Extreme habitable environments : a bridge between astrophysics and astrobiology This book presents the current trends of research in exoplanets with an aim to understand the big question: are we alone in the universe . This gives us the insight towards the fascinating extreme habitable worlds in which specific extremophiles potentially could survive. The content briefly describes the similarity indexing techniques such as: Earth Similarity Index (ESI) to find Earth like exoplanets Mars Similarity Index (MSI) to find Mars like exoplanets and also indexing applied for specific water extremophile (i.e. Tardigrades) to potentially survive on exoplanets. These tardigrades are indexed as Active Tardigrade Index (ATI) and Cryptobiotic Tardigrade Index (CTI). Basically the book is a bridge between astronomy astrophysics and current astrobiology studies