Every eight hours, the Jupiter-sized world WASP-39b passes between its star and Earth. During those brief transits, a fraction of starlight filters through the planet's atmosphere before reaching our telescopes. That filtered light carries molecular fingerprints—absorption signatures encoding the chemical composition of an atmosphere 700 light-years away. This is transit spectroscopy, and it has transformed exoplanet science from a census-taking exercise into genuine atmospheric characterization.

The technique exploits a fundamental property of light-matter interaction: different molecules absorb different wavelengths. When starlight passes through a planetary atmosphere, water vapor absorbs at specific infrared wavelengths, sodium at its characteristic yellow doublet, methane at distinct carbon-hydrogen stretching frequencies. By comparing the star's spectrum during and outside transit, we extract these absorption features—effectively reading the chemical recipe of alien air.

JWST has elevated this technique from promising to revolutionary. Its infrared sensitivity and spectral resolution have revealed atmospheric compositions with unprecedented detail, detecting carbon dioxide, sulfur dioxide, and water in exoplanet atmospheres for the first time. But interpreting these spectra requires understanding the complex physics connecting observed absorption depths to actual atmospheric properties. The translation from spectrum to atmospheric structure involves radiative transfer, atmospheric scale height, and the ever-present complication of clouds and hazes that can obscure the very features we seek to detect.

The geometry of transit spectroscopy determines what we can learn. As a planet crosses its star, a thin atmospheric annulus—the terminator region—intercepts starlight. The apparent size of this annulus varies with wavelength because atmospheric opacity varies with wavelength. At wavelengths where molecular absorption is strong, the atmosphere becomes opaque at higher altitudes, making the planet appear slightly larger. At wavelengths where the atmosphere is relatively transparent, starlight penetrates deeper before being absorbed, and the planet appears slightly smaller.

This wavelength-dependent transit depth produces the transmission spectrum. Absorption features appear as increases in apparent planetary radius at specific wavelengths. The amplitude of these features depends on the atmospheric scale height—the altitude over which atmospheric pressure decreases by a factor of e. Scale height itself encodes atmospheric temperature and mean molecular weight: hotter, lighter atmospheres have larger scale heights and produce stronger spectral features.

The relationship is quantifiable. For a hydrogen-dominated hot Jupiter atmosphere at 1000 K, the scale height approaches 500 kilometers. Each scale height of absorption at a particular wavelength adds roughly 2H to the apparent planetary radius, where H is the scale height. For a Jupiter-sized planet around a Sun-sized star, this translates to transit depth variations of tens to hundreds of parts per million—detectable with space-based facilities like JWST and Hubble.

Extracting actual molecular abundances from these features requires sophisticated atmospheric retrieval models. These models simulate radiative transfer through parameterized atmospheric structures, varying temperature profiles, chemical abundances, and cloud properties to find combinations that reproduce observed spectra. The approach is fundamentally inverse: we observe the integrated effect of atmospheric opacity along the line of sight and work backward to infer the atmospheric properties responsible.

Critical to interpretation is understanding what altitude range transmission spectroscopy actually probes. The technique is most sensitive to pressures between roughly 1 millibar and 1 microbar—the upper atmosphere where the atmosphere transitions from optically thick to optically thin. This is far above the pressure levels where most atmospheric mass resides, making transmission spectroscopy a probe of atmospheric composition at altitude rather than bulk composition.

Takeaway

Transmission spectroscopy measures how a planet's apparent size varies with wavelength—molecular absorption at specific wavelengths makes the atmosphere opaque at higher altitudes, encoding composition and temperature structure in wavelength-dependent transit depths.

Hot Jupiters—gas giants orbiting their stars in days rather than years—were the first exoplanets amenable to atmospheric characterization. Their extended, hydrogen-dominated atmospheres and high temperatures produce strong spectral features. WASP-39b, HD 209458b, and HD 189733b have become atmospheric laboratories, their repeated transits building up signal-to-noise ratios sufficient for detailed chemical analysis.

Water vapor dominates hot Jupiter transmission spectra. The molecule's strong infrared absorption bands at 1.4, 1.8, and 2.7 micrometers produce unmistakable features in JWST's near-infrared coverage. Water abundance provides constraints on atmospheric metallicity—the enhancement of heavy elements relative to hydrogen compared to stellar values. Jupiter's atmosphere shows roughly 3× solar metallicity; hot Jupiter water abundances ranging from sub-solar to highly super-solar suggest diverse formation histories.

