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We provide a preliminary estimate of the performance of reflex astrometry onEarth-like planets in the habitable zones of nearby stars. In Monte Carloexperiments, we analyze large samples of astrometric data sets with low tomoderate signal-to-noise ratios. We treat the idealized case of a single planetorbiting a single star, and assume there are no non-Keplerian complications oruncertainties. The real case can only be more difficult. We use periodogramsfor discovery and least-squares fits for estimating the Keplerian parameters.We find a completeness for detection compatible with estimates in theliterature. We find mass estimation by least squares to be biased, as has beenfound for noisy radial-velocity data sets; this bias degrades the completenessof accurate mass estimation. When we compare the true planetary position withthe position predicted from the fitted orbital parameters, at future times, wefind low completeness for an accuracy goal of 0.3 times the semimajor axis ofthe planet, even with no delay following the end of astrometric observations.Our findings suggest that the recommendation of the ExoPlanet Task Force(Lunine et al. 2008) for "the capability to measure convincingly wobblesemi-amplitudes down to 0.2 $\mu$as integrated over the mission lifetime," maynot be satisfied by an instrument characterized by the noise floor of the SpaceInterferometry Mission, $\sigma_\mathrm{floor}\approx0.035\mu$as. An important,unsolved, strategic challenge for the exoplanetary science program is figuringout how to predict the future position of an Earth-like planet with accuracysufficient to ensure the efficiency and success of the science operations forfollow-on spectroscopy, which would search for biologically significantmolecules in the atmosphere.

On the Completeness of Reflex Astrometry on Extrasolar Planets near the Sensitivity Limit