Minimum blackbody temperature and corresponding moon flux fraction in M for a total observation time of 6 h on ELT/METIS as a function of semi-major axis for 5σ detectability of a moon around ε Indi Ab for an Io-sized, Mars-sized, and Earth-sized moon (top), with a corresponding first-order estimate of the required eccentricity (bottom). The grey region delineates the area bounded by the edge-on and face-on cases (p = 2/π and p = 1); the red and blue lines correspond to the inclination-averaged case (p ≲ 0.842). Semi-major axes and eccentricities of the seven largest Solar System moons Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton are marked by their first letter(s). The post-capture migration path of Triton assuming conservation of angular momentum (such as in Ross & Schubert 1990) is also marked to illustrate the observational potential of captured moons. The moons used for further analysis in Sects. 3.2 and 3.3 are marked in green in the top plot. For all three moons over all explored semi-major axes, the minimum 5σ-detectable temperature corresponds to a spectroastrometric signal of roughly 0.013 mas.