What a Biosignature Is and Why Claiming One in Astronomy Is the Riskiest Detection in Science

Seven words can end a career or ignite a thousand follow-up papers: "We may have detected signs of life." In September 2020, a team led by astronomer Jane Greaves announced the detection of phosphine gas in the atmosphere of Venus. Phosphine, on Earth, is produced almost exclusively by anaerobic bacteria and industrial processes. Venus has neither factories nor, as far as anyone knew, the right chemistry to make it abiotically. The announcement landed like a thunderclap. Then came the scrutiny. Within months, independent teams reanalysing the same data from the James Clerk Maxwell Telescope and the Atacama Large Millimeter Array found the signal was far weaker than initially reported, possibly an artefact of data processing. The phosphine claim did not die cleanly, but it shrank. What the episode clarified, permanently, is that a biosignature is not a fact. It is a hypothesis that must survive every alternative explanation first.
A biosignature is any measurable property of a planet, its atmosphere, its surface, or its light, that life could plausibly have produced and that cannot be fully explained by geology or chemistry alone. The definition is deliberately careful. Scientists divide biosignatures into several categories. Gaseous biosignatures are molecules in an atmosphere: oxygen, ozone, methane, nitrous oxide, dimethyl sulphide. Surface biosignatures include the "red edge," a sharp spike in infrared reflectance that vegetation produces because chlorophyll absorbs red light but reflects near-infrared. Temporal biosignatures are patterns that change with seasons or cycles, the kind of rhythmic fluctuation a barren rock does not produce. No single one of these is proof. The power of a biosignature case comes from combinations: oxygen and methane together, for instance, should react and cancel each other out over geological time. Finding both simultaneously in an atmosphere means something is continuously replenishing them. That something, on Earth, is life. On an unknown exoplanet, it is a question mark that demands an answer.
Oxygen is the molecule most people associate with life. It makes up 21 percent of Earth's atmosphere and is produced in enormous quantities by photosynthesis. The instinct is to look for it elsewhere and declare success. The problem is abiotic oxygen production. A planet with a lot of water and a strong ultraviolet flux from its star can have water molecules photolysed, split apart, with hydrogen escaping to space and oxygen accumulating. This process, called abiotic oxygen buildup, could produce an oxygen-rich atmosphere on a dead world. The exoplanet GJ 1132b, orbiting a red dwarf about 41 light-years from Earth, may have experienced exactly this. Ozone, oxygen's three-atom cousin, has the same problem. Detecting it via spectroscopy is easier than detecting molecular oxygen directly, but it inherits the same ambiguity. The lesson is not that oxygen is useless as a biosignature. It is that oxygen alone, without context, without companion molecules, without understanding the star's UV output and the planet's water history, tells you very little. The claim has to be built, not announced.
When starlight passes through a planet's atmosphere during a transit, different molecules absorb specific wavelengths. The resulting spectrum, a pattern of dips and peaks across wavelengths, is a chemical fingerprint. This technique, transmission spectroscopy, is how astronomers read atmospheres they will never physically visit. The James Webb Space Telescope, which began science operations in 2022, is the most powerful instrument ever built for this work. Its infrared sensitivity can detect molecules like carbon dioxide, water vapour, methane, and sulphur dioxide in the atmospheres of exoplanets orbiting distant stars. In 2023, JWST confirmed carbon dioxide in the atmosphere of the exoplanet WASP-39b, a hot gas giant. That is not a biosignature, but it demonstrated the instrument's capability. The harder target is a small, rocky planet in the habitable zone of its star, the kind where liquid water could exist. TRAPPIST-1e, one of seven Earth-sized planets orbiting an ultracool red dwarf 39 light-years away, is among the candidates JWST is studying. Getting a clean spectrum from a planet that small requires dozens of transit observations stacked together. A single transit yields noise. The signal only emerges from patience and accumulation. India's astronomical community watches this closely. ISRO's AstroSat, launched in 2015, has contributed to multi-wavelength studies of stellar behaviour, and understanding how a host star's ultraviolet and X-ray output affects an orbiting planet's atmosphere is directly relevant to biosignature interpretation. A star that bathes its planet in extreme UV radiation can strip an atmosphere or drive abiotic chemistry that mimics life. Knowing the star is not a background task. It is half the problem.

The stakes are asymmetric. If you announce a biosignature detection and you are wrong, the damage is not just to your reputation. It erodes public trust in the scientific process, consumes telescope time on follow-up that goes nowhere, and makes funding bodies cautious about the next serious candidate. The 1996 Allen Hills meteorite case is the oldest lesson. NASA scientists announced that the Martian meteorite ALH84001 contained structures that resembled fossilised microbial life. The claim was global news. Over the following decade, each proposed biosignature in that rock, the polycyclic aromatic hydrocarbons, the magnetite crystals, the carbonate globules, was shown to have plausible abiotic explanations. The claim was never formally retracted, but the scientific consensus moved firmly against it. The standard that has emerged from these episodes is sometimes called the Carl Sagan standard, after his dictum that extraordinary claims require extraordinary evidence. In practice, this means a detection must survive: alternative abiotic chemistry explanations, instrument calibration errors, contamination from Earth, stellar activity mimicking atmospheric signals, and independent confirmation from a second telescope or method. That is not a high bar set to frustrate discovery. It is the minimum required before the word "life" earns its place in a headline.
The phosphine episode on Venus and the ALH84001 meteorite case look like failures. They were not. Each one sharpened the criteria, forced better instrument calibration protocols, and produced a clearer map of what a genuine detection would need to show. A biosignature claim that survives the gauntlet those episodes created will be the most verified finding in the history of science, because the field learned, from the wrong answers, exactly what right looks like.