Biosignatures

Gases in a planet's atmosphere that life could produce and maintain — especially a chemical disequilibrium, like oxygen and methane together, that non-living chemistry would quickly erase. Detected by the spectroscopy of a planet's light.

A signal that mimics life but is made by non-living processes — abiotic oxygen from water splitting, geological methane, or a mis-measured spectral feature. Ruling out false positives is the hardest and most important part of any claim of life; the Venus phosphine debate is a recent lesson.

Chemical fingerprints of life in rock or ice — the isotopic ratios life prefers, the handedness (chirality) of its molecules, and specific organic byproducts — the kind of evidence the Perseverance rover seeks and caches on Mars.

Structures preserved in rock that life can build — layered stromatolites, biominerals, and microfossils. Perseverance is exploring an ancient Martian river delta where such signs might be preserved.

Signs of life on a surface — pigments, the 'red edge' reflectance of vegetation, or patterns and textures a biosphere leaves behind — sought both on exoplanets and on the surface of Mars.

A sign not of life but of technology — a radio or optical signal, an artificial light, an industrial pollutant, or a megastructure. The search for technosignatures is the search for intelligence, led by SETI and Breakthrough Listen.