenon typically permits detectable secondary eclipses at such wavelengths, which may be used to infer atmospheric composition. In this paper, we explore the effects of emission from the nightside of the exoplanet on the primary transit light curve, which is essentially a self-blend. Allowing for nightside emission, an exoplanet's transit depth is no longer exclusively a function of the ratio-of-radii. The nightside of an exoplanet is emitting flux, and the contrast to the star's emission is of the order of similar to 10-3 for hot Jupiters. Consequently, we show that the transit depth in the mid-infrared will be attenuated due to flux contribution from the nightside emission by similar to 10-4. We show how this effect can be compensated for in the case where exoplanet phase curves have been measured, in particular for HD 189733b. For other systems, it may be possible to make a first-order correction by using temperature estimates of the planet. Unless the effect is accounted for, transmission spectra will also be polluted by nightside emission, and we estimate that a Spitzer broad-band spectrum on a bright target is altered at the 1 Sigma level. Using archived Spitzer measurements, we show that the effect respectively increases the 8.0-mu m and 24.0-mu m transit depths by 1 Sigma and 0.5 Sigma per transit for HD 189733b. Consequently, we estimate that this would be similar to 5-10 Sigma effect for near future James Webb Space Telescope observations.