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The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet inthe habitable zone around a nearby star. Mass is the most important parameterof a planet and can only be measured by observing the motion of the star aroundthe planet-star center of mass. A single image of a planet, however, does notprovide evidence that the planet is Earth mass or that it is in a habitablezone orbit. The planet's orbit, however, can be measured either by imaging theplanet at multiple epochs or by measuring the position of the star at multipleepochs by space-based astrometry. The measurement of an exo-planet's orbit bydirect imaging is complicated by a number of factors: (1) the inner workingangle (IWA); (2) the apparent brightness of the planet depending on the orbitalphase; (3) confusion arising from the presence of multiple planets; and (4) theplanet-star contrast. In this paper we address the question: "Can a priorastrometric mission that can identify which stars have Earthlike planetssignificantly improve the science yield of a mission to image exo-Earths?" Wefind that the Occulting Ozone Observatory (a small external occulter missionthat cannot measure spectra) could confirm the orbits of ~4 to ~5 times as manyexo-Earths if an astrometric mission preceded it to identify which stars hadsuch planets. We find that in the case of an internal coronagraph, a survey ofthe nearest ~60 stars could be done with a telescope of half the size if anastrometric mission had first identified the presence of Earth-like planets inthe habitable zone and measured their orbital parameters.

The Synergy of Direct Imaging and Astrometry for Orbit Determination of exo-Earths