For much of the 20th century, mainstream scientists have considered the search for alien life to be the realm of dreamers, conspiracy theorists and quacks. But recent discoveries forced the scientific community to reevaluate the circumstances in which life can exist and, with that, reconsider the once nearly unthinkable possibility that there might be alien life in our own planetary backyard. In this Aeon interview, the UK space scientist Monica Grady discusses major developments in the search for extraterrestrial life over the past few decades, the most promising planetary candidates that could harbour life in our solar system, and raises some of the ethical and existential implications of humanity’s search for life beyond Earth.