The He I 10830 line in exoplanets, triggered by coronal emission

Despite of early efforts back in 2000 to find the He I 10830 line (actually a triplet) in transiting exoplanets, it was not until 2018 that the line was finally observed in an exoplanet. An increasing number of detections in transiting exoplanets is allowing us to trace an unambiguous relation between this line and the incoming XUV irradiation from the corona of the star. The stellar radiation below 504 Å ionizes He atoms in the planet atmosphere, producing the He I 10830 Å triplet after recombination takes place. This line is becoming a useful tool to measure the effects of photoevaporation in planets, and even to detect the presence of winds. In contrast with the H Lyman-α line, the He triplet is not absorbed by the interstellar medium, posing it as the best tracer of photoevaporation for the future. The analysis of the stellar coronal emission is essential to predict the line detection in exoplanets. The careful work carried out in the X-exoplanets database provides an easy and reliable way to calculate the expected He-ionizing XUV stellar flux