Abstract:  One of the most promising theoretical framework to understand the observed deficiency of close-in super-Earth/sub-Neptune planets in the Exoplanet "Radius Valley", is the atmospheric photo-evaporation model. Atmospheres of planets with low density and high enough ultraviolet irradiation will not have stable thermospheres. Instead, they will drive an outflow similar to Parker wind. These evaporating atmospheres will have an extended exosphere making them ideal for detection via transmission spectroscopy. Traditionally, excess Ly alpha absorption from the neutral Hydrogen was used to detect these exo-spheres during transit from the Hubble Space Telescope. 

Recently, resonant scattering absorption of meta-stable Helium atoms at 10830 Angstrom have become a powerful probe to detect these extended exo-spheres. This, being a near-infrared line observable from ground and unaffected by inter-stellar medium, has numerous advantages over Ly alpha. In this seminar, after a brief introduction to the topic, I shall talk about our observational program to detect these evaporating exo-spheres using Habitable-zone Planet Finder (HPF) from Hobby Eberly Telescope. I shall also talk more about our exciting first detection of He10830 in a warm Neptune orbiting the M-dwarf GJ 3470 using HPF.