Exoplanet Atmospheres: How Light Reveals What Is in the Air

Astronomers study exoplanet atmospheres by splitting light into a spectrum and looking for the fingerprints of molecules. A planet’s air changes the starlight that reaches us, and the patterns in that change reveal which gases are present, how hot the layers are and sometimes whether clouds are in the way.

No telescope can photograph a planet’s clouds from many light years away, so the whole field depends on subtle differences in brightness and colour. This page walks through the main techniques and the problems they face.

Why a spectrum is a fingerprint

Each molecule absorbs or emits light at particular wavelengths. Water, carbon dioxide, methane and carbon monoxide all have characteristic sets of lines, especially in the infrared. When a spectrum shows a dip or bump at those wavelengths, that molecule is probably present.

The same principle has been used for more than a century to analyse stars. The difficulty with a planet is that its signal is buried in the light of a star millions of times brighter.

Transmission spectroscopy

When a planet passes in front of its star, a thin ring of its atmosphere is lit from behind. At wavelengths where a molecule absorbs strongly, the atmosphere looks slightly thicker, so the planet blocks a little more starlight. Plotting the transit depth against wavelength gives a transmission spectrum.

The first detection of an exoplanet atmosphere used this idea. In 2002, a team using the Hubble Space Telescope reported sodium in the atmosphere of HD 209458 b, a hot Jupiter. Since then, larger telescopes have measured water and other species in many giant planets.

Transmission only probes the upper atmosphere along the planet’s edge. High clouds or haze can flatten the spectrum and hide what lies below.

Emission and eclipse spectroscopy

When a planet passes behind its star (a secondary eclipse), the total light dips slightly because the planet’s own glow disappears. Comparing the brightness just before and during the eclipse isolates the light from the planet’s dayside.

This shows how hot the dayside is and what absorbs at different heights. A temperature that rises with altitude, called a thermal inversion, leaves a distinct pattern. Phase curves, which follow the system’s brightness around a full orbit, add information about how heat moves from the day side to the night side.

High-resolution spectroscopy and cross-correlation

Space telescopes have limited spectral resolution, so ground-based instruments take a different approach. They observe at very high resolution and use the planet’s changing speed along the line of sight.

A planet’s orbital motion shifts its spectral lines by many kilometres per second back and forth over the orbit, while the star’s lines and Earth’s atmosphere stay almost fixed. Astronomers remove the unmoving components and then compare what remains with a template spectrum of a molecule. Summing the match over hundreds or thousands of lines produces a peak at the right velocity, even if no single line is visible.

That is the logic behind cross-correlation, and a worked example is shown on the page cross-correlation example. The method has been used for hot gas giants and has also been applied to directly imaged, young planets.

What the space telescopes added

The James Webb Space Telescope, launched in 2021, observes in the infrared at higher sensitivity than earlier instruments. Early results published in 2022 and 2023 included the detection of carbon dioxide in the atmosphere of the hot Saturn WASP-39 b, as reported by the JWST early-release science team and covered by NASA. That kind of measurement gives information about the amount of heavy elements in a planet’s atmosphere compared with its host star.

The main obstacles

• Clouds and haze can mute spectral features.
• Stellar activity, such as spots and flares, mimics or hides planetary signals.
• Small planets have thin atmospheres relative to their size, so signals are tiny.
• Telluric absorption from Earth’s own atmosphere contaminates ground-based data.
• Model dependence: converting a spectrum into abundances requires assumptions about temperature and pressure.

Because of these issues, results are generally reported with uncertainties, and independent teams often reanalyse the same data.

Where this fits

The overview in how astronomers read the atmospheres of distant planets covers the general idea. This page goes into the individual techniques and their limits.

Frequently asked questions

What are exoplanet atmospheres made of?

It varies widely. Hot giants show hydrogen-dominated air with water, carbon monoxide and sometimes sodium or potassium. For small rocky planets, composition is much harder to measure and remains an active research area.

Can astronomers detect life from an atmosphere?

Not directly. Some gases are of interest as possible biosignatures, but non-living processes can produce many of them, so any claim would need strong supporting evidence and careful scrutiny.

Why is infrared light so important?

Many key molecules have strong absorption bands in the infrared. Planets also emit most of their thermal glow there, and the contrast with the star is better than in visible light.

Does the planet need to transit?

For transmission and eclipse methods, yes. High-resolution techniques and direct imaging can work for non-transiting planets in some cases, but transiting systems remain the easiest targets.

Summary

Spectra turn faint changes in starlight into a chemical inventory of distant air. The three main routes, transmission, emission and high-resolution cross-correlation, complement each other, and their results are strongest when combined. The home page lists related material on the subject.