Abstract
Abstract We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define η ⊕ as the HZ occurrence of planets with radii between 0.5 and 1.5 R ⊕ orbiting stars with effective temperatures between 4800 and 6300 K. We find that η ⊕ for the conservative HZ is between (errors reflect 68% credible intervals) and planets per star, while the optimistic HZ occurrence is between and planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ∼6 pc away and there are ∼4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.
Highlights
One of the primary goals of the Kepler mission (Borucki et al 2010; Koch et al 2010; Borucki 2016) is to determine the frequency of occurrence of habitablezone rocky planets around Sun-like stars, known as “η⊕”
We present occurrence rates for rocky planets in the habitable zones (HZ) of main-sequence dwarf stars based on the Kepler data release 25 (DR25) planet candidate catalog and Gaia-based stellar properties
We perform our analysis for the two extrapolation cases, and find that their difference in habitable zone occurrence rates is small relative to our uncertainties. In this way we provide a bounded estimate of habitable zone occurrence rates using instellation flux, rather than the unbounded extrapolation resulting from using orbital period
Summary
One of the primary goals of the Kepler mission (Borucki et al 2010; Koch et al 2010; Borucki 2016) is to determine the frequency of occurrence of habitablezone rocky planets around Sun-like stars, known as “η⊕”. Measuring η⊕ informs theories of planet formation, helping us to understand why we are here, and is an important input to mission design for instruments designed to detect and characterize habitable-zone planets such as LUVOIR (The LUVOIR Team 2019) and HabEX (Gaudi et al 2020). Kepler’s strategy to measure η⊕ was to continuously observe >150,000 Solar-like main-sequence dwarf stars (primarily F, G, and K) with a highly sensitive photometer in Solar orbit, identifying planets through the detection of transits. The planet catalog in the final Kepler data release 25 (DR25)
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