1. Compare transit depths rather than photograph an atmosphere

During a transit, some starlight passes through the atmosphere at the planet's day–night boundary. Absorption or scattering that removes more light at a particular wavelength makes the transit appear deeper there. Madhusudhan's specialist review distinguishes this transmission measurement from dayside emission spectroscopy. It is not a direct image of the entire atmosphere or surface.

For a uniformly bright stellar disk and a planet fully within that disk, a simple model gives δ(λ) ≈ [Rₚ,eff(λ)/R★]². R★ is stellar radius and Rₚ,eff is the effective radius blocking light at that wavelength. Real analyses also fit transit geometry and limb darkening, the variation in brightness toward the star's edge. The approximation does not imply that every actual light curve of an atmosphere-free planet is wavelength-independent.

2. A depth change is not the same numerical radius change

Suppose, illustratively, depths at two wavelengths are 1.00% and 1.02%. Their difference is 0.02 percentage points, or 200 ppm of the total stellar brightness. In the simple model, the radius ratios are √0.0100 = 0.1000 and √0.0102 ≈ 0.1010. Reading 200 ppm directly as a change in planetary radius therefore mixes quantities. These numbers illustrate units and the squared relationship; they are not measurements of a particular planet.

3. What was actually compared for WASP-39b

Rustamkulov and colleagues observed one transit on 10 July 2022 with JWST NIRSpec PRISM, obtaining a 0.5–5.5 μm transmission spectrum. They detected the CO₂ feature at 4.3 μm but no significant CH₄ absorption expected at 3.3 μm. That non-detection does not establish that methane is absent everywhere on the planet.

The paper distinguishes strong evidence for a CO₂ feature from estimates of atmospheric abundances. Evidence that a model containing a molecule fits better does not by itself provide its quantity: temperature, clouds and other constituents must be fitted together. A weaker feature at 4.05 μm was interpreted as a candidate SO₂ signal in this paper. It should not be listed with the same certainty as CO₂.

4. A useful order for reading a spectrum

First check whether the vertical axis gives transit depth or radius ratio, and which wavelengths are covered. Then separate measured points and error bars from model curves, and inspect how a model changes when the molecule of interest is omitted. Finally check assumptions about clouds, temperature and stellar light used in abundance estimation.

Madhusudhan explains that high cloud decks can mute molecular features, while abundances and clouds or hazes can produce similar spectra. A flat curve is not automatically evidence of no atmosphere, and detecting one molecule is not detecting life. Each conclusion must stay within the question the observation addressed.

5. Sources

Rustamkulov et al.: a particular JWST observation and model comparisons for WASP-39b. Madhusudhan: a specialist review of observing methods and atmospheric retrieval.