On Alien Planets, Nitrogen May Be A Sign of Habitability

We might commonly think of Earth as having an oxygen-dominated atmosphere, but in reality the molecule makes up only a fifth of our air. Astrobiologists are beginning to see nitrogen — and not just oxygen — as a key indicator of a planet's habitability. Nitrogen is essential for life on Earth and could signal an atmosphere thick enough to stabilize liquid water on a planet's surface, fundamental to creating habitable conditions.