The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data (2010.14812v2)

Abstract: We present occurrence rates for rocky planets in the habitable zones (HZ) 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 $\eta_\oplus$ as the HZ occurrence of planets with radius between 0.5 and 1.5 $R_\oplus$ orbiting stars with effective temperatures between 4800 K and 6300 K. We find that $\eta_\oplus$ for the conservative HZ is between $0.37^{+0.48}_{-0.21}$ (errors reflect 68\% credible intervals) and $0.60^{+0.90}_{-0.36}$ planets per star, while the optimistic HZ occurrence is between $0.58^{+0.73}_{-0.33}$ and $0.88^{+1.28}_{-0.51}$ 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 using both a Poisson likelihood Bayesian analysis and 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 about 6 pc away, and there are about 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.

• The paper quantifies the occurrence rate of rocky habitable-zone planets around solar-like stars using Kepler DR25 and Gaia data.
• It employs instellation flux analysis with Poisson likelihood and Approximate Bayesian Computation to correct for biases and catalog incompleteness.
• Findings indicate occurrence rates (η⊕) of 0.37–0.60 (conservative) and 0.58–0.88 (optimistic), informing the design of future exoplanet missions.

Analyzing the Occurrence of Rocky Habitable Zone Planets from Kepler Data

This paper, authored by Bryson et al., presents a detailed analysis of the occurrence rates of rocky planets within the habitable zones (HZ) of solar-like stars, leveraging data from the Kepler mission. The primary aim is to quantify the frequency with which Earth-sized planets in potentially habitable zones occur around main-sequence dwarf stars, specifically those that are similar to the sun, i.e., F, G, and K stars.

Methodology and Models

The paper employs data from Kepler's DR25 planet candidate catalog and corroborates it with Gaia-based stellar parameters. The authors present an analysis based on instellation flux — the flux of stellar radiation received by a planet — allowing them to dynamically track planets in HZs of various stars. They define the parameter $\eta_\oplus$, which quantifies the rate of occurrence of such rocky, habitable-zone (HZ) planets around stars with effective temperatures ranging from 4800K to 6300K and planet radii between 0.5 and 1.5 Earth radii.

In their approach, two statistical methods are used — Poisson likelihood Bayesian analysis and Approximate Bayesian Computation (ABC) — to derive the planet occurrence rates from the available data. These methods correct for biases introduced by catalog completeness and reliability limitations. By utilizing these probabilistic models, they account for the observational gaps and errors inherent in the Kepler dataset.

Results

The authors report an estimated range for $\eta_\oplus$, the occurrence rate for planets in the conservative habitable zone, between 0.37 and 0.60 planets per star, whereas for an optimistic HZ model, this estimate stands between 0.58 and 0.88 planets per star. The variability in these estimates is primarily due to the assumptions made concerning completeness extrapolation for those orbital periods not directly covered by the available data.

A significant challenge highlighted is the large uncertainties in occurrence rates, attributed to the relatively small number of directly detected small HZ planets. This necessitates careful completeness corrections to derive reliable statistics.

Implications

The findings have a dual impact: advancing theoretical models of planet formation and guiding the design of future astronomical missions focused on detecting and characterizing exoplanets in habitable zones, such as potential successors to the Kepler mission or future initiatives like LUVOIR or HabEX.

Future Directions

This paper underlines the necessity for improved observational data, either through enhanced algorithms for existing datasets or future missions with longer observational timelines to increase completeness and thereby reduce uncertainties. Only through such measures can more precise estimations of exoplanetary habitability rates be attained, offering firmer ground for both scientific inquiry and the search for extraterrestrial life.

The research underscores the nuanced understanding required of the interplay between stellar characteristics and planetary habitability. It also hints at further refining the models accounting for varied stellar properties that might impact the instellation flux, potentially affecting the perceived habitability of exoplanets.

In conclusion, the methodologies and analysis presented in this paper significantly contribute to the groundwork for estimating how common Earth-like planets may be in our galaxy, marking an important step in the broader endeavor of astrobiology and planetary science.