Nautilus Array Targets Astrometric Reflex Motion for First Scalable Exomoon Survey

The paper outlines a staged campaign that begins with one or two apertures monitoring the closest imaged planets such as those around epsilon Indi or 51 Eri. Astrometric signals from moons grow linearly with moon mass and semimajor axis while falling with system distance, so the nearest, lowest-mass planets are optimal. Continuous monitoring over multiple orbital periods distinguishes the planetary wobble from stellar activity and proper motion. Simulations cited in the work indicate that a 0.3 microarcsecond noise floor suffices for sub-Earth-mass detections at 5-10 pc.

Nautilus's modular design directly addresses the main barrier identified in earlier astrometric studies: the requirement for years-long, high-cadence observations that cannot be scheduled on shared facilities. This approach complements transit and radial-velocity methods, which lose sensitivity for wide orbits, and extends demographic coverage to moons around planets already characterized by high-contrast imaging. Synergy with a starshade for Earth-like planets around the same stars creates a unified nearby-system survey architecture.

The principal limitation remains the current absence of demonstrated 0.1 microarcsecond astrometric performance on space-based arrays; ground-based adaptive-optics results have not yet bridged this gap. Full validation will require on-sky demonstration during the initial Nautilus phase. A successful pilot would justify expanding the array while parallel high-contrast spectroscopy constrains moon-host planet masses and atmospheres.

Next steps include refining target lists from ongoing direct-imaging surveys and integrating moon-induced photocenter jitter into existing Nautilus yield simulations. The architecture thereby converts an opportunistic search into a systematic demographic probe.