Exomoon habitability constrained by energy flux and orbital stability

Abstract

Context. Detecting massive satellites that orbit extrasolar planets has now become feasible, which led naturally to questions about the habitability of exomoons. In a previous study we presented constraints on the habitability of moons from stellar and planetary illumination as well as from tidal heating. Aims. Here I refine our model by including the effect of eclipses on the orbit-averaged illumination. I then apply an analytic approximation for the Hill stability of a satellite to identify the range of stellar and planetary masses in which moons can be habitable. Moons in low-mass stellar systems must orbit their planet very closely to remain bounded, which puts them at risk of strong tidal heating. Methods. I first describe the effect of eclipses on the stellar illumination of satellites. Then I calculate the orbit-averaged energy flux, which includes illumination from the planet and tidal heating to parametrize exomoon habitability as a function of stellar mass, planetary mass, and planet-moon orbital eccentricity. The habitability limit is defined by a scaling relation at which a moon loses its water by the runaway greenhouse process. As a working hypothesis, orbital stability is assumed if the moon's orbital period is less than 1/9 of the planet's orbital period. Results. Due to eclipses, a satellite in a close orbit can experience a reduction in orbit-averaged stellar flux by up to about 6%. The smaller the semi-major axis and the lower the inclination of the moon's orbit, the stronger the reduction. I find a lower mass limit of ≈0.2 M for exomoon host stars that allows a moon to receive an orbit-averaged stellar flux comparable to the Earth's, with which it can also avoid the runaway greenhouse effect. Precise estimates depend on the satellite's orbital eccentricity. Deleterious effects on exomoon habitability may occur up to ≈0.5 M if the satellite's eccentricity is 0.05. Conclusions. Although the traditional habitable zone lies close to low-mass stars, which allows for many transits of planet-moon binaries within a given observation cycle, resources should not be spent to trace habitable satellites around them. Gravitational perturbations by the close star, another planet, or another satellite induce eccentricities that likely make any moon uninhabitable. Estimates for individual systems require dynamical simulations that include perturbations among all bodies and tidal heating in the satellite.

Evaluating galactic habitability using high-resolution cosmological simulations of galaxy formation

“…Exomoons may also provide sites for biospheres, and their SHZs are complicated even further by the effects of tidal heating (Heller & Barnes, 2013b;Forgan & Kipping, 2013), frequent eclipses (Heller, 2012) and by infrared radiation from the host planet (Heller & Barnes, 2013a;Forgan & Yotov, 2014). These SHZs implicitly assume that magnetic fields can shield the moon from high energy radiation, which bears its own caveats (Heller & Zuluaga, 2013).…”

Constraints on the Habitability of Extrasolar Moons

“…As we virtually decrease the stellar mass and as we move our planet-moon binary towards the star to remain the IHZ, the star also forces the satellite's orbit to become more and more eccentric. We expect that for stellar masses below about 0.2 M no habitable Super-Ganymede exomoon can exist in the stellar IHZ due strong tidal dissipation (Heller 2012). Figure 3 shall illustrate our gedankenexperiment.…”

“…In the right panel, the moon's orbit is tilted by 45 • against the circumstellar orbit and eclipses occur rarely (for satellite eclipses see Fig. 1 in Heller 2012). Illumination from the planet overcompensates for the small reduction of stellar illumination and makes the sub-planetary point the warmest spot on the moon.…”

“…Space for habitable orbits about a planet decreases when the planet's Hill radius moves inward and when the habitable edge moves outward. This is what happens when we virtually move a given planet-moon binary from the IHZ of G a star to that of a K star and finally into the IHZ of an M star (Heller 2012). Shrinking the planet's Hill sphere means that any moon must orbit the planet ever closer to remain gravitationally bound.…”

“…With the detection of an exomoon in the stellar irradiation habitable zone (IHZ) at the horizon, exomoon habitability is now drawing scientific and public attention. Yet, investigations on exomoon habitability are rare (Reynolds et al 1987;Scharf 2006;Heller 2012). These studies have shown that illumination from the planet, satellite eclipses, tidal heating, and constraints from orbital stability have fundamental effects on the habitability of moons -at least as important as irradiation from the star.…”