Starspot Jitter in Photometry, Astrometry and Radial Velocity Measurements

Analytical relations are derived for the amplitude of astrometric, photometric, and radial velocity ( RV) perturbations caused by a single rotating spot. The relative power of the starspot 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 degrees at 10 pc is predicted to have an 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 than the sensitivity of prospective ultra-precise RV observations of nearby stars.