Planets around other stars are far too faint and too close to their suns to photograph in detail. Yet astronomers can still tell what some of them are made of, how fast they rotate and even how the wind blows in their skies. The key is spectroscopy: spreading light into its component colours and reading what is written in it.
Light as a fingerprint
Every molecule absorbs or emits light at particular wavelengths. Water, carbon monoxide, methane and sodium each leave a distinctive pattern of dark or bright lines. When starlight passes through a planet’s atmosphere, or when the planet’s own glow is recorded, those patterns can be matched to laboratory data to name the gases present.
Why “high resolution” matters
Low-resolution spectra show broad bumps. High-resolution spectra split the light so finely that individual lines become visible. This brings several advantages:
- Lines from a planet can be told apart from lines from the star and from Earth’s own atmosphere, because the planet’s motion shifts them by a different amount.
- Many weak lines can be combined to build a signal that no single line could provide.
- The exact shape of the lines carries information about winds and rotation.
The Doppler shift
A planet orbiting its star moves towards us and away from us over the course of its orbit. Its light is shifted slightly in wavelength, just as the pitch of a passing siren changes. Tracking that shift separates the planet from the almost static telescope-and-sky background.
Cross-correlation in brief
Because the planetary lines are buried in noise, researchers compare the observed spectrum with a template built from a model of the expected molecules. The template is shifted through a range of velocities, and the agreement is recorded each time. A peak at one particular velocity indicates that the molecule is present and moving at that speed.
Three ways to catch a planet
- Transits. When a planet crosses in front of its star, a little starlight filters through the upper atmosphere.
- Day-side emission. Observations near the moment the planet passes behind the star help isolate the glow of its sunlit side.
- Direct imaging. Young, widely separated giants can be separated from the star in the image itself, and their spectra then recorded.
What spin tells us
When a young giant planet or brown dwarf rotates, the motion broadens its spectral lines. Measuring that broadening gives the rotation speed. Comparing many objects can show how spin relates to mass, age and orbital distance, which in turn tells us about how such bodies gather material and lose angular momentum while they form.
Limits and future goals
Today’s methods work best on large, hot worlds because they produce the strongest signals. Smaller and cooler planets, closer to Earth in size, remain a challenge. Larger telescopes and improved instruments are expected to push that boundary, and the long-term hope of the field is to search for chemical signs that might point to life.
Further exploration
To see this kind of research in practice, read the overview of the publications or the worked cross-correlation example.