A Catalog of Kepler Habitable Zone Exoplanet Candidates (1608.00620v2)

Abstract: The NASA Kepler mission has discovered thousands of new planetary candidates, many of which have been confirmed through follow-up observations. A primary goal of the mission is to determine the occurrance rate of terrestrial-size planets within the Habitable Zone (HZ) of their host stars. Here we provide a list of HZ exoplanet candidates from the Kepler Data Release 24 Q1-Q17 data vetting process. This work was undertaken as part of the Kepler Habitable Zone Working Group. We use a variety of criteria regarding HZ boundaries and planetary sizes to produce complete lists of HZ candidates, including a catalog of 104 candidates within the optimistic HZ and 20 candidates with radii less than two Earth radii within the conservative HZ. We cross-match our HZ candidates with the Data Release 25 stellar properties and confirmed planet properties to provide robust stellar parameters and candidate dispositions. We also include false positive probabilities recently calculated by Morton et al. (2016) for each of the candidates within our catalogs to aid in their validation. Finally, we performed dynamical analysis simulations for multi-planet systems that contain candidates with radii less than two Earth radii as a step toward validation of those systems.

An Overview of Kepler Habitable Zone Exoplanet Candidates

The paper delineates a systematic pursuit of terrestrial-sized exoplanets located within the habitable zones (HZ) of their host stars using data from the Kepler Space Telescope. The core objective of the research was to determine the frequency of such planets, termed as eta-Earth ($\eta_\oplus$). The Kepler mission, through the Data Release 24 Q1-Q17 vetting process, triggered by the Kepler Habitable Zone Working Group, enabled the identification of HZ exoplanet candidates. The catalog produced includes 104 candidates within the optimistic HZ and 20 candidates with radii less than 2 Earth radii in the conservative HZ, thereby fostering a significant leap in cataloging potentially habitable planets.

Kepler Data and Candidate Vetting

Data from the NASA Exoplanet Archive, primarily the DR24 table of candidates, served as the foundation for the identification process. This was supplemented with the most recent stellar parameters from the DR25 catalog to ensure a reliable list of HZ candidates with well vetted properties. The false positive probabilities (FPP) calculated in recent studies were also included in the analysis to solidify the identification of these candidates.

Habitable Zone Boundaries

The research revisits the conventional idea of the HZ, defined as the region around a star where liquid water could persist on a planet's surface. The paper adopts different criteria for HZ boundaries considering updated climate models - both 1-D and evolving 3-D models. The conservative HZ is defined as 0.99-1.7 AU while the optimistic HZ extends to 0.75-1.8 AU. Adjustments to HZ boundaries based on greenhouse gases and the albedo effect under varying stellar irradiance were also considered significant.

Candidate Categorization and Distribution

Candidates were categorized into four groups concerning size and location within the HZ, namely:

• Category 1: In the conservative HZ and $R_p < 2 R_\oplus$.
• Category 2: In the optimistic HZ and $R_p < 2 R_\oplus$.
• Category 3: In the conservative HZ and any size.
• Category 4: In the optimistic HZ and any size.

The research uncovered 20 exoplanets in Category 1 and 104 across Category 4. Detailed tables listing identifiers, stellar properties, and corresponding planetary parameters were composed for these categories. There was no substantial difference in radius distribution between candidates within the HZ and the broader Kepler candidate population, aligned with the completion limitations and potential systematic noise impacting the data.

Dynamical Stability Assessments

A pivotal part of validating these candidates lies within their dynamical stability, particularly for multi-planet systems. For planets below 2 Earth radii within the optimistic HZ, comprehensive dynamical simulations were undertaken. Assumptions regarding orbital eccentricities and planetary compositions were scrutinized for their effect on system stability over Gyr timescales.

Implications and Future Work

The categorization and validation exercises expand the characterization of planetary objects within the HZ, providing a groundwork for follow-up observations and further analysis. Importantly, this endeavor underscores the necessity of utilizing candidate catalogs to refine target selection criteria for direct imaging and characterization of exoplanets. The synthesis of results suggests that, considering observational constraints, Earth-sized planets in the HZ of Sun-like stars could be relatively common, which has profound implications for both future observational missions and the quest for extraterrestrial life.

The findings and methods from this paper pose questions for future endeavors, particularly concerning the influence of more detailed atmospheric compositions and climatic feedback mechanisms on the exact definition of the HZ. As models continue to evolve, the observational capabilities will also need alignment to ensure robust risk assessments and characterization of potential habitats beyond Earth.