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An exoplanet transmission spectrum is the measured exoplanet transit depth as a function of wavelength. Light from the star passing through the limb of an exoplanet atmosphere is absorbed or scattered differently in different spectral bands. At a strongly absorbing wavelength, a ray becomes opaque farther from the centre, making the effective planetary radius larger.

The annulus model for transmission spectroscopy estimates the extra blocked area, while the slant optical depth of an isothermal atmosphere explains why this method can detect tenuous gas. Molecular-band amplitudes depend on the atmospheric scale height, abundances, and exoplanet cloud deck. Stellar contamination of a transmission spectrum and reference-pressure degeneracy in transmission spectroscopy limit how directly this observable can be translated into a unique composition.

Atmospheric haze consists of small suspended particles that scatter or absorb radiation, often formed through atmospheric photochemistry rather than direct condensation of a locally abundant vapour. In an exoplanet atmosphere, transport, growth and settling determine the particles' altitude and size distribution.

A haze can add a wavelength-dependent continuum to an exoplanet transmission spectrum and conceal molecular bands. The scattering slope of a transmission spectrum connects an idealized extinction law with the atmospheric scale height, but a measured slope alone does not establish a particular haze composition. Stellar contamination of a transmission spectrum can also alter the slope. Distinguishing these explanations requires spectral coverage and a joint model of the star and planet.

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.