In the face of climate change, deforestation and biodiversity loss, creating a sustainable version of civilization is one of humanity’s most urgent tasks. But when confronting this immense challenge, we rarely ask what may be the most pressing question of all: How do we know if sustainability is even possible? Astronomers have inventoried a sizable share of the universe’s stars, galaxies, comets, and black holes. But are planets with sustainable civilizations also something the universe contains? Or does every civilization that may have arisen in the cosmos last only a few centuries before it falls to the climate change it triggers?