The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity (1501.01623v2)

Abstract: We present an improved estimate of the occurrence rate of small planets orbiting small stars by searching the full four-year Kepler data set for transiting planets using our own planet detection pipeline and conducting transit injection and recovery simulations to empirically measure the search completeness of our pipeline. We identified 156 planet candidates, including one object that was not previously identified as a Kepler Object of Interest. We inspected all publicly available follow-up images, observing notes, and centroid analyses, and corrected for the likelihood of false positives. We evaluated the sensitivity of our detection pipeline on a star-by-star basis by injecting 2000 transit signals into the light curve of each target star. For periods shorter than 50 days, we find 0.56 (+0.06/-0.05) Earth-size planets (1-1.5 Earth radii) and 0.46 (+0.07/-0.05) super-Earths (1.5-2 Earth radii) per M dwarf. In total, we estimate a cumulative planet occurrence rate of $2.5\pm0.2$ planets per M dwarf with radii 1-4 Earth radii and periods shorter than 200 days. Within a conservatively defined habitable zone based on the moist greenhouse inner limit and maximum greenhouse outer limit, we estimate an occurrence rate of 0.16 (+0.17/-0.07) Earth-size planets and 0.12 (+0.10/-0.05) super-Earths per M dwarf habitable zone. Adopting the broader insolation boundaries of the recent Venus and early Mars limits yields a higher estimate of 0.24 (+0.18/-0.08) Earth-size planets and 0.21 (+0.11/-0.06) super-Earths per M dwarf habitable zone. This suggests that the nearest potentially habitable non-transiting and transiting Earth-size planets are $2.6\pm0.4$ pc and 10.6 (+1.6/-1.8) pc away, respectively. If we include super-Earths, these distances diminish to $2.1\pm0.2$ pc and 8.6 (+0.7/-0.8) pc.

• The paper refines the occurrence rates of habitable exoplanets around M dwarfs using the complete Kepler dataset.
• The paper employs a custom transit detection pipeline combined with empirical transit injection tests to accurately measure detection sensitivity.
• The paper finds that M dwarfs potentially host up to 2.5 small planets per star, underscoring their importance for future habitability studies.

Estimation of Habitable Planets Orbiting M Dwarfs Using Kepler Data

The research by Dressing and Charbonneau provides a refined analysis of the occurrence rate of potentially habitable exoplanets orbiting M dwarfs using the complete dataset from the Kepler mission. Through a meticulous application of a bespoke transit detection pipeline coupled with empirical measurements of detection sensitivity via transit injection and recovery simulations, the authors reassess previous estimations of exoplanet populations around low-mass stars.

The paper's methodological rigor is exemplified by its comprehensive use of the entire four-year Kepler dataset, offering a substantial basis for its conclusions. The authors identified 156 exoplanet candidates, of which one was a novel identification. Detection sensitivity was evaluated using star-specific transit injection tests, addressing both photometric data integrity and potential biases in detection efficiency.

Key results indicate that M dwarfs host +0.06 Earth-sized and +0.07 super-Earth-sized planets with orbital periods less than 50 days per star. Furthermore, for radii between 1 and 4 Earth radii and periods up to 200 days, an occurrence rate of approximately 2.5 planets per M dwarf is determined. Within a conservatively defined habitable zone, the occurrence rate is estimated around +0.17 Earth-sized and +0.10 super-Earth-sized planets per star, which slightly increases under broader insolation limits.

These findings carry significant implications for M dwarf system studies, suggesting the relatively high prevalence of small planets with potential habitability. This prevalence underscores the strategic value of M dwarfs as subjects in the pursuit of biosignature detection and characterization, leveraging future observational technologies to exploit this abundance effectively. These results provide pivotal insights for upcoming missions and represent a crucial reference point for framing scientific strategies targeting exoplanet habitability around M dwarfs.

Moreover, the analysis addresses inherent systematic biases arising from stellar parameter uncertainties. By incorporating empirical measurements into their models, the authors provide a nuanced understanding that fine-tunes search completeness and consequently enriches the estimates of habitable planet frequencies.

Going forward, the implications of this paper can shape the future trajectory of observational campaigns targeting small stars. By characterizing occultations in this stellar category, scientists can refine models of planet formation and evolution. Future advancements in direct imaging capabilities and high-precision radial velocity methods will further validate these findings, potentially unveiling more about atmospheric properties and composition, thus refining habitability constraints.

In summation, this paper substantiates the potential ubiquity of habitable planets around M dwarfs, fortifying theoretical models and guiding empirical pursuits in exoplanet research. Its methodological foresight and nuanced dataset utilization set a benchmark for subsequent explorations in the broader astronomical community.