Accounting for multiplicity in calculating eta Earth

Abstract

Using the updated exoplanet population parameters of our previous study, which includes the planetary radius updates from Gaia DR2 and an inferred multiplicity distribution, we provide a revised η_⊕ calculation. This is achieved by sampling planets from our derived population model and determining which planets meet our criterion for habitability. To ensure robust results, we provide probabilities calculated over a range of upper radius limits. Our most optimistic criterion for habitability provides an η_⊕ value of 0.34± 0.02 planets/star. We also consider the effects of multiplicity and the number of habitable planets each system may contain. Our calculation indicates that 6.4± 0.5{{ per cent}} of GK dwarfs have more than one planet within their habitable zone. This optimistic habitability criterion also suggests that 0.036± 0.009{{ per cent}} of solar-like stars will harbour five or more habitable planets. These tightly packed highly habitable systems should be extremely rare, but still possible. Even with our most pessimistic criterion, we still expect that 1.8± 0.2{{ per cent}} of solar-like stars harbour more than one habitable planet.