= Solution
\b[Transmission spectroscopy, eclipse/emission spectroscopy and direct-imaging spectroscopy] probe atmospheres in complementary geometries.
A <exoplanet transmission spectrum> uses the wavelength dependence of primary-transit depth to measure slant-path absorption at the limb. Its long path length gives sensitivity to trace absorbers, but only transiting systems qualify; <exoplanet clouds>, stellar heterogeneity and the reference-radius/abundance degeneracy complicate interpretation, and the result samples the terminator rather than the dayside.
An <exoplanet secondary eclipse> isolates the dayside thermal/reflected spectrum by removing the planet behind the star. It constrains <brightness temperatures>, molecules and <thermal inversions>, but the planet/star contrast is small, a suitable orbital geometry is needed, and temperature-abundance degeneracies remain. A <exoplanet thermal phase curve> extends this method to longitudinal information.
<Exoplanet direct imaging> spatially separates a planet from its star and permits repeated spectroscopy without requiring a transit. It is effective for young, luminous, widely separated giant planets, but demands high contrast and angular separation; old small planets close to their stars are difficult. Inferred gravities, radii and masses can depend strongly on atmosphere and evolutionary models.
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