Stellar contamination of a transmission spectrum

Stellar contamination of a transmission spectrum arises when the spectrum behind a transiting planet differs from the spectrum averaged over the whole stellar disc. For a small opaque planet, neglecting limb darkening, the approximate depth isHere $I_{\lambda }$ is specific intensity. Unocculted cool starspots change the denominator in a wavelength-dependent way, creating apparent planetary features even if $R_p$ is constant. Crossing a starspot changes the numerator and can produce a visible anomaly in the light curve.^[ref]

Repeated exoplanet transits, monitoring of the host star, and joint stellar and planetary modelling help test this effect. Small rotational variability alone does not establish that the stellar surface is uniform.