Constraining Exoplanet Mass from Transmission Spectroscopy

Weighing Up Exoplanets The mass of a planet is important to know, but it is difficult to determine for an exoplanet. If a transmission spectrum of an exoplanet is available, de Wit and Seager (p. 1473) show that it is possible to determine its mass based on the properties of its atmosphere. The method is suited for low-density planets orbiting bright or large stars, and it is complementary to other mass-retrieval methods. The mass of an exoplanet can be derived based on the properties of its atmosphere. Determination of an exoplanet’s mass is a key to understanding its basic properties, including its potential for supporting life. To date, mass constraints for exoplanets are predominantly based on radial velocity (RV) measurements, which are not suited for planets with low masses, large semimajor axes, or those orbiting faint or active stars. Here, we present a method to extract an exoplanet’s mass solely from its transmission spectrum. We find good agreement between the mass retrieved for the hot Jupiter HD 189733b from transmission spectroscopy with that from RV measurements. Our method will be able to retrieve the masses of Earth-sized and super-Earth planets using data from future space telescopes that were initially designed for atmospheric characterization.