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Malti🔭 Exoplanet Atmospheres — Transmission Spectroscopy & Biosignatures
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.
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.
🌌 Transit Spectroscopy
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.
🌏 Biosignature Gases
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.
🔭 JWST Capabilities
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.
🎮 How to Use
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.
About Exoplanet Transmission Spectroscopy
This simulation models how astronomers probe exoplanet atmospheres while a planet transits its star. As starlight grazes the upper atmosphere, molecules absorb at characteristic wavelengths, deepening the apparent transit slightly where they absorb. The baseline transit depth is δ = (R_p/R_*)², and the extra atmospheric signal scales with the atmospheric scale height H = kT/(mg). The result is a transmission spectrum of transit depth versus wavelength from roughly 0.5 to 20 microns.
You choose a planet preset (Earth twin, Venus twin, Hot Jupiter or TRAPPIST-1e), or set the planet radius (0.5–12 R⊕) and scale height (1–500 km) directly, then toggle which molecules populate the atmosphere. Optional overlays show the JWST noise floor and highlight biosignature gases against an Earth reference. This mirrors the real technique JWST uses to characterise small worlds, and it is central to the search for habitable planets and signs of life beyond the Solar System.
Frequently Asked Questions
What is transmission spectroscopy?
It is a technique for detecting an exoplanet's atmosphere during a transit. When the planet passes in front of its star, a thin ring of starlight filters through the atmosphere. Molecules absorb specific wavelengths, so the planet appears very slightly larger at those colours. Measuring transit depth across wavelength reveals which gases are present.
How does the simulation calculate the transit depth?
The baseline transit depth is the planet-to-star area ratio, δ = (R_p/R_*)², shown as a percentage in the readout. Here the reference star radius is fixed at about 8 Earth radii. The extra atmospheric signal is added on top in parts per million, scaling with the scale height and planet radius, and only at wavelengths where the selected molecules absorb.
What does the scale height control do?
The scale height H is the vertical distance over which atmospheric pressure falls by a factor of e, given by H = kT/(mg). A larger H makes the atmosphere puffier, so absorption features become deeper and easier to detect. The slider spans 1 to 500 km, from a compact Earth-like atmosphere up to a hot, low-gravity gas giant.
What do the planet presets represent?
Earth twin uses R_p = 1.0 R⊕, H = 8.5 km with H₂O, O₃, CH₄ and N₂O. Venus twin is a thick CO₂ atmosphere. Hot Jupiter is a large, puffy planet (about 11 R⊕, H = 400 km) with H₂O and CO₂. TRAPPIST-1e uses R_p = 0.92 R⊕ with H₂O and CO₂, matching a real, much-studied rocky world around an M dwarf.
Which gases count as biosignatures here?
The simulation flags H₂O, O₃, CH₄ and N₂O as biologically interesting. Ozone implies oxygen from photosynthesis, methane can come from methanogens, and nitrous oxide from denitrifying microbes. No single gas is conclusive on its own; the combination of methane and oxygen-bearing species is especially compelling because they react and would be depleted without continuous replenishment.
What does the JWST noise floor overlay show?
Toggling it draws a dashed line representing the instrument's measurement limit. Features that rise above this floor are detectable; those below are lost in the noise. The model assumes a per-observation precision of roughly 25 ppm and stacks about ten transits, which is comparable to what JWST achieves on favourable targets.
Why is the spectrum tilted upward at short wavelengths?
That blue slope is Rayleigh scattering. Small particles and molecules scatter light with a cross-section proportional to λ⁻⁴, so shorter (bluer) wavelengths are scattered more strongly. This makes the atmosphere look slightly larger and the transit deeper towards the blue end, independent of any specific absorption band.
How accurate is this model?
It is a faithful conceptual illustration rather than a research-grade radiative transfer code. The transit-depth and scale-height formulae are correct, and the molecular absorption bands are placed at realistic wavelengths. However, band strengths, noise and habitability scoring are simplified for clarity and interactivity, so the numbers are indicative rather than publication-quality.
What is the detection significance value?
The detection readout, shown in sigma (σ), compares the atmospheric signal to the JWST noise. Roughly, it divides the signal in ppm by the per-transit noise after combining several transits. Values above about 3σ turn green to indicate a confident detection, while lower values are marked as marginal.
Why does combining methane and oxygen matter for life?
Methane and oxygen react together quickly in a planetary atmosphere, so they cannot both persist at high abundance without being constantly resupplied. On Earth, biology replenishes both. Seeing them together on an exoplanet would therefore be a strong chemical-disequilibrium hint of an active biosphere, which is why the simulation rewards that pairing in its habitability score.
Exoplanet transmission spectroscopy, biosignatures, JWST data and habitability zone analysis.
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