Combining photometry and astrometry to improve orbit retrieval of directly imaged exoplanets

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ABSTRACT Future missions like Roman, HabEx, and LUVOIR will directly image exoplanets in reflected light. While current near-infrared direct imaging searches are only sensitive to young, self-luminous planets whose brightness is independent of their orbital phase, reflected light direct imaging will reveal changes in planet brightness throughout an orbit due to phase variations. One of the first objectives will be determining the planet’s orbit via astrometry, the projected position of the planet with respect to its host star in the sky plane. We show that phase variations can significantly improve the accuracy and precision of orbital retrieval with two or three direct images. This would speed up the classification of exoplanets and improve the efficiency of subsequent spectroscopic characterization. We develop a forward model to generate synthetic observations of the 2D astrometry and the planet/star flux ratio. Synthetic data are fitted with Keplerian orbits and Henyey–Greenstein phase variations to retrieve orbital and phase parameters. For astrometric uncertainties of 0.01 au in projected separation and flux ratio uncertainties of 10−12, using photometry in orbit retrieval improves the accuracy of semimajor axis by 47 per cent for two epochs and 61 per cent for three epochs if the phase curves have a known shape, but unknown amplitude. In a realistic scenario where phase curve shape and amplitude are a priori unknown, photometry improves accuracy by 16 per cent for two epochs and 50 per cent for three epochs. In general, we find that if the planetary flux is measured to better than 10σ at multiple epochs, it usefully contributes to orbit retrieval.