Simulating Roman Astrometry of Terra Hunting Stars

Discovering Solar-system analogs may be an essential step in finding habitable planets. However, Earth-mass planets on year-long orbits and gas giants on decade-long orbits lie at the edge of current detection limits. The Nancy Grace Roman Space Telescope, scheduled to launch in late 2025, will be capable of precision astrometry using its wide field imager. For bright stars, Roman may be able to achieve astrometric accuracy of 1-10 μas. The Terra Hunting Experiment (THE), starting in 2022, will take nightly radial velocity observations of at least 40 bright G and K dwarfs for 10 years, in search of planets that are Earth-like in mass and temperature. We have simulated an observing program that combines Roman astrometry observations with THE radial velocity measurements. These astrometric measurements can break the mass-inclination degeneracy in radial velocity observations. Additionally, astrometry and radial velocity measurements are complementary: the radial velocity method is most sensitive to planets close to their host star, while the astrometric signal is largest for more distant planets. Definitive astrometric measurements require that we trace stellar motion over a significant fraction of the orbital period of the planet. By combining measurements from a Roman astrometry observing program with Gaia data, we can increase the baseline of astrometric observations in order to discover gas giants on Jupiter-like orbits. As Jupiter has played a critical role in the habitability of Earth, discovering Jupiter-analogues around THE stars could help in determining the habitability of Earth-analogues that may be discovered.