Pass starlight through a prism and the rainbow that comes out has gaps in it. Dark lines, thin and sharp, mark wavelengths where something has swallowed the light. Those gaps are the heart of absorption spectroscopy, and they are how astronomers now read the air of planets they will never visit.
The European Space Agency lays out the idea plainly. Light is split into its wavelengths, much as raindrops split sunlight. Some of those wavelengths, in the infrared and ultraviolet, are invisible to us, though detectors can still measure them. Molecules and atoms in a star’s outer layer absorb certain wavelengths, so the spectrum has places where light is missing. Where a line falls tells you what kind of particle absorbed the light.
What a star’s lines give away
Stars turn out to be surprisingly easy to read. ESA notes that cooler stars carry molecules in their atmospheres, hotter stars carry atoms, and the hottest have only ions, which are atoms stripped of electrons. So the same lines hint at temperature. Magnetic fields leave a mark too: in their presence a single absorption line splits into several thinner ones, and the stronger the field, the further apart those thin lines spread.
Reading a planet’s atmosphere
Planets are harder, because their light is swamped by the star. The trick relies on geometry. In some systems the planet passes in front of its star as it orbits, and the star dims slightly. During that transit a little starlight filters through the planet’s atmosphere, if it has one. The spectrum Ariel records is then the star’s combined with the atmosphere’s. ESA calls this transmission spectroscopy.
Then comes the second half. When the planet slips fully behind the star, in eclipse, no light passes through its atmosphere and the spectrum shows the star alone. Subtract the eclipse spectrum from the transit spectrum and what remains are the absorption lines of the planet’s atmosphere. According to ESA, that can reveal what the atmosphere is made of, its temperature and the presence of a magnetic field.
ESA’s Ariel mission is built around this work, with spectrometers and photometers made for the purpose. Because Ariel will return to some stars more than once, repeating the transit and eclipse measurements, astronomers can follow changes over time. They expect to use those repeat observations to understand exoplanet weather patterns and study how clouds are distributed. A separate method, the phase curve, skips spectra and tracks the combined brightness of star and planet as the planet goes through phases like the Moon. That shows how reflective the planet is, and can hint at whether its atmosphere is cloudy or hazy.
Why the gases matter
Spectroscopy Online, in a March 2025 overview by Jerome Workman, Jr., describes spectroscopy as a critical tool for working out the composition of exoplanet atmospheres. Analyse the light passing through or reflected off a planet’s atmosphere, it says, and scientists can pick out specific gases that bear on habitability. Detecting oxygen, carbon dioxide and water vapor, for instance, could indicate conditions friendly to life.
The same overview points to instruments such as SPHERE, which offer detailed spectral analysis, and to artificial intelligence and machine learning that help pull faint signals out of huge astronomical datasets. It also names NASA’s Roman Space Telescope and ESA’s PLATO mission among those expected to deepen understanding of exoplanet atmospheres and how planets form.
Sources
- ESA – Studying exoplanet atmospheres with Ariel
- Exoplanet Discovery Using Spectroscopy | Spectroscopy Online
