Orbital Inclinations, True Masses, and System Architectures of Long-Period Giant Planets: New Constraints with Hipparcos-Gaia Astrometry

Radial velocity surveys have uncovered a growing number of long period (> 5 year) giant exoplanets in recent years, but measuring the orbital inclinations of these objects remains challenging. Gaia will provide inclinations and true masses for many of these long-period companions, but this data is not yet available. However, combining proper motion data from Gaia and the earlier Hipparcos mission allows for measurements of orbital inclinations in favourable cases. We highlight examples where Hipparcos-Gaia astrometry provides constraints on mutual inclinations in systems that also include transiting planets, such as the π Mensae system. We also discuss systems with transiting planets where astrometric accelerations suggest the presence of unidentified long-period companions. Lastly we discuss cases where Hipparcos-Gaia astrometry reveals that supposed long-period planets are not planets at all, but are instead stars or brown dwarfs observed at near-polar orbital inclinations. These results provide a dress rehearsal for the full release of Gaia astrometry which will provide constraints for comparatively lower mass planets at shorter orbital periods, providing deep constraints on the architectures of exoplanetary systems.
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