What does a world need before life can live there?

Before life can live somewhere, a world needs three things. NASA's astrobiology roadmap names them: extended regions of liquid water, conditions favorable for building complex organic molecules, and energy sources to sustain metabolism. Energy is non-negotiable: an absolute requirement for life is an energy source. A puddle full of ingredients with nothing to power the chemistry stays a puddle.

The ingredients are the easy part. Carbon, hydrogen, oxygen and nitrogen are among the most common reactive elements in the universe, and together they make up over 96% of Earth's biomass. Carbon can bond with itself into a huge variety of intricate structures, which makes it ideal for the machinery of living cells, and hydrogen and oxygen, in the form of water, make the solvent in which that machinery works. Sulfur and phosphorus, needed for proteins and for DNA and RNA, are not rare either.

The hard part is having all three at once, in the same place. That is why liquid water is called necessary but not sufficient: a wet world can still lack energy or chemistry. And the places that qualify keep turning up in odd locations. Starting in the 1980s and 1990s, biologists found microbial life in acidic, scorching or high-pressure niches that looked inhospitable, and some scientists even concluded that life may have begun in hydrothermal vents beneath the ocean. The checklist did not change. What changed was how many places could satisfy it.