Is It Possible to Detect Airless Exomoons Through Thermal Phase Curves?

More than 200 moons exist in our solar system, but no exomoon has been confirmed to date. What kind of exoplanetary systems are possible to host exomoons, and what are the possible ways to detect the exomoons? We conduct N-body orbital simulations for three representative cases that are close to their host stars, and find that the possibility of exomoon existence varies across different systems. TRAPPIST-1 e and GJ 1214 b are possible to host exomoons, although the exomoon orbital stability zones are narrow and close to the planets. WASP-121 b is unlikely to host exomoons because the planetary radius is nearly half of the Hill radius, and beyond the Hill sphere, the the star's gravitational influence dominates the exomoon. Close-in airless exomoons maintain large temperature difference between the day and night hemispheres. The large day-night temperature contrast of the exomoon significantly amplifies the total thermal phase curve amplitude of an exomoon-exoplanet system, especially for large, airless exomoons orbiting exoplanets with atmospheres. When the hypothetical exomoon transits or is blocked by the exoplanet, the transit depth varies with the planetary phase, and the occultation depth varies with the exomoon's phase. For an Earth-sized exomoon orbiting GJ 1214 b, the occultation signal can reach 100 ppm. Without extracting the exomoon signal, retrieving the planetary temperature distribution from observed thermal phase curve is likely to overestimate the planetary day-night temperature contrast, and underestimate the planetary horizontal heat transport. With longer observation time and greater time resolution from infrared space telescopes in the future, detecting exomoons through thermal phase curves is possible.