The Occurrence Rate of Small Planets around Small Stars (1302.1647v2)

Abstract: We use the optical and near-infrared photometry from the Kepler Input Catalog to provide improved estimates of the stellar characteristics of the smallest stars in the Kepler target list. We find 3897 dwarfs with temperatures below 4000K, including 64 planet candidate host stars orbited by 95 transiting planet candidates. We refit the transit events in the Kepler light curves for these planet candidates and combine the revised planet/star radius ratios with our improved stellar radii to revise the radii of the planet candidates orbiting the cool target stars. We then compare the number of observed planet candidates to the number of stars around which such planets could have been detected in order to estimate the planet occurrence rate around cool stars. We find that the occurrence rate of 0.5-4 Earth radius planets with orbital periods shorter than 50 days is 0.90 (+0.04/-0.03) planets per star. The occurrence rate of Earth-size (0.5-1.4 Earth radius) planets is constant across the temperature range of our sample at 0.51 (+0.06/-0.05) Earth-size planets per star, but the occurrence of 1.4-4 Earth radius planets decreases significantly at cooler temperatures. Our sample includes 2 Earth-size planet candidates in the habitable zone, allowing us to estimate that the mean number of Earth-size planets in the habitable zone is 0.15 (+0.13/-0.06) planets per cool star. Our 95% confidence lower limit on the occurrence rate of Earth-size planets in the habitable zones of cool stars is 0.04 planets per star. With 95% confidence, the nearest transiting Earth-size planet in the habitable zone of a cool star is within 21 pc. Moreover, the nearest non-transiting planet in the habitable zone is within 5 pc with 95% confidence.

• The paper refines stellar parameters for 3897 cool dwarf stars using Kepler data, leading to reduced estimates for stellar radii and consequently smaller radii for 95 planet candidates.
• The study calculates an occurrence rate of approximately 0.90 planets (0.5-4 Earth radii) per star for cool stars with orbital periods less than 50 days, including 0.51 Earth-size planets (0.5-1.4 Earth radii) per star.
• Analysis suggests the nearest transiting Earth-size planet in a cool star’s habitable zone is likely within 21 parsecs, offering a target for future observational missions like JWST.

Analysis of the Occurrence Rate of Small Planets Around Cool Stars

The paper "The Occurrence Rate of Small Planets around Small Stars" by Dressing and Charbonneau offers a detailed investigation into the prevalence of small planets orbiting dwarf stars, specifically utilizing data from the Kepler mission to refine the understanding of such planet occurrences. The authors focus on enhancing the characterization of cool stars, defined as those with temperatures below 4000K, and provide crucial insights into the frequency and distribution of planets around these stars.

Key Findings

1. Redefinition of Stellar Parameters: Utilizing photometric data from the Kepler Input Catalog (KIC), the authors refined the stellar parameters of 3897 cool dwarf stars, reducing the stellar radii and temperatures compared to the initial KIC estimates. The paper highlights that previous radius estimations were significantly larger—by approximately 31% on average—than what the revised calculations indicate.
2. Planetary Parameters: For 64 host stars and 95 planet candidates, the authors re-evaluated the planetary characteristics using the revised stellar parameters. This led to recalculations that generally diminished the radius of planet candidates by 29%, emphasizing an adjustment in earlier planetary data due to the improved stellar metrics.
3. Planet Occurrence Rate: The paper reports the occurrence rate of small planets in the range of 0.5-4 Earth radii with orbital periods less than 50 days to be approximately 0.90 planets per star. Earth-size planets (0.5-1.4 Earth radii) show an occurrence rate of 0.51 planets per star in the cool star sample, a notable consistency across the temperature spectrum of the evaluated stars.
4. Habitability and Proximity: Two Earth-size planet candidates in the habitable zones warrant an occurrence rate estimate for such planets around cool stars to be 0.15 Earth-size planets per star. The analysis projects, with 95% confidence, the nearest transiting Earth-size planet in a cool star’s habitable zone to lie within 21 parsecs.
5. Dependency on Stellar Temperature: The occurrence rate for stars cooler than 3723K reveals that Earth-size planets have a consistent occurrence rate across the temperature range of the paper, implying that a rise in the occurrence rate of larger planets diminishes at lower temperatures. This attribute correlates with theories of planet formation that suggest lower surface densities in cooler stars’ circumstellar disks might inhibit giant planet formation.

Implications and Speculations

The implications of this paper are multifaceted:

• Theoretical Considerations:

The research confirms and extends theoretical models suggesting that M-dwarfs have a propensity to host small, rocky planets rather than gas giants. It supports the notion that planetary systems around cooler stars can differ significantly in structure from those around hotter, solar-like stars.

• Observational Strategies:

The neighborhood of 21-parsec proximity to habitable-zone Earth-size planets around M-dwarfs is significant for future observational missions targeting exoplanet atmospheres and surface conditions. This metric provides a targeted direction for current and upcoming observational resources, like the James Webb Space Telescope (JWST), which will deeply investigate these proximities to paper atmospheric properties.

• Future Research Trajectories:

The methodology and findings invite further examination of stellar atmospheres and improved modeling of M-dwarfs, contributing to the systematic accuracy in categorizing these celestial bodies and their accompanying exoplanets. Continued refinements in stellar characterizations and models could enhance predictions of planetary configurations and their habitability potential.

Overall, this paper reaffirms the importance of red dwarf stars in the search for exoplanets, specifically those potentially akin to Earth, providing a framework for guiding future explorations and theoretical developments in exoplanet science.