  The occurrence of a planet transiting in front of its host star offers the
opportunity to observe the planet's atmosphere filtering starlight. The
fraction of occulted stellar flux is roughly proportional to the optically
thick area of the planet, the extent of which depends on the opacity of the
planet's gaseous envelope at the observed wavelengths. Chemical species, haze,
and clouds are now routinely detected in exoplanet atmospheres through rather
small features in transmission spectra, i.e., collections of planet-to-star
area ratios across multiple spectral bins and/or photometric bands.
Technological advances have led to a shrinking of the error bars down to a few
tens of parts per million (ppm) per spectral point for the brightest targets.
The upcoming James Webb Space Telescope (JWST) is anticipated to deliver
transmission spectra with precision down to 10 ppm. The increasing precision of
measurements requires a reassessment of the approximations hitherto adopted in
astrophysical models, including transit light curve models. Recently, it has
been shown that neglecting the planet's thermal emission can introduce
significant biases in the transit depth measured with the JWST/Mid-InfraRed
Instrument, integrated between 5 and 12 $\mu$m. In this paper, we take a step
forward by analyzing the effects of the approximation on transmission spectra
over the 0.6-12 $\mu$m wavelength range covered by various JWST instruments. We
present open source software to predict the spectral bias, showing that, if not
corrected, it may affect the inferred molecular abundances and thermal
structure of some exoplanet atmospheres.
