Determining the Detectability of a Planet Transiting a Star of Extragalactic Origin

The search for planets orbiting other stars has recently expanded to include stars from galaxies other than the Milky Way. Claims of planets of extragalactic origin have been heavily debated in the past. With the launch of the TESS and Gaia surveys, a large sample of light curves and stellar kinematic measurements could be used to identify planet candidates around stars of extragalactic origin. In this study, 1,080 evolved stars observed by Gaia and NASA’s TESS missions with kinematics indicative of extragalactic origin were searched for planet transits. A series of injection and recovery tests were performed to measure the sensitivity of the TESS data and completeness of the sample. Although no planet transits were detected in this sample the limits found here on planet occurrence are consistent with previous studies of planet occurrence around similar host stars. Furthermore, metallicity and planet occurrence tend to be strongly correlated. As stars in the halo tend to be lower metallicity, it is therefore predicted that finding a planet transiting this population of stars will be more difficult.  In addition, it is likely that some of the stars in our sample do not originate from other galaxies. Thus, we predict ~65,000 stars must be searched with similar precision to likely detect a planet of extragalactic origin, which may be possible with TESS and Gaia data available in the near future.