By Orbital Team
Oxygen, methane, and phosphine have all been proposed as biosignature gases, molecules that on Earth are produced overwhelmingly by living organisms. Detecting one in an exoplanet's atmosphere is exciting, and also, on its own, close to meaningless as proof of life.
The problem is that every proposed biosignature gas also has at least one plausible abiotic source. Oxygen can build up in a planet's atmosphere through the photolysis of water vapor by starlight, with no biology involved, particularly around certain types of small, active stars. Phosphine has proposed volcanic and photochemical pathways that do not require life either.
This means a single gas detection is never sufficient on its own. What the field actually looks for is a combination of gases that would be difficult to sustain together through non biological chemistry alone, such as oxygen and methane coexisting at levels that would otherwise react away, a combination that on Earth is maintained only because both are constantly replenished by living organisms.
Context matters as much as chemistry: the type of host star, its age, its flare activity, and the planet's likely geological history all shift how plausible an abiotic explanation is for any given detection.
The honest current state of the field is that no detection to date has cleared the bar required to rule out every abiotic explanation. That is not a failure, it is the field correctly refusing to declare victory on a question that, if answered wrong publicly, would be difficult to walk back credibly.