2022MNRAS.512.5228S

In the search for life in the Universe, exoplanets represent numerous natural experiments in planet formation, evolution, and the emergence of life. This raises the fascinating prospect of evaluating cosmic life on a statistical basis. One key statistic is the occurrence rate of life-bearing worlds, f_L, the 'frequency of life' term in the famous Drake Equation. Measuring f_L would give profound insight into how common life is and may help us to constrain origin-of-life theories. I propose f_L as the goal for the DRAKE mission (Dedicated Research for Advancing Knowledge of Exobiology): a transit spectroscopy survey of M-dwarf habitable zone terrestrial planets. I investigate how the uncertainty on the observed value of f_L scales with sample size. I determine that sampling error dominates over observational error and that the uncertainty is a function of the observed f_L value. I show that even small sample sizes can provide significant constraints on f_L, boding well for the transit spectroscopy approach. I perform a feasibility study of the DRAKE mission using a nominal instrument design and mission plan. Due to low observing efficiencies, DRAKE may need to be incorporated into a wider-ranging deep-space or lunar observatory. A 50-planet survey could constrain f_L to <= 0.06 (at 95 per cent confidence) if the sample f_L = 0, or 0.03-0.2 if the sample f_L = 0.1. This can be achieved (on average) in 10 yr using a 17-m telescope with an unrestricted field-of-regard. DRAKE is a viable approach to attempting the first experimental measurement of f_L.