Possibility for albedo estimation of exomoons: Why should we care about M dwarfs?

TL;DR

This paper evaluates the feasibility of using occultation light curves to estimate the albedo of icy exomoons, finding it challenging around solar-like stars but potentially possible around small M dwarfs with next-generation telescopes.

Contribution

The study provides a quantitative analysis of the detectability of exomoon albedo using occultation light curves with upcoming telescopes, highlighting the importance of stellar type and observational noise.

Findings

Albedo estimation is not feasible for moons of solar-like stars.

Small M dwarfs are better candidates for albedo measurements.

Photon noise is a limiting factor in near-infrared occultation observations.

Abstract

Occultation light curves of exomoons may give information on their albedo and hence indicate the presence of ice cover on the surface. Icy moons might have subsurface oceans thus these may potentially be habitable. The objective of our paper is to determine whether next generation telescopes will be capable of albedo estimations for icy exomoons using their occultation light curves. The success of the measurements depends on the depth of the moon's occultation in the light curve and on the sensitivity of the used instruments. We applied simple calculations for different stellar masses in the V and J photometric bands, and compared the flux drop caused by the moon's occultation and the estimated photon noise of next generation missions with 5 $\sigma$ confidence. We found that albedo estimation by this method is not feasible for moons of solar-like stars, but small M dwarfs are better candidates for such measurements. Our calculations in the J photometric band show that E-ELT MICADO's photon noise is just about 4 ppm greater than the flux difference caused by a 2 Earth-radii icy satellite in a circular orbit at the snowline of an 0.1 stellar mass star. However, considering only photon noise underestimates the real expected noise, because other noise sources, such as CCD read-out and dark signal become significant in the near infrared measurements. Hence we conclude that occultation measurements with next generation missions are far too challenging, even in the case of large, icy moons at the snowline of small M dwarfs. We also discuss the role of the parameters that were neglected in the calculations, e.g. inclination, eccentricity, orbiting direction of the moon. We predict that the first albedo estimations of exomoons will probably be made for large icy moons around the snowline of M4 -- M9 type main sequence stars.