  'Most habitable worlds in the cosmos will have no remotely detectable signs
of life' is proposed as a biological hypothesis to be tested in studies of
exoplanets. Habitable planets could be discovered elsewhere in the Universe,
yet there are many hypothetical scenarios whereby the search for life on them
could yield negative results. Scenarios for habitable worlds with no remotely
detectable signatures of life include: planets that are habitable, but have no
biosphere (Uninhabited Habitable Worlds); planets with life, but lacking any
detectable surface signatures of that life (laboratory examples are provided)
and planets with life, where the concentration of atmospheric gases produced or
removed by biota are impossible to disentangle from abiotic processes because
of the lack of detailed knowledge of planetary conditions (the 'problem of
exoplanet thermodynamic uncertainty'). A rejection of the hypothesis would
require that the origin of life usually occurs on habitable planets, that
spectrally detectable pigments and/or metabolisms that produce unequivocal
biosignature gases (e.g. oxygenic photosynthesis) usually evolve and that the
organisms that harbour them usually achieve a sufficient biomass to produce
biosignatures detectable to alien astronomers.
