Spectro-polarimetry of Earth-like (Exo)planets: Science and Techniques

More than 5000 exoplanets have been discovered so far and several have been characterized for their atmospheres too. The future efforts to search and characterize nearby Earth-like planets will be based on directly detecting the reflected light from the planet. The reflected light spectrum will have signatures of gases and clouds. In this talk, I will explain about the spectrum and polarization of light reflected from the planetary surface and the atmosphere; and discuss on how these can be used to infer the presence of clouds, oceans or biosignature gases in the planet. Before understanding the reflected light from other temperate planets, it is crucial to study and retrieve the information contained in the disk-integrated spectrum of Earth itself. Earth, being the only known planet to host water and life, serves as a unique testbed for such a study. We propose a spectro-polarimeter experiment to be conducted from Lunar orbit which will help us in understanding the disc-integrated spectrum and polarization of Earth. In the end of the talk I will also highlight about futuristic instrument techniques, such as stellar coronagraphs, which are being pursued worldwide to help us see the reflected light from the habitable planets around nearby stars.