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Exoplanet characterization combines transit photometry (radius), radial-velocity (mass), direct imaging (young massive planets), and spectroscopy to determine atmospheric composition, cloud properties, temperature, and surface gravity. Mass and radius determine planet type (terrestrial, super-Earth, sub-Neptune, gas giant) and infer internal structure.
From exoplanet detection methods, you know how we find planets around other stars — transit photometry measures the dip in starlight as a planet crosses the star's face, radial velocity measures the star's wobble from the planet's gravitational tug, and direct imaging captures light from the planet itself. Characterization is the next step: once you know a planet exists, what can you actually learn about it? The answer, remarkably, is quite a lot — and spectroscopy is the tool that makes it possible.
The most fundamental characterization comes from combining mass (from radial velocity) and radius (from transit depth) to calculate bulk density. This single number immediately tells you what kind of planet you are looking at. A density near 5.5 g/cm³ (like Earth) indicates a rocky, iron-core world. A density below 2 g/cm³ suggests a thick gaseous or volatile-rich envelope — the planet is a sub-Neptune or gas giant. Densities between these extremes might indicate a water world or a rocky core with a modest atmosphere. This mass-radius relationship creates a classification scheme: terrestrial planets (Earth-like rock and metal), super-Earths (larger rocky worlds up to ~1.6 Earth radii), sub-Neptunes (with substantial hydrogen-helium or water envelopes), and gas giants (dominated by hydrogen and helium, like Jupiter and Saturn). The boundary between super-Earths and sub-Neptunes — the so-called radius valley near 1.5–2 Earth radii — is one of the most important discoveries in exoplanet science, suggesting that atmospheric escape sculpts the planet population.
Spectroscopy transforms characterization from bulk properties to atmospheric chemistry. During a transit, starlight filters through the planet's atmosphere, and different molecules absorb at characteristic wavelengths — water vapor at 1.4 and 2.7 μm, CO₂ at 4.3 μm, methane at 3.3 μm, sodium and potassium at visible wavelengths. By comparing the transit depth at many wavelengths, astronomers construct a transmission spectrum that reveals which molecules are present. The James Webb Space Telescope has made this routine for giant planets and is beginning to probe smaller worlds. For hot Jupiters, JWST has detected water, CO₂, SO₂, and even silicate clouds. Emission spectroscopy — measuring the planet's own thermal radiation by observing the brightness drop when the planet passes behind the star — provides complementary information about temperature structure and heat redistribution.
The ultimate goal is characterizing potentially habitable rocky planets: measuring their surface temperature, detecting water vapor, and searching for atmospheric biosignatures. This remains at the frontier of current capabilities. Small rocky planets have thin atmospheres that produce tiny spectral signals — parts per million of the total starlight — demanding extraordinary instrumental precision. Clouds and hazes can mute spectral features, and degeneracies between atmospheric composition and cloud coverage make interpretation ambiguous. Nevertheless, the pathway from detection to characterization to habitability assessment is now well established, and each generation of telescopes pushes the boundary toward smaller, cooler, more Earth-like worlds.