An Understanding of the Shoulder of Giants: Jovian Planets around Late K Dwarf Stars and the Trend with Stellar Mass

TL;DR

This study investigates the occurrence rate of giant planets around late K dwarf stars using four years of Doppler data, revealing a trend of increasing giant planet frequency with stellar mass and highlighting discrepancies with Kepler data.

Contribution

It provides the first estimate of giant planet occurrence around late K dwarfs and compares Doppler and Kepler survey results, suggesting a positive correlation with stellar mass.

Findings

4.0+/-2.3% of late K dwarfs host giant planets with periods <245 days.

Kepler survey estimates a lower occurrence rate than Doppler data, but the difference depends on system specifics.

The results support a trend of increasing giant planet occurrence with stellar mass.

Abstract

Analyses of exoplanet statistics suggest a trend of giant planet occurrence with host star mass, a clue to how planets like Jupiter form. One missing piece of the puzzle is the occurrence around late K dwarf stars (masses of 0.5-0.75Msun and effective temperatures of 3900-4800K). We analyzed four years of Doppler radial velocities data of 110 late K dwarfs, one of which hosts two previously reported giant planets. We estimate that 4.0+/-2.3% of these stars have Saturn-mass or larger planets with orbital periods <245d, depending on the planet mass distribution and RV variability of stars without giant planets. We also estimate that 0.7+/-0.5% of similar stars observed by Kepler have giant planets. This Kepler rate is significantly (99% confidence) lower than that derived from our Doppler survey, but the difference vanishes if only the single Doppler system (HIP 57274) with completely resolved orbits is considered. The difference could also be explained by the exclusion of close binaries (without giant planets) from the Doppler but not Kepler surveys, the effect of long-period companions and stellar noise on the Doppler data, or an intrinsic difference between the two populations. Our estimates for late K dwarfs bridge those for solar-type stars and M dwarfs and support a positive trend with stellar mass. Small sample size precludes statements about finer structure, e.g. a "shoulder" in the distribution of giant planets with stellar mass. Future surveys such as the Next Generation Transit Survey and the Transiting Exoplanet Satellite Survey will ameliorate this deficiency.