Upcoming space-based photometric satellites offer the possibility of detecting continuum flux variability at the micro-magnitude ( \mu { mag } ) level .
We show that the Doppler flux variability induced by the reflex motion of stars due to planetary companions has an amplitude of ( 3 - \alpha ) K / c , where K is the reflex radial velocity amplitude and \alpha \approx d \ln F _ { \nu } / d \ln \nu is the logarithmic slope of the source spectral flux in the observed frequency band .
For many of the known close-in planetary systems with periods P \lesssim 0.2 ~ { } { yr } , the periodic Doppler variability , { \cal O } ( \mu { mag } ) , is significant relative to the variability caused by reflected light from the planetary companion .
For companions with P \gtrsim 0.2 ~ { } { yr } , the Doppler signal is larger than the reflected light signal .
We show that the future photometric satellites should reach the sensitivity to detect this Doppler variability .
In particular , the Kepler 
satellite should have the photon noise sensitivity to detect at a signal-to-noise ratio S / N \gtrsim 5 , all planets with minimum mass M _ { p } \sin i \gtrsim 5 M _ { J } and P \lesssim 0.1 ~ { } { yr } around the \sim 10 ^ { 4 } 
main-sequence stars with spectral types A through K and apparent magnitude V < 12 in its field-of-view .