Periodic Flux Variability of Stars due to the Reflex Doppler Effect Induced by Planetary Companions

Upcoming space-based photometric satellites offer the possibility ofdetecting continuum flux variability at the micro-magnitude level. We show thatthe Doppler flux variability induced by the reflex motion of stars due toplanetary companions has an amplitude of (3-alpha)K/c, where K is the reflexradial velocity amplitude and alpha is the logarithmic slope of source spectralflux in the observed frequency band. For many of the known close-in planetarysystems with periods less than ~0.2 years, the periodic Doppler variability isof order a micromagnitude, and is significant relative to the variabilitycaused by reflected light from the planetary companion. For companions withperiods greater than ~0.2 years, the Doppler signal is larger than thereflected light signal. We show that the future photometric satellites shouldreach the sensitivity to detect this Doppler variability. In particular, theKepler satellite should have the photon-noise sensitivity to detect at asignal-to-noise ratio > 5, all planets with minimum mass greater than 5 Jupitermasses, and periods less than 0.1 years around the 10,000 main-sequence starswith spectral types A through K and apparent magnitude V<12 in itsfield-of-view.Comment: 4 pages, 2 figures. Revised version, minor changes. Accepted to ApJL, to appear in the May 10, 2003 issu