Astrobiologists Rethink Biosignatures as Oxygen Proves an Unreliable Marker of Life

The presence of oxygen in an exoplanet atmosphere may be an unreliable biological signature, according to a study led by the University of California, Santa Cruz. A computational thermo-geochemical-climatic model, published in AGU Advances, simulated the atmospheric evolution of rocky planets around a Sun-like star over 4.5 billion years — the age established for Earth — while varying the initial inventory of volatile compounds. It produced three scenarios in which a planet develops oxygen-rich atmospheres of abiotic origin. This is a modelling result, not an observation, and no planetary measurement underpins it.

The finding cuts against the assumption that oxygen alone signals biology, and it lands alongside a mirrored concern. Astrobiologists already worry about false positives — signals that wrongly suggest life — but the opposite error, the false negative, may be equally insidious: results indicating absence even where life is present. The roster of gaseous molecules used as biosignatures — oxygen, carbon dioxide, methane, ozone, ammonia, phosphine, methyl chloride, ethane and methanethiol — is itself in flux, with researchers at the University of California, Riverside proposing further additions. A separate line of work warns that organic molecules, though the basis of all known life, are also produced abiotically by volcanic and hydrothermal activity.

The rethinking has redirected attention to where evidence is best preserved. A study by researchers at Penn State University and NASA, published in Astrobiology, concludes that amino acids — the building blocks of proteins — survive better in Martian permafrost and ice caps than on the rocky surface, making the frozen ground the preferred target for biomarker hunting. On Enceladus, an international team drawn from ten institutes reported phosphorus, an element essential to life as we know it, in the form of sodium phosphates, recovered from Cassini data gathered during the mission's exploration of the Saturn system between 2004 and 2017. Two studies from the Planetary Sciences and Remote Sensing group at Freie Universität Berlin conclude that a dedicated Enceladus mission would find biosignatures more easily than previously assumed.

Whether such targets are reachable is another matter. Work on Europa and Enceladus ocean exploration notes the physical limits on returning scientific data from remote spacecraft as a principal obstacle. The nearer-term milestone is observational. NASA's Nancy Grace Roman Space Telescope, launched on 30 August 2026 aboard a SpaceX Falcon Heavy from Kennedy Space Center, is on a three-month journey to the Sun-Earth L2 point. Its coronagraph is designed to block starlight so that faint companions can be seen; a team led by SETI Institute scientist Dr. Maggie Turnbull will use it. NASA expects the observatory's first images in early 2027.