Transit spectroscopy · Biosignatures · Scale height · JWST detection · Habitability
Simulate how astronomers detect exoplanet atmospheres via transmission spectroscopy during transit. See how atmospheric composition leaves absorption lines in the stellar spectrum, and explore biosignature gases that could indicate life.
When a planet transits its star, starlight filters through the atmosphere. The transit depth δ = (R_p/R_*)² gives the planet-to-star area ratio. Atmospheric absorption adds δ_atm = 2H·R_p/R_*² at wavelengths where molecules absorb. The scale height H = kT/(mg) depends on temperature T, molecular mass m, and surface gravity g. JWST detects H₂O in TRAPPIST-1e over ~10 transits.
Biosignatures are gases that indicate biological processes: O₂ (photosynthesis), O₃ (ozone from O₂), CH₄ (methanogenesis — interesting when combined with O₂), N₂O (denitrification), H₂O (life as we know it). No single gas is definitive; the combination of CH₄ + O₂ is especially compelling since they react abiotically. Context — stellar type, planet mass, orbit — matters greatly.
The James Webb Space Telescope operates 0.6–28 μm and can achieve transit depth precision of 20–50 ppm per observation. For TRAPPIST-1e (R_p ≈ 0.92 R⊕, around an M dwarf), the transit depth is ~0.7%. H₂O features at 1.4 and 1.9 μm are detectable with ~10 transits. CO₂ at 4.3 μm is detectable even in a thin atmosphere. Rayleigh scattering σ∝λ⁻⁴ adds a blue slope.
Select a planet preset (Earth twin, Venus twin, Hot Jupiter, TRAPPIST-1e) or adjust planet radius and scale height manually. Toggle molecules to add their absorption features to the transmission spectrum. Enable JWST noise floor to see detectability. Toggle biosignature highlighting to identify life-indicating features. Watch the transit light curve animation and read detection significance (σ) and habitability score.