What unique planetary property does astrometry determine that radial velocity only estimates as minimum mass?
The planet's true mass
Astrometry offers a crucial advantage over methods like radial velocity because it allows astronomers to determine the exoplanet's true mass. Radial velocity methods only provide a minimum mass estimate because they cannot independently determine the orbital inclination—the tilt of the orbit relative to our line of sight. When an orbit is viewed nearly face-on, the radial velocity signal (movement toward or away from us) is severely underestimated. Astrometry, by measuring the sideways wobble of the star, captures the necessary component of motion. When astrometric data is combined with radial velocity measurements, the orbital inclination can be solved for, yielding the planet’s actual, or true, mass. This is vital for distinguishing between a true, low-mass planet and a more massive companion like a brown dwarf.
