The Search for Life on Other Planets: Sulfur-Based, Silicon-Based, Ammonia-Based Life

The search for extraterrestrial life is one of the most challenging and interesting scientific themes of the 21st century. This search has been guided by our understanding of the life's nature. Up to now, we only know life on Earth, which uses water as a solvent and the building blocks of which are based on carbon and oxygen. Hence, the search for extraterrestrial life has been the search for life as we know it as based on life which lives on Earth. However, living systems that may have originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products.Therefore, we need to expand the boundaries of our Earth-centric concept of life and be open-minded and aware of the most general features of living systems. Life forms based on silicon, ammonia, and sulfur are among those who may have evolved on other worlds, and these possibilities are discussed.