Eclipse spectroscopy

Some exoplanet systems are oriented such that we can see a planet pass in front of its host star as it orbits. When this happens, we notice a slight dimming of the star as the planet blocks some starlight. During such a transit, part of the starlight passes through the atmosphere of the planet (if the planet has one). This changes the spectrum of starlight that Ariel measures; the resulting spectrum is that of the star combined with that of the planet’s atmosphere. We call this method of exploring exoplanets ‘transmission spectroscopy’.

During ‘eclipse spectroscopy’ the spectrum of starlight is measured when the orbiting planet is completely hidden by its star. The spectrum shows the star alone as no light passes through the planet’s atmosphere.

By subtracting the spectrum in eclipse from that in transit, astronomers can see the absorption lines resulting only from the planet’s atmosphere. This can reveal what the atmosphere is made of, its temperature, the presence of a magnetic field, and more.