  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 lightcurve, 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 ~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 ~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 broadband spectrum on a
bright target is altered at the 1-sigma level. Using archived Spitzer
measurements, we show that the effect respectively increases the 8.0um and
24.0um transit depths by 1-sigma and 0.5-sigma per transit for HD 189733b.
Consequently, we estimate that this would be 5-10 sigma effect for near-future
JWST observations.
