Scaling K2. III. Comparable Planet Occurrence in the FGK Samples of Campaign 5 and Kepler

TL;DR

This study compares planet occurrence rates between K2 Campaign 5 and Kepler samples, finding similar overall occurrence suggesting Kepler data may serve as a Galactic baseline, with hints of metallicity influence on planet formation.

Contribution

It provides the first direct comparison of planet occurrence rates between K2 Campaign 5 and Kepler, highlighting similarities and potential metallicity effects.

Findings

Similar planet occurrence rates in K2 and Kepler samples.

Evidence supporting Kepler as a Galactic baseline.

Weak metallicity-driven formation trend observed.

Abstract

Using our K2 Campaign 5 fully automated planet detection data set (43 planets), which has corresponding measures of completeness and reliability, we infer an underlying planet population model for the FGK dwarfs sample (9,257 stars). Implementing a broken power-law for both the period and radius distribution, we find an overall planet occurrence of $1.00^{+1.07}_{-0.51}$ planets per star within a period range of 0.5-38 days. Making similar cuts and running a comparable analysis on the Kepler sample (2,318 planets; 94,222 stars), we find an overall occurrence of $1.10\pm0.05$ planets per star. Since the Campaign 5 field is nearly 120 angular degrees away from the Kepler field, this occurrence similarity offers evidence that the Kepler sample may provide a good baseline for Galactic inferences. Furthermore, the Kepler stellar sample is metal-rich compared to the K2 Campaign 5 sample, thus a finding of occurrence parity may reduce the role of metallicity in planet formation. However, a weak ($1.5\sigma$) difference, in agreement with metal-driven formation, is found when assuming the Kepler model power-laws for the K2 Campaign 5 sample and optimizing only the planet occurrence factor. This weak trend indicates further investigation of metallicity dependent occurrence is warranted once a larger sample of uniformly vetted K2 planet candidates are made available.