Astrobiology is the study of life off of Earth. None has been found so it is a bit speculative. The study includes:
Reasoning out the conditions that might support life is of interest as is the reasoning out the possible signs, i.e., biosignatures. Life is thought to be most likely on a planet or moon when carbon is present, which is the element most likely to be the basis of very large, complex molecules, and when liquid water, a very effective solvent, is present, more likely with both.