A biosignature is any measurable sign that life is or was present — from oxygen-methane mixtures in an exoplanet's atmosphere to fossil textures in Martian rock. Modern telescopes hunt them in starlight filtered through distant atmospheres; rovers cache samples that may hold them.
Understanding Biosignature
Reading an atmosphere in transit
When a planet crosses its star, a sliver of starlight grazes its atmosphere; molecules absorb signature wavelengths, deepening the transit at those colours by parts in ten thousand. Stacking many transits builds the spectrum — JWST's haul on temperate worlds includes carbon dioxide, methane and water — but small rocky planets around quiet stars remain at sensitivity's edge: the oxygen-on-an-Earth-twin measurement is the next-generation flagship's defining task.
The context problem
Every proposed biosignature now ships with its abiotic impostors catalogued, and assessment frameworks weigh detection quality, environmental plausibility and alternative chemistry before biology earns consideration. The famous cautionary tales — Martian methane's contested wisps, phosphine on Venus rising and falling with reanalysis — taught the field its posture: extraordinary claims triaged through ordinary, grinding verification.