Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin

TL;DR

This study searches for transiting planets around extragalactic halo stars using Gaia and TESS data, setting upper limits on planet occurrence and predicting the need for larger samples for future detections.

Contribution

It introduces a method to constrain planet occurrence rates around extragalactic stars using combined Gaia and TESS data, and provides upper limits based on non-detections.

Findings

No planets detected in the sample.

Upper limit on hot Jupiter occurrence is 0.52%.

Approximately 7,000 stars need to be surveyed for potential detections.

Abstract

The search for planets orbiting other stars has recently expanded to include stars from galaxies outside the Milky Way. With the TESS and Gaia surveys, photometric and kinematic information can be combined to identify transiting planet candidates of extragalactic origin. Here, 1,080 low-luminosity red giant branch stars observed by Gaia and TESS with kinematics suggesting a high likelihood of extragalactic origin were searched for planet transits. Transit injection-recovery tests were performed to measure the sensitivity of the TESS data and completeness of the transit search. Injected signals of planets larger than Jupiter with orbital periods of 10 days or less were recovered in $\approx$44% of cases. Although no planet transits were detected in this sample, we find an upper limit on planet occurrence of 0.52% for hot Jupiters, consistent with previous studies of planet occurrence around similar host stars. As stars in the halo tend to be lower metallicity, and short period giant planet occurrence tends to be strongly correlated with stellar metallicity, we predict that relative to the Galactic disk population, a smaller fraction of halo stars will host planets detectable by transit surveys. Thus, applying the known planet occurrence trends to potential planet detection around halo stars, we predict $\gtrsim$7,000 stars must be searched with similar cadence and precision as the stars studied here before a detection of a planet of extragalactic origin is likely. This may be possible with future data releases from the TESS and Gaia missions.