Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data (1102.0541v2)

Abstract: On 1 February 2011 the Kepler Mission released data for 156,453 stars observed from the beginning of the science observations on 2 May through 16 September 2009. There are 1235 planetary candidates with transit like signatures detected in this period. These are associated with 997 host stars. Distributions of the characteristics of the planetary candidates are separated into five class-sizes; 68 candidates of approximately Earth-size (radius < 1.25 Earth radii), 288 super-Earth size (1.25 Earth radii < radius < 2 Earth radii), 662 Neptune-size (2 Earth radii < radius < 6 Earth radii), 165 Jupiter-size (6 Earth radii < radius < 15 Earth radii), and 19 up to twice the size of Jupiter (15 Earth radii < radius < 22 Earth radii). In the temperature range appropriate for the habitable zone, 54 candidates are found with sizes ranging from Earth-size to larger than that of Jupiter. Five are less than twice the size of the Earth. Over 74% of the planetary candidates are smaller than Neptune. The observed number versus size distribution of planetary candidates increases to a peak at two to three times Earth-size and then declines inversely proportional to area of the candidate. Our current best estimates of the intrinsic frequencies of planetary candidates, after correcting for geometric and sensitivity biases, are 6% for Earth-size candidates, 7% for super-Earth size candidates, 17% for Neptune-size candidates, and 4% for Jupiter-size candidates. Multi-candidate, transiting systems are frequent; 17% of the host stars have multi-candidate systems, and 33.9% of all the candidates are part of multi-candidate systems.

• The paper presents an extensive analysis identifying 997 planetary candidates among 156,453 stars using four months of Kepler data.
• It classifies candidates into five size groups, revealing that over 74% are smaller than Neptune, with detailed frequency estimates.
• It outlines rigorous vetting methods like centroid analysis and radial velocity measurements, setting the stage for future exoplanet studies.

Analysis of Kepler's First Four Months of Planetary Candidate Data

The paper presents an extensive analysis of planetary candidates identified by the Kepler mission during its first four months of scientific operation. Covering data from May to September 2009, it provides a comprehensive overview of the transit-like signatures detected in 156,453 stars, identifying 997 host stars associated with planetary candidates. The paper involves an in-depth classification of these candidates by size and provides frequency estimates accounting for geometric and sensitivity biases.

Size Distribution and Frequency of Planetary Candidates

The planetary candidates have been categorized into five size classes: Earth-size, super-Earth-size, Neptune-size, Jupiter-size, and candidates larger than Jupiter. Key findings indicate that more than 74% of the planetary candidates are smaller than Neptune, highlighting the capability of the Kepler data to detect smaller exoplanets. For detailed categorization, 68 candidates are approximately Earth-sized, 288 are super-Earth-sized, 662 are Neptune-sized, 165 are Jupiter-sized, and 19 are larger than Jupiter. Particularly noteworthy is the calculated intrinsic frequency of these candidates, with Neptune-sized bodies showing the highest occurrence.

Habitable Zone Candidates

In the context of searching for habitable worlds, the analysis identifies 54 candidates residing within their respective stars' habitable zones, with six being less than twice the size of Earth. However, the frequency and identification of these Earth-like candidates indicate that small candidates are significantly underrepresented when compared to larger ones, primarily due to detection limitations and observational biases.

Methodological Overview

Within the paper, considerable attention is given to the methodology employed for candidate validation and the effort to minimize false positives. The Kepler team utilized high precision RV measurements and transit timing variations while also developing a robust centroid analysis to distinguish between true planets and background eclipsing binaries. Despite these rigorous vetting processes, the paper presents an estimated false positive rate and acknowledges false positives as a persistent challenge.

Implications and Future Directions

The analysis provides a baseline for understanding the distribution and characteristics of exoplanets, establishing important implications for the paper of planetary system architectures and formation theories. It suggests a promising future as improvements in data reduction and analysis techniques are expected to unveil even more candidates, indicating that many smaller and less common candidates may still be detected with continued observations.

Additionally, the frequency estimates for close-in, small-period candidates suggest potentially distinct population dynamics resulting from planet migration or formation processes.

Multi-Planetary Systems

The paper further discusses the prevalence of multi-candidate systems, with 17% of host stars showing multiple transit-like signatures. The simultaneous detection of multiple candidates around a single star offers a unique opportunity to examine planetary systems' co-planar nature and dynamics, reinforcing existing models of planetary formation while providing insights into possible gaps or anomalies in these models.

Conclusion

This rigorous analysis of the initial four months of Kepler data marks a foundational step in the statistical paper of exoplanets. It significantly informs the design of future missions aimed at direct imaging or in-depth spectroscopic studies of exoplanetary atmospheres. Moving forward, as the vetting process matures and more data becomes available, the Kepler mission's foundational results will be pivotal in confirming the frequency of Earth-size planets in the galaxy, thereby fostering a deeper understanding of planetary habitability and the potential for life elsewhere in the universe.