Direct imagine of exoplanets is a very challenging task. This is especially true for exoplanets that are found within the Habitable Zone of their host stars. The reason for this is that a star is many orders of magnitude brighter than a planet and when observed over interstellar distances, the angular separation between the star and planet is very small. Nonetheless, it does become less difficult to directly image a planet that is located further from its host star because an increase in the star-planet angular separation places the planet further from the glare of its host star. However, such exoplanets tend to be gas giant planets that are located well outside the Habitable Zone of their host stars. Within our solar system, Jupiter and Saturn are typical examples of such planets.
Although gas giant planets are unlikely places for life to exist, they can have systems of moons where life may exist on some of them. Beyond the Habitable Zone, insolation becomes inadequate to warm an exomoon sufficiently for liquid water to exist on its surface. As a result, tidal heating serves as a viable mechanism to provide the extra amount of energy needed to raise the equilibrium temperature of the exomoon. In order for tidal heating to occur, the orbit of an exomoon around a gas giant planet needs to have some amount of eccentricity. Furthermore, for an exomoon to maintain the orbital eccentricity necessary to sustain tidal heating, it needs to be in resonance with at least one or more exomoons that are orbiting the same gas giant planet. A classic example in our solar system is the satellite system of Jupiter where the moons Io, Europa and Ganymede are in a 1:2:4 orbital period resonance. This sustains Io’s tidal heating by keeping its orbit from ever circularizing and makes Io the most volcanically active world in our solar system.
An exomoon with one-tenth Earth’s mass is probably the minimum mass for a potentially habitable tidally heated exomoon. This is because a less massive exomoon will be unable to hold onto an atmosphere if it is tidally heated to an average surface temperature that is sufficient to sustain liquid water. However, such massive moons do not exist in our solar system and even the largest moons are no more than a few percent of Earth’s mass. Regardless of that, it is not unrealistic to consider the existence of much more massive exomoons given the unexpected diversity of exoplanets that have already been discovered.
Consider an Earth-sized exomoon with an Earth-like density and an orbital eccentricity of just 0.005. This Earth-sized exomoon will have to orbit a Jupiter-mass gas giant planet at a distance of roughly 1.1 million kilometres in order for tidal heating to be sufficient to sustain an average surface temperature of 300 K. At this temperature, liquid water can exist on the surface of the exomoon. If the planet-moon distance were halved, tidal heating will be so effective that the surface temperature of the Earth-sized exomoon will be a blistering 1000 K, making it uninhabitable. In comparison, Jupiter’s moon Io orbits Jupiter at an average distance of 422,000 kilometres, with an orbital eccentricity of 0.004. Observing at an infrared wavelength of ~14 micrometres, a 300 K tidally heated Earth-sized exomoon will appear as bright as the Jupiter-mass gas giant planet it is orbiting.
Direct imaging of tidally heated exomoons can be a lot less challenging than the direct imaging of similar sized Habitable Zone exoplanets. This is because a tidally heated exomoon can exist at an arbitrarily large distance from its host star since its mean surface temperature is largely determined by tidal heating rather than insolation from its host star. In comparison, a Habitable Zone exoplanet around a Sun-like star is restricted to distances of ~1 AU from its host star (1 AU is the average Earth-Sun distance) since its mean surface temperature is largely determined by the amount of insolation it receives from its host star. As a result, a tidally heated exomoon can be a lot easier to image directly because it can be much further away from the overwhelming glare of its host star. To conclude, the direct imaging of habitable exomoons can happened long before it is technological possible to directly image habitable exoplanets.
Mary Anne Peters and Edwin L. Turner (2012), “On the Direct Imaging of Tidally Heated Exomoons”, arXiv:1209.4418 [astro-ph.EP]