Thousands of planets beyond our solar system are now known, and the galaxy is likely to contain vastly more. But despite that growing census, Earth remains the only world where life has been confirmed.
Scientists search in several ways: examining nearby worlds for signs of past or present microbes, studying exoplanet atmospheres for possible biosignatures, and listening for signals from technology. A planet in a habitable zone may allow liquid water under some conditions; that alone does not show that life is there.
No credible, independently confirmed evidence of extraterrestrial life has been found. The unknowns are enormous: how readily life begins, how long it lasts, and whether it commonly becomes complex or detectable. Even a promising chemical signal can have nonliving explanations, so researchers need multiple lines of evidence.