The Synergy of Direct Imaging and Astrometry for Detection of exo-Earths


Joseph Catanzarite and Michael Shao


Jet Propulsion Laboratory, California Institute of Technology


The holy grail of exoplanet searches is an image of an Earth mass planet orbiting in the habitable zone around a nearby star. A single image of such a planet, however, does not provide evidence that the planet is of Earth mass or that it is in a habitable zone orbit. The measurement of an exo-planet’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 exceeds the IWA. Secondly, a planet’s apparent brightness and color depend on the phase angle, just as the moon is much brighter at full moon than at half moon phase. 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. For a coronagraphic mission, the detection of a planet and measurement of its orbit requires a moderately large number of images, compromising the ability of some types of coronagraphs from searching a large number of stars for exo-Earths. We examine the effect of realistic planet semimajor axis and mass distributions on imaging surveys. We investigate the synergy between astrometry and imaging, and we conclude that this synergy offers the most efficient and realistic road to detect and characterize exo-Earths in habitable zone around nearby stars. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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