We provide a preliminary estimate of the performance of reflex astrometry on
Earth-like planets in the habitable zones of nearby stars. In Monte Carlo
experiments, we analyze large samples of astrometric data sets with low to
moderate signal-to-noise ratios. We treat the idealized case of a single planet
orbiting a single star, and assume there are no non-Keplerian complications or
uncertainties. The real case can only be more difficult. We use periodograms
for discovery and least-squares fits for estimating the Keplerian parameters.
We find a completeness for detection compatible with estimates in the
literature. We find mass estimation by least squares to be biased, as has been
found for noisy radial-velocity data sets; this bias degrades the completeness
of accurate mass estimation. When we compare the true planetary position with
the position predicted from the fitted orbital parameters, at future times, we
find low completeness for an accuracy goal of 0.3 times the semimajor axis of
the 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 wobble
semi-amplitudes down to 0.2 μas integrated over the mission lifetime," may
not be satisfied by an instrument characterized by the noise floor of the Space
Interferometry Mission, σfloor​≈0.035μas. An important,
unsolved, strategic challenge for the exoplanetary science program is figuring
out how to predict the future position of an Earth-like planet with accuracy
sufficient to ensure the efficiency and success of the science operations for
follow-on spectroscopy, which would search for biologically significant
molecules in the atmosphere.Comment: v2: 16 pages, 4 figures; ApJ accepte