Spectro-polarimetric signatures of the pale blue dot: from planets to exoplanets

Abstract

When it comes to understanding the habitability of planets, the context of the third planet in our Solar System—our Earth—is of paramount importance. Understanding the climate of Earth and other planets in our Solar System is a continuously evolving subject and is a central theme of space exploration for several space agencies around the world. The theoretical understanding of this subject is often confronted with the new observations from spacecrafts and telescopes. Few deeper questions of humanity, which fuel these developments and explorations are about the future of our own species and the possibility of life beyond our planet. In last few decades, our understanding of climate has deepened and our methods to search for exoplanets have significantly improved. Two recent Nobel Prizes in Physics reflect these advances: in 2019, Michel Mayor and Didier Queloz were recognized for the discovery of the first exoplanet orbiting a Sun-like star, and in 2021, Syukuro Manabe and Klaus Hasselmann were honored for developing models that underpin our understanding of Earth’s climate. Manabe & Hasselmann gave us the laws to understand climates; Mayor & Queloz gave us new worlds to apply them to. With these tools and methods in hand, it is an exciting time to go one step deeper on the questions related to the presence of another habitable planet i.e Earth 2.0. Building upon this global and interdisciplinary progress, the present thesis examines how Earth itself can serve as a natural benchmark for such searches. Earth’s globally integrated signatures, when observed as those of a distant planet, can serve as a reference for identifying Earth-like planets among other types of worlds.

This thesis involves the development of a radiative transfer modeling framework together with a novel instrument concept to observe Earth. The observations of Earth-as an exoplanet by this instrument, called SHAPE (Spectropolarimetry of HAbitable Planet Earth), are finally interpreted within the framework of the modeling studies presented in this thesis. The analysis highlights distinct spectro-polarimetric signatures across different phase angles, which can help identify the presence of oceans and clouds in disc-integrated observations of exoplanets. This represents the central development and overarching theme of the thesis. Along the way, we also explore characteristic signatures of (exo)planetary atmospheres that can be probed with contemporary and future telescopes, contributing to a deeper understanding of planetary environments