False negatives in the search for extraterrestrial life

One of the central goals of astrobiology is to test the hypothesis that extraterrestrial life exists. In practice, this means seeking imperfect proxies for life, or ‘biosignatures’. Experience shows that ambiguous and contestable results are common in this field. Many astrobiologists are highly attuned to the possibility of ‘false positive’ results that incorrectly indicate the discovery of life. But what if we fail to detect life that is (or was) actually present? Such ‘false negatives’ are bound to arise and they matter because they would represent failures to recognize past or present existence of life. Unlike false positives, false negatives are not currently high on research agendas, as they do not pose immediate risks. Here we identify multiple potential sources of false negatives in the search for life, including factors related to the abundance, activity, appearance and location of life; the preservation and detectability of its observable traces; and the limitations inherent in our detection methods. We call for the development of a deliberate research strategy that systematically addresses these risks. Identifying and constraining such sources is essential to reduce the likelihood of overlooking genuine evidence of life.