ASTROMETRIC JITTER OF THE SUN AS A STAR

The daily variation of the solar photocenter over some 11 yr is derived from the Mount Wilson data reprocessed by Ulrich et al. to closely match the surface distribution of solar irradiance. The standard deviations of astrometric jitter are 0.52 mu AU and 0.39 mu AU in the equatorial and the axial dimensions, respectively. The overall dispersion is strongly correlated with solar cycle, reaching 0.91 mu AU at maximum activity in 2000. The largest short-term deviations from the running average (up to 2.6 mu AU) occur when a group of large spots happen to lie on one side with respect to the center of the disk. The amplitude spectrum of the photocenter variations never exceeds 0.033 mu AU for the range of periods 0.6-1.4 yr, corresponding to the orbital periods of planets in the habitable zone. Astrometric detection of Earth-like planets around stars as quiet as the Sun is not affected by star spot noise, but the prospects for more active stars may be limited to giant planets.