Limits on Planetary Companions from Doppler Surveys of Nearby Stars

Abstract

Most of our knowledge of planets orbiting nearby stars comes from Doppler surveys. For spaced-based, high-contrast imaging missions, nearby stars with Doppler-discovered planets are attractive targets. The known orbits tell imaging missions where and when to observe, and the dynamically-determined masses provide important constraints for the interpretation of planetary spectra. Quantifying the set of planet masses and orbits that could have been detected will enable more efficient planet discovery and characterization. We analyzed Doppler measurements from Lick and Keck Observatories collected by the California Planet Survey. We focused on stars that are likely targets for three space-based planet imaging mission concepts studied by NASA--WFIRST-AFTA, Exo-C, and Exo-S. The Doppler targets are primarily F8 and later main sequence stars, with observations spanning 1987-2014. We identified 76 stars with Doppler measurements from the prospective mission target lists. We developed an automated planet search and a methodology to estimate the pipeline completeness using injection and recovery tests. We applied this machinery to the Doppler data and computed planet detection limits for each star as a function of planet minimum mass and semi-major axis. For typical stars in the survey, we are sensitive to approximately Saturn-mass planets inside of 1 AU, Jupiter-mass planets inside of ~3 AU, and our sensitivity declines out to ~10 AU. For the best Doppler targets, we are sensitive to Neptune-mass planets in 3 AU orbits. Using an idealized model of Doppler survey completeness, we forecast the precision of future surveys of non-ideal Doppler targets that are likely targets of imaging missions.