Characterization Methods

The statistical machinery that turns a spectrum into numbers: models of an atmosphere are compared against the data, usually with Bayesian inference, to find the mix of gases, temperatures, and clouds that best explains it — along with honest uncertainties. Retrieval is how a wiggly spectrum becomes a measured abundance.

Measuring the light a planet itself emits — usually its thermal glow — to probe the temperature structure and composition of its dayside. Because the planet's emission is measured against the star, it is easiest for hot planets, and it reveals whether temperature rises or falls with altitude in ways transmission spectra cannot.

A ground-based technique that resolves thousands of individual spectral lines and uses the planet's changing Doppler shift along its orbit to separate its light from the star's and the Earth's atmosphere. Cross-correlating against molecular templates detects specific gases and even measures atmospheric winds; it is a headline science case for the coming extremely large telescopes.

Tracking a planet's brightness through its whole orbit as its illuminated fraction changes, like the phases of the Moon. The shape of the curve reveals the temperature difference between day and night sides and how efficiently winds carry heat around the planet — a direct window onto atmospheric circulation.

A spectroscopic signature seen during transit: as the planet crosses the face of its rotating star, it blocks first the approaching then the receding limb, distorting the star's measured velocity. The shape of that distortion reveals whether the planet's orbit is aligned with the star's spin — a key clue to how the system formed and migrated.

The moment a planet passes behind its star. The dip in the system's combined light equals the light the planet was contributing, so subtracting the two isolates the planet's thermal emission and reflected light — the basis of emission spectroscopy and dayside temperature measurements.

The workhorse of exoplanet atmospheres: as a planet transits its star, a sliver of starlight filters through the ring of atmosphere at its edge, and the gases there imprint absorption features that vary with wavelength. Comparing the planet's apparent size across a spectrum reveals which molecules — water, carbon dioxide, methane — are present. JWST has made this a routine, high-precision technique.