Astrometric detection of exo-Earths in the presence of stellar noise

Astrometry from space is capable of making extremely precise measurements of the positions of stars, at angular precision of well below 1 micro-arcsecond (uas) at each visit. Hundreds of visits over a period of five years could achieve a relative astrometric precision for the mission of below 0.05 uas; this is well below the astrometric signature of 0.3 uas for a Sun-Earth system at a distance of 10 pc. The Sun's photometric fluctuations on time scales from days to years are dominated by the rotation and evolution of stellar surface features (sunspots and faculae). This flux variability is a source of astrophysical noise in astrometric as well as radial velocity (RV) measurements of the star. In this paper we describe a dynamic starspot model that produces flux variability which is consistent with the measured photometric power spectra of the Sun and several other stars. We use that model to predict the jitter in astrometric and RV measurements due to starspots. We also employ empirical stellar activity models to estimate the astrometric jitter of a much larger sample of stars. The conclusion of these simulations is that astrometric detection of planets in the habitable zones of solar-type stars is not severely impacted by the noise due to starspots/faculae, down to well below one Earth mass.