A transiting planet does more than dim its star - the starlight that grazes its atmosphere on the way to a telescope gets colored by whatever gases are up there. Toggle a molecule below and scrub the transit to see when that signal actually appears.
Published figures: WASP-39b, a highly inflated "hot Saturn" about 700 light-years away, 4.055-day orbit; JWST's 2022 Early Release Science observations gave the first definitive detection of carbon dioxide in an exoplanet atmosphere, plus water vapor and sulfur dioxide (Nature 2023 papers, NASA JWST press materials).
Drag to orbit and scroll or pinch to zoom. Toggle water, carbon dioxide, sulfur dioxide or methane, scrub the transit slider, or jump straight to mid-transit.
Exoplanet Atmosphere Spectroscopy 3D Explorer
This browser explorer shows how astronomers read an exoplanet's atmosphere without ever landing on it: toggle water, carbon dioxide, sulfur dioxide or methane and scrub the transit slider - the spectrum panel only shows absorption bumps while the planet is actually crossing in front of its star, because that is the only moment starlight passes through the atmosphere on its way to a telescope.
The technique is called transmission spectroscopy. Every molecule absorbs light at its own characteristic wavelengths, so the size of the dip at each wavelength reveals which gases are present and roughly how much of each. In 2022, JWST's Early Release Science program pointed four instruments at WASP-39b, a highly inflated "hot Saturn" about 700 light-years away that completes an orbit every 4.055 days, and produced the first definitive detection of carbon dioxide in any exoplanet atmosphere.
- Toggle water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2) or methane (CH4) to see each molecule's real absorption bands
- Scrub the transit slider or press Play orbit to watch the signal appear only during transit
- Jump straight to mid-transit, when the atmosphere's limb is most fully sampled
- Read the live facts panel for each molecule's detection status on WASP-39b
- Drag to orbit the 3D view, scroll or pinch to zoom
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Figure | Published value |
|---|---|
| Target studied | WASP-39b, a "hot Saturn" about 700 light-years away |
| Orbital period | 4.055 days (near-circular orbit) |
| Approximate transit depth | about 2.1% (planet-to-star radius ratio squared) |
| Carbon dioxide (CO2) band | about 4.3 micrometers - first definitive exoplanet detection, JWST 2022 |
| Water (H2O) bands | about 1.4, 1.9 and 2.7 micrometers - detected |
| Sulfur dioxide (SO2) band | about 4.05 micrometers - detected; first evidence of exoplanet atmospheric photochemistry |
| Methane (CH4) band | about 3.3 micrometers - not strongly detected on WASP-39b |
| JWST wavelength coverage used | NIRISS about 0.6-2.8 um; NIRSpec about 0.6-5.3 um |
The methane result is as informative as the detections: its near-absence, alongside the sulfur dioxide signal, told researchers the atmosphere is not in simple chemical equilibrium - sunlight is actively driving chemistry high in the clouds. For the light-curve side of a transit - detecting a planet at all and sizing it from the dip alone - see the Exoplanet Transit 3D Explorer.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation - the spectrum panel is a smooth model calibrated to the real published band positions and WASP-39b detections, not raw JWST instrument data or a live data-reduction pipeline.
Frequently Asked Questions
What is transmission spectroscopy?
It is the technique of measuring how much a planet dims its star at many different wavelengths during a transit. Each gas in the planet's atmosphere absorbs specific wavelengths, so the pattern of dips across the spectrum reveals which molecules are present.
Why does the spectrum only show bumps during transit?
Because that is the only time starlight actually grazes the planet's atmosphere on its way to a telescope. Outside transit the star's light reaches the telescope directly, with no atmospheric signal to measure.
What did JWST actually find on WASP-39b?
In 2022, JWST's Early Release Science program used four instruments to observe WASP-39b and produced the first definitive detection of carbon dioxide in any exoplanet atmosphere, plus water vapor and sulfur dioxide. The sulfur dioxide was the first evidence of sunlight-driven atmospheric chemistry found on an exoplanet.
Why does methane matter if it was not detected?
A weak or missing methane signal is itself a clue. Alongside the sulfur dioxide detection, the lack of a strong methane feature told researchers the atmosphere is not in simple chemical equilibrium - ongoing photochemistry is reshaping its composition.
How is this different from the Exoplanet Transit explorer?
The Exoplanet Transit explorer teaches the light-curve dip itself - detecting a planet exists and estimating its size from how much light it blocks. This page teaches what that dip looks like wavelength by wavelength, which is how astronomers read out an atmosphere's composition.
Is this real JWST data?
No. It is an educational explorer using a smooth model calibrated to the real published band positions and WASP-39b detections, not raw instrument data or a live data-reduction pipeline.