A planet casts a slightly different shadow in each color
Picture a planet crossing the face of a distant star. Its solid-looking body hides some starlight. Around its edge, a much thinner ring of light passes through gas before reaching us. A wavelength that the gas absorbs strongly loses more light, so the planet appears a little larger at that wavelength.
The telescope collects the light of the unresolved star and planet together. It measures how much the star dims during transit, the passage of the planet across the stellar disk. Repeating that measurement at many wavelengths gives a transmission spectrum. The name describes the light’s journey, even though the plotted quantity is usually the amount blocked.
That choice of vertical axis explains a common surprise: an absorption band points upward. Its height measures the extra starlight removed by the gas.
Reading: Feinstein et al. (2023) · Early Release Science of WASP-39 b with JWST NIRISS · Heng and Kitzmann (2017) · Transmission spectra and the radius–pressure degeneracy
A molecule has a family of absorption bands
Water molecules can rotate and vibrate in many related ways. Different changes absorb different photon energies. At modest spectral resolution, many individual absorption lines blend into broad bands. Together their locations form a recognizable pattern.
JWST began an 8.2-hour observation of WASP-39 b on 26 July 2022. The NIRISS instrument followed a transit across a broad near-infrared range. In the published spectrum, water bands near 0.9, 1.15, 1.4 and 1.8 micrometres appear together. Several independent reductions recovered the broad structure.
This is the advantage of seeing a family. A proposed molecule must account for several locations and their relative strengths at once. If a model explains one high point but predicts another strong band where the spectrum stays low, the rest of the spectrum has supplied a useful test.
Reading: Feinstein et al. (2023) · Early Release Science of WASP-39 b with JWST NIRISS · WASP-39 b NIRISS author data · Zenodo 7709712 and fixed repository commit
Clouds change how much of the atmosphere we can see
A grazing ray takes a long route through the edge of an atmosphere. Even thin gas can absorb appreciably along that route. Warm gas also extends farther upward than otherwise similar cold gas, because thermal motion supports a larger vertical pressure scale.
Now raise an opaque cloud. It cuts off the deepest rays first. The relatively transparent wavelengths begin to see the cloud itself, while strong molecular bands may already be absorbing higher above it. The low parts of the transit spectrum rise and the contrast between peaks and windows shrinks.
A flatter spectrum can therefore accompany water that is still present. The band contrast depends on the available path through the gas. Temperature, molecular content and the chosen reference size of the planet all enter that comparison.
The required precision is small on an everyday scale. If an apparent radius grows by one part in a thousand, the blocked area grows by about two parts in a thousand. For a transit that originally hides two percent of the star, that is roughly forty parts per million of the star’s total light.
Reading: Heng and Kitzmann (2017) · Transmission spectra and the radius–pressure degeneracy · Feinstein et al. (2023) · Early Release Science of WASP-39 b with JWST NIRISS
The star is part of the measurement
The planet blocks a strip of stellar surface, but the comparison light comes from the whole visible star. Those two light sources can have different spectra. A cool region outside the planet’s path contributes less light at some wavelengths without being crossed by the planet.
Dividing the missing light by this dimmer whole-star reference makes the transit look deeper. If the cool region has its own molecular absorption, some spectral structure can enter the measured transit spectrum through the star. This is called the transit light source effect.
Its importance depends on the star, the temperature contrast and the projected area of the cool regions. Rackham and colleagues found that molecular-band contamination is generally weak for ordinary activity levels in F-, G- and K-type stars. Cooler or unusually active stars require closer attention.
Reading: Rackham, Apai and Giampapa (2019) · The Transit Light Source Effect II · Husser et al. (2013) · A PHOENIX library of stellar atmospheres and spectra
Read the pattern before choosing an atmosphere
Start with the wavelength pattern. Then compare the strong bands with the weaker bands and the spaces between them. Ask whether changing cloud height can flatten some features while preserving others. Finally, check how a different stellar surface changes the same wavelength range.
The WASP-39 b study used a much broader collection of atmospheric models than a single water-and-gray-cloud calculation. Its cloud treatment and composition were tested against the full spectrum. A small teaching model is useful for tracing a ray into an apparent radius; a physical interpretation of a real planet also needs the assumptions behind each omitted opacity and fitted stellar parameter.
Several bands make the distant gas recognizable. Their changing contrast then becomes a way to ask where the gas ends, where clouds interrupt the view and how the star colors the light that arrives.
Reading: Feinstein et al. (2023) · Early Release Science of WASP-39 b with JWST NIRISS · Rackham, Apai and Giampapa (2019) · The Transit Light Source Effect II