New Discoveries Renew Questions About Extraterrestrial Life

What is the chance that extra-terrestrial life exists? It is a question that we astronomers often get asked, and I always respond by saying that we have no idea. To predict the probability of something occurring, you need to start with some relevant, prior knowledge. For instance, meteorologists can figure out how likely it is to rain tomorrow because they already have lots of data about weather patterns. But in the case of predicting how common it is for life to arise in the universe, we currently only have one datum: the existence of life on earth. That's just not enough.