How do we detect exoplanets’ atmospheres?

The methods to detect exoplanets (planets that are found beyond the solar system) are various, and the used technique varies enormously in each case. Thanks to new technologies, more than 5,200 exoplanets have been detected until this moment. However, the number of planets we have been able to analyse the composition of their atmosphere is very reduced, as it presents a great technological challenge.

The chemical composition of an atmosphere can tell us a lot about the physical conditions of the planet, which could possibly contain signs of life.

Atmosphere detection in exoplanets

As we explained in a previous post, the most used method, and the one with which we have discovered more exoplanets, is the transit of planets around other stars. This method consists in observing the small drops in the brightness of a star when a planet passes in front of it (this is called a transit).

While a part of the light is blocked by the planet, an even smaller part goes through the planet’s atmosphere. When this happens, the present molecules in the atmosphere absorb part of the light spectrum from the star (each element absorbs a specific wavelength), whereas other types of light can go through it without any problem.

From the Earth, we can obtain the light spectrum coming from the planet, which shows us which wavelengths have been absorbed by its atmosphere. From here we are capable of deducing which elements are present in the atmosphere of the observed planet.

However, the fraction of stellar light that goes through the atmosphere of a transiting exoplanet is very small. This limits the telescopes and instruments that can be used for this purpose, as they have to be very powerful to detect, amongst all the light from the star, that small part that goes through the atmosphere.

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