THE SYNERGY OF DIRECT IMAGING AND ASTROMETRY FOR ORBIT DETERMINATION OF EXO-EARTHS

Abstract

The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in the habitable zone (HZ) around a nearby star. Mass is one of the most important characteristics of a planet and can only be measured by observing the motion of the star around the planet-star center of gravity. The planet's orbit can be measured either by imaging the planet at multiple epochs or by measuring the position of the star at multiple epochs by space-based astrometry. The measurement of an exoplanet's orbit by direct imaging is complicated by a number of factors. One is the inner working angle (IWA). A space coronagraph or interferometer imaging an exo-Earth can separate the light from the planet from the light from the star only when the star-planet separation is larger than the IWA. Second, the apparent brightness of a planet depends on the orbital phase. A single image of a planet cannot tell us whether the planet is in the HZ or distinguish whether it is an exo-Earth or a Neptune-mass planet. Third is the confusion that may arise from the presence of multiple planets. With two images of a multiple planet system, it is not possible to assign a dot to a planet based only on the photometry and color of the planet. Finally, the planet-star contrast must exceed a certain minimum value in order for the planet to be detected. The planet may be unobservable even when it is outside the IWA, such as when the bright side of the planet is facing away from us in a crescent phase. In this paper we address the question: Can a prior astrometric mission that can identify which stars have Earth-like planets significantly improve the science yield of a mission to image exo-Earths? In the case of the Occulting Ozone Observatory, a small external occulter mission that cannot measure spectra, we find that the occulter mission could confirm the orbits of ~4 to ~5 times as many exo-Earths if an astrometric mission preceded it to identify which stars had such planets. In the case of an internal coronagraph we find that a survey of the nearest ~60 stars could be done with a telescope half the size if an astrometric mission had first identified the presence of Earth-like planets in the HZ and measured their orbital parameters.