Reading atmospheres

15 chapters, each an animated explainer with its physics and sources. Every drawing says whether it is an illustration, a model from stored values, or real data, and by how much it is exaggerated. Only the transit chapter is something SetuHQ itself does.

Chapter 11 of 15 · explainer: SetuHQ does not do this

Reading atmospheres

During a transit, a sliver of starlight passes through the planet's atmosphere at its edge. Where molecules absorb, the atmosphere is opaque higher up, so the planet looks slightly bigger at those wavelengths. SetuHQ does not do spectroscopy: this explains how JWST-class telescopes read atmospheres.

Illustration
A planet's limb with its thin atmosphere drawn ten times too thick, starlight passing through it. Two spectrum bars from 0.5 to 5.1 micrometres in false colour: starlight only, and during transit, which gains dark absorption bands at the real positions of sodium, potassium, water, methane, carbon dioxide and carbon monoxide as the planet crosses.

One scale height H = k_B T/(μ m_H g) of atmosphere changes the transit depth by about 2 H Rp/R★². Strong features span a few scale heights. Hot, puffy, hydrogen-rich planets around small stars give the biggest signals (Seager & Sasselov 2000; Winn 2010).

QuantityValueFrom
WASP-39 b scale height (μ = 2.3)930 kmexoplanet.eu Mp = 0.28 MJ, Rp = 1.27 RJ; Teq 1116 K (Faedi et al. 2011) [calc]
WASP-39 b: depth change per scale height436 ppmR★ = 0.895 R☉ [calc]
WASP-39 b: CO₂ at 4.3 µm26σJWST ERS Team 2023
pi Men c: per scale height, H₂-rich17 ppmTeq 1147 K (exoplanet.eu) [calc]
pi Men c: per scale height, steam (μ = 18)2.1 ppm[calc]
pi Men c: SetuHQ's stored transit depth241 ppmsetuhq run (real data)

The bars are schematic: band positions are real (NIST CCCBDB fundamentals and Schwieterman et al. 2018), shapes and strengths are not. False colour: infrared is invisible to the eye. Which vibration makes each band ›

A real spectrum: WASP-39 b with JWST

Real data, cited
JWST NIRSpec PRISM transmission spectrum of WASP-39 b: 36 binned points with error bars and the paper's best-fit model; the deepest point is at 4.37 micrometres, the carbon dioxide band.

Real data, cited. Carter et al. 2024, Nature Astronomy 8, 1008, doi:10.1038/s41550-024-02292-x; data: Products and Models for "A benchmark JWST near-infrared spectrum for the exoplanet WASP-39 b", Zenodo record 10161743, licence CC-BY-4.0. File ZENODO/4_TRANSMISSION_SPECTRA/Fixed_LimbDarkening/NIRSpec_PRISM/bins_scale5.csv, fetched 2026-10-19; Depth = (Rp/R★)²; error = 2·(Rp/R★)·σ, the larger of the published lower and upper errors. The line is the paper's best-fit model, not SetuHQ's.

Atmosphere Lab

Switch molecules on and off and change the temperature, the mean molecular weight and how strong the bands are. The lab computes the scale height and the depth per scale height from WASP-39 b's stored mass and radii, then adds each molecule's bands. It is a toy model, not a radiative-transfer calculation: band centres are real, band widths are chosen for display, and strengths are in scale heights set by the slider.

Model from stored values
The Atmosphere Lab: a model transmission spectrum from 0.5 to 5.4 micrometres for the molecules switched on, at the chosen temperature, mean molecular weight and band strength, with WASP-39 b's JWST points overlaid when loaded.
Planets in this chapterWASP-39 b ›HD 209458 b ›HD 189733 Ab ›K2-18 b ›
Sources

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