PROBING EXOPLANET ATMOSPHERES
Webb uses a method called transit spectroscopy to work out what molecules are present in an exoplanet’s atmosphere. A transit occurs when a planet moves across the face of its parent star as seen from the perspective of the telescope observing it. One effect of this is to produce a small decrease in the apparent brightness of the star, and this gives us one of the most useful methods of discovering exoplanets in the first place. All the planets found by the TESS and Kepler spacecraft, for example, were detected by observing the small dips in brightness occurring during a transit.
There is another side effect of transits that a powerful telescope like Webb can exploit. If the exoplanet happens to have an atmosphere, then some of the star’s light will pass through this during a transit. But the light doesn’t pass through unhindered, because the gas molecules in the atmosphere will absorb a fraction of it. When this happens, it’s possible to determine exactly which gases are involved because a particular molecule only absorbs light at certain well-defined wavelengths. As a result, when astronomers look at the spectrum of light – a technique known as spectroscopy – they see characteristic dips at the wavelengths where light has been absorbed. The pattern of these dips then allows them to work out which gases are present in the planet’s atmosphere.
Spectroscopy is a well-established astronomical technique when applied to the composition of stars. But it becomes much harder in the case of exoplanets due to the tiny amount of light involved. Fortunately, Webb is sensitive enough that it should be able to measure the spectra of even relatively small planets like Earth.