Alkali metals sodium and potassium produce absorption features at optical wavelengths, their broad pressure-broadened wings extending across hundreds of nanometers. The detection of sodium in HD 209458b's atmosphere in 2002 marked the first exoplanet atmospheric detection. These alkali features are sensitive to temperature structure and cloud coverage—sharp, deep features indicate clear atmospheres, while muted features suggest high-altitude aerosol layers.

More exotic detections reveal high-temperature chemistry. Titanium oxide and vanadium oxide, detected in ultrahot Jupiters like WASP-121b, act as strong optical absorbers that can drive thermal inversions—temperature profiles that increase with altitude in the upper atmosphere. These inversions fundamentally alter atmospheric dynamics and chemistry, creating day-night temperature contrasts exceeding 1000 K on the most irradiated worlds.

Carbon dioxide and sulfur dioxide detections by JWST have opened new windows into atmospheric chemistry. Sulfur dioxide in particular appears to be a photochemical product—created when stellar ultraviolet radiation dissociates water and hydrogen sulfide, allowing sulfur and oxygen to combine. Its presence indicates active atmospheric photochemistry and provides constraints on the ultraviolet radiation environment these planets experience.

Takeaway

Hot Jupiter atmospheric compositions record planetary formation and migration history: water abundances constrain metallicity and formation location, alkali metals probe temperature structure, and photochemical products like sulfur dioxide reveal ongoing atmospheric processing by stellar radiation.

The clearest hot Jupiter transmission spectra show sharply defined absorption features with depths matching cloud-free atmospheric models. But many observed spectra are disappointingly muted—features shallower than expected or entirely absent. The culprit is nearly always aerosols: clouds of condensed particles or photochemical hazes that add wavelength-independent opacity, obscuring the molecular absorption features that encode atmospheric composition.

The physics is straightforward. If cloud or haze particles reside above the altitudes where molecular absorption occurs, they block starlight before it can interact with absorbing molecules. The atmosphere appears as a featureless gray absorber regardless of its underlying composition. GJ 1214b, a sub-Neptune that should show strong water features if hydrogen-dominated, instead displays an essentially flat transmission spectrum consistent with high-altitude hazes—frustrating attempts to determine whether its atmosphere is hydrogen-rich or water-dominated.

Cloud composition varies with atmospheric temperature. Hot Jupiters may host silicate clouds—condensed magnesium silicates and iron droplets forming at temperatures around 1500-2000 K. Cooler planets may develop sulfide clouds or water clouds at appropriate pressure-temperature conditions. Modeling predicts these cloud species, but direct compositional identification remains challenging because condensate particles produce broad, featureless scattering signatures.

Several strategies are emerging to overcome aerosol interference. High-resolution spectroscopy can detect narrow molecular absorption lines that remain visible even when broadband features are muted. Observations at longer infrared wavelengths probe deeper into atmospheres because scattering efficiency decreases at longer wavelengths. Phase curve observations—measuring planetary emission throughout the orbit—access the hotter, potentially clearer dayside atmosphere where thermal emission spectroscopy probes different altitude ranges.

JWST's mid-infrared capabilities have proven particularly valuable. The instrument's coverage beyond 5 micrometers reaches wavelengths where many cloud species become less opaque, revealing spectral features hidden at shorter wavelengths. The CO2 detection in WASP-39b's atmosphere came from JWST's mid-infrared observations, demonstrating the value of wavelength coverage extending beyond the optical and near-infrared windows where clouds most strongly affect transmission spectra.

Takeaway

Aerosols—condensate clouds and photochemical hazes—can obscure the molecular absorption features that encode atmospheric composition. Overcoming this interference requires multi-wavelength observations, high spectral resolution, and emission spectroscopy techniques that probe different atmospheric regions.

Transit spectroscopy has transformed exoplanets from points of light into characterized worlds with known atmospheric compositions. The technique's power lies in its simplicity: molecules absorb at specific wavelengths, producing features in transmission spectra that encode atmospheric chemistry and structure. JWST's observations have realized the technique's potential, detecting species from water and carbon dioxide to photochemical products like sulfur dioxide.

Yet interpretation remains complex. Clouds and hazes obscure features, retrieval models face degeneracies between composition and temperature structure, and transmission spectra probe limited altitude ranges. Hot Jupiters, with their extended atmospheres and favorable observation geometries, have been the proving ground for these techniques, but the ultimate targets are smaller, cooler worlds—potentially habitable planets where atmospheric composition might indicate biological activity.

The chemical recipes of alien atmospheres reveal planetary formation histories, ongoing atmospheric processes, and the radiation environments planets experience. Each detection adds to our understanding of how planetary atmospheres form and evolve across the galaxy's extraordinary diversity of worlds.