How to Weigh a Star Using a Moon

TL;DR

This paper demonstrates that in transiting star-planet-moon systems, the absolute masses and radii of all components can be determined using lightcurve analysis, radial velocities, and dynamical effects, enabling precise characterization.

Contribution

It introduces a method to derive absolute dimensions of star, planet, and moon in transiting systems with moons, expanding the potential for system calibration and satellite composition analysis.

Findings

Absolute masses and radii of star, planet, and moon are determinable.

The method allows for calibration of stellar evolution models.

Dynamical effects enable mass measurement of the moon.

Abstract

We show that for a transiting exoplanet accompanied by a moon which also transits, the absolute masses and radii of the star, planet and moon are determinable. For a planet-star system, it is well known that the density of the star is calculable from the lightcurve by manipulation of Kepler's Third Law. In an analogous way, the planetary density is calculable for a planet-moon system which transits a star, and thus the ratio-of-densities is known. By combining this ratio with the observed ratio-of-radii and the radial velocity measurements of the system, we show that the absolute dimensions of the star and planet are determinable. This means such systems could be used as calibrators of stellar evolution. The detection of dynamical effects, such as transit timing variations, allows the absolute mass of the moon to be determined as well, which may be combined with the radius to infer the satellite's composition.