Analytical relations are derived for the amplitude of astrometric , photometric and radial velocity perturbations caused by a single rotating spot .
The relative power of the star spot jitter is estimated and compared with the available data for \kappa ^ { 1 } Ceti and HD 166435 , as well as with numerical simulations for \kappa ^ { 1 } Ceti and the Sun .
A Sun-like star inclined at i = 90 \arcdeg at 10 pc is predicted to have a RMS jitter of 0.087 \mu as in its astrometric position along the equator , and 0.38 m s ^ { -1 } in radial velocities .
If the presence of spots due to stellar activity is the ultimate limiting factor for planet detection , the sensitivity of SIM Lite to Earth-like planets in habitable zones is about an order of magnitude higher that the sensitivity of prospective ultra-precise radial velocity observations of nearby stars .