A Dynamical Method for Measuring the Masses of Stars with Transiting Planets

As a planet transits the face of a star, it accelerates along theline-of-sight. The changing delay in the propagation of photons produces anapparent deceleration of the planet across the sky throughout the transit. Thispersistent transverse deceleration breaks the time-reversal symmetry in thetransit lightcurve of a spherical planet in a circular orbit around a perfectlysymmetric star. For `hot Jupiter' systems, ingress advances at a higher ratethan egress by a fraction of 10^{-4}-10^{-3}. Forthcoming space telescopes suchas Kepler or COROT will reach the sensitivity required to detect thisasymmetry. The scaling of the fractional asymmetry with stellar mass M andplanetary orbital radius R as M/R^2 is different from the scaling of theorbital period as (M/R^3)^{-1/2}. Therefore, this effect constitutes a newmethod for a purely dynamical determination of the mass of the star, which iscurrently inferred indirectly with theoretical uncertainties based on spectralmodeling. Radial velocity data for the reflex motion of the star can then beused to determine the planet's mass. Although orbital eccentricity couldintroduce a larger asymmetry than the light propagation delay, the eccentricityis expected to decay by tidal dissipation to negligible values for a close-inplanet with no perturbing third body. Future detection of the eclipse of aplanet's emission by its star could be used to measure the light propagationdelay across the orbital diameter, 46.7(R/7x10^{11}cm) seconds, and alsodetermine the stellar mass from the orbital period.Comment: 4 pages, 2 figures, submitted to ApJ