Astrometric exomoon detection by means of optical interferometry

Context. With no conclusive detection to date, the search for exomoons, satellites of planets orbiting other stars, remains a formidable challenge. Detecting these objects, compiling a population-level sample and constraining their occurrence will inform planet and moon formation models and shed light on moon habitability. Aims. Here, we demonstrate the possibility of a moon search based on astrometric time series data, repeated measurements of the position of a given planet relative to its host star. The perturbing influence of an orbiting moon induces a potentially detectable planetary reflex motion. Methods. Based on an analytical description of the astrometric signal amplitude, we placed the expected signatures of putative moons around real exoplanets into context with our current and future astrometric measurement precision. Modelling the orbital perturbation as a function of time, we then simulated the detection process given different target system configurations, instrumental measurement precisions and numbers of observational epochs to obtain the first astrometric exomoon sensitivity curves. Results. The astrometric technique already allows for the detection and characterisation of favourable moons around giant exoplanets and brown dwarfs. Since the detection sensitivity of this method is mainly governed by the achievable astrometric precision, long-baseline interferometry lends itself ideally to this pursuit. We find that, on the basis of 12 epochs obtained with VLTI/GRAVITY, it is already today possible to infer at a confidence of 5 σ the presence of a 0.14 M Jup satellite at a separation of 0.39 AU around AF Lep b. Future facilities offering better precision will refine our sensitivity in both moon mass and separation from the host planet by several orders of magnitude. Conclusions. The astrometric method of exomoon detection, especially when applied to interferometric observations, provides a promising avenue towards making the detection of these elusive worlds a reality and efficiently building a sample of confirmed objects. With a future facility that achieves an astrometric precision of 1 μas, probing for Earth-like moons within the habitable zone of a given star will become a realistic proposition.