As a planet transits the face of a star , it accelerates along the line-of-sight .
The changing delay in the propagation of photons produces an apparent deceleration of the planet across the sky throughout the transit .
This persistent transverse deceleration breaks the time-reversal symmetry in the transit lightcurve of a spherical planet in a circular orbit around a perfectly symmetric star .
For “ hot Jupiter ” systems , ingress advances at a higher rate than egress by a fraction \sim 10 ^ { -4 } – 10 ^ { -3 } .
Forthcoming space telescopes such as Kepler or COROT will reach the sensitivity required to detect this asymmetry .
The scaling of the fractional asymmetry with stellar mass M _ { \star } and planetary orbital radius a as \propto M _ { \star } / a ^ { 2 } is different from that of the orbital period as \propto ( M _ { \star } / a ^ { 3 } ) ^ { -1 / 2 } .
Therefore , this effect constitutes a new method for a purely dynamical determination of the mass of the star , which is currently inferred indirectly with theoretical uncertainties based on spectral modeling .
Radial velocity data for the reflex motion of the star can then be used to determine the planet ’ s mass .
Although orbital eccentricity could introduce a larger asymmetry than the light propagation delay , the eccentricity is expected to decay by tidal dissipation to negligible values for a close-in planet with no perturbing third body .
Future detection of the eclipse of a planet ’ s emission by its star could be used to measure the light propagation delay across the orbital diameter , 46.7 ( a / 7 \times 10 ^ { 11 } ~ { } { cm } ) seconds , and also determine the stellar mass from the orbital period .