On the Direct Imaging of Tidally Heated Exomoons

Abstract

We demonstrate the ability of existing and planned telescopes, on the ground and in space, to directly image tidally heated exomoons orbiting gas-giant exoplanets. Tidally heated exomoons can plausibly be far more luminous than their host exoplanet and as much as 0.1% as bright as the system's stellar primary if it is a low mass star. Because emission from exomoons can be powered by tidal forces, they can shine brightly at arbitrarily large separations from the system's stellar primary with temperatures of several hundreds degrees Kelvin or even higher in extreme cases. Furthermore, these high temperatures can occur in systems that are billions of years old. Tidally heated exomoons may thus be far easier targets for direct imaging studies than giant exoplanets which must be both young and at a large projected separation from their primary to be accessible to current generation direct imaging studies. For example, the (warm) Spitzer Space Telescope and the next generation of ground based instruments could detect an exomoon roughly the size of the Earth at a temperature ~600K and a distance ~5 parsecs. Future mid-infrared space telescopes, such as JWST and SPICA, will be capable of directly imaging tidally heated exomoons around the nearest two dozen stars with brightness temperature >300K and R>1R_earth orbiting at >12AU from the primary star at a 5sigma confidence level in a 10,000 second integration. In addition it is possible that some of the exoplanets which have already been directly imaged are actually tidally heated exomoons or blends of such objects with hot young planets. If such exomoons exist and are sufficiently common (i.e. nearby), it may well be far easier to directly image an exomoon with surface conditions that allow the existence of liquid water than it will be to resolve an Earth-like planet in the classical Habitable Zone of its primary.