Exoplanet Atmospheres | mysimulator.uk

Transmission spectroscopy: reading starlight through an edge of air

Most confirmed exoplanets are found by the transit method: watching a star's brightness dip slightly as a planet crosses in front of it. During that transit, a thin sliver of starlight does not hit the planet's opaque disc — it grazes straight through the ring of atmosphere at the planet's limb on its way to the telescope. Different gases absorb light at very specific wavelengths, so the transit looks slightly deeper at wavelengths a particular gas absorbs than at wavelengths it does not, because the effective opaque radius of the planet is larger at those absorbed wavelengths.

live demo · a transit dimming a distant star● LIVE

transit depth(λ) = ( R_planet(λ) / R_star )²

R_planet(λ) is slightly larger at wavelengths the atmosphere absorbs strongly
→ subtract the "clear" baseline spectrum from the in-transit spectrum
→ the residual bumps are the atmosphere's chemical fingerprint

This measures a difference of parts in ten thousand in starlight brightness across many transits — an extraordinarily small, patient signal that requires either a very bright, nearby star or a space telescope free of atmospheric interference of its own.

What JWST changed

The James Webb Space Telescope's infrared instruments, particularly NIRSpec and NIRISS, cover the wavelength range where water, carbon dioxide, methane and other molecules have their strongest absorption features, with far greater sensitivity and wavelength coverage than earlier space telescopes designed primarily for other purposes. JWST observations of exoplanets such as WASP-39b delivered the first unambiguous detection of carbon dioxide and sulfur dioxide in an exoplanet atmosphere, with the sulfur dioxide in particular signalling photochemistry — sunlight-driven chemical reactions happening in the planet's upper atmosphere, not just its bulk composition.

Biosignatures: gases that are hard to explain without life

A biosignature gas is one whose sustained presence is difficult to explain through geology or chemistry alone. Oxygen is the classic example on Earth, continuously replenished by photosynthesis; without a biological source it reacts away with surface rocks and volcanic gases within a relatively short geological time. The trouble is that oxygen alone is not conclusive — it can also build up abiotically on planets that lost most of their hydrogen to space, since ultraviolet light splits water vapour and the leftover oxygen accumulates. The stronger case is a disequilibrium pair: oxygen and methane found together, since the two react with each other and would disappear within a geologically short window unless both are being actively, continuously replenished — a combination that is much harder to sustain without an ongoing biological (or at least highly unusual geochemical) source.

The habitable zone: necessary, not sufficient

The habitable zone (sometimes called the Goldilocks zone) is the range of orbital distances from a star where a planet with an Earth-like atmosphere could sustain liquid water on its surface — close enough that it is not permanently frozen, far enough that it does not boil away or trigger a runaway greenhouse. It is calculated purely from stellar brightness and orbital distance and says nothing about whether the planet actually has an atmosphere, what that atmosphere contains, or whether a protective magnetic field exists. Venus and Mars both lie near the edges of the Sun's habitable zone, and neither has surface liquid water today — a reminder that "in the habitable zone" is only a first filter, not a determination of habitability.

Frequently asked questions

How can telescopes detect gases in an atmosphere trillions of kilometres away?

By transmission spectroscopy. When a planet transits in front of its star, a sliver of starlight grazes through the thin ring of atmosphere at the planet's edge on its way to the telescope, and specific gases absorb specific wavelengths of that light. Comparing the star's spectrum during transit to its spectrum just before or after reveals a wavelength-dependent dip pattern that acts as a chemical fingerprint for the gases present, even though no light from the planet itself is being directly imaged.

Does detecting oxygen or methane in an exoplanet atmosphere prove there is life?

No, and this is the central caution in the field. Both gases can also be produced abiotically — oxygen through UV splitting of water or CO2 on planets that have lost most of their hydrogen, methane through volcanic and geochemical processes. What makes a stronger case is finding oxygen and methane together in the same atmosphere, because on their own they react with each other and disappear within a geologically short time, so persistent, simultaneous detection implies both are being continuously replenished, which is much harder to explain without biology.

Is a planet in the habitable zone automatically habitable?

No. The habitable zone is defined purely by orbital distance and stellar brightness — the range where a planet with an Earth-like atmosphere could sustain liquid surface water. It says nothing about whether the planet actually has an atmosphere at all, what that atmosphere is made of, or whether the planet has a magnetic field to protect it from stellar radiation. Venus and Mars both sit near the edges of the Sun's habitable zone and neither has surface liquid water, which is exactly why 'habitable zone' is treated as a necessary but not sufficient condition.

Try it live

Everything above runs in your browser — open Exoplanet Atmospheres and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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Exoplanet Atmospheres: Transmission Spectroscopy and Biosignatures