  Here we advocate an observational strategy to help prioritize exoplanet
observations. It starts with more easily obtained observational data, and ranks
exoplanets for more difficult follow-up observations based on the likelihood of
avoiding planets for which oxygen is a false positives or even an inconclusive
signature of life. We find that for oxygen to be a robust biosignature, both
land and surface water must be present. Landless exoplanets have much slower
biogeochemical cycles, so while oxygenic photosynthesizing life could exist on
such planets, it could not produce oxygen at a rate competitive with abiotic
rates such as photolysis. These habitable planets, whose life would not be
detectable, should be avoided.
