Large exomoons around giant planets in the habitable zone of their host stars could serve as habitats for extraterrestrial life. Such an exomoon would need to have at least twice the mass of Mars or so (i.e. ~0.2 Earth masses) for it to be habitable. For comparison, Ganymede, the largest moon in the Solar System, is roughly 1/40 the mass of Earth. In addition, habitability requires a surface temperature that cannot be too high or too low. This is governed not just by stellar radiation from the host star, but also by stellar light reflected from the giant planet, thermal radiation from the giant planet itself and tidal heating.
Figure 1: Artist’s impression of a giant planet hosting a system of moons. Credit: Kevin Sherman.
Over time, a gaseous giant planet contracts and releases thermal energy as it converts gravitational potential energy into heat. In a paper by Heller & Barnes (2013), the authors investigate how thermal radiation from a shrinking gaseous giant planet could drive a runaway greenhouse effect for an Earth-like exomoon if it is in a close enough orbit around the giant planet. This effect is particularly significant for a young giant planet during the first few hundred million years or so. During this period, the young and hot giant planet is cooling at a more rapid rate, and consequently releases a greater deal of thermal radiation.
To illustrate the combined effects of stellar radiation, thermal radiation from the giant planet and tidal heating, Heller & Barnes (2013) introduced five possible states for an exomoon: (1) Tidal Venus, (2) Tidal-Illumination Venus, (3) Super-Io, (4) Tidal Earth and (5) Earth-like. For these states, a Tidal Venus and a Tidal-Illumination Venus are uninhabitable, while a Super-Io, a Tidal Earth, and an Earth-like moon could be habitable. In the study, a rocky Earth-type exomoon orbiting a giant planet with a mass 13 times that of Jupiter is considered. Besides an Earth-type exomoon, a Super-Ganymede (i.e. a large exomoon with composition similar to Ganymede) is also considered.
At a distance of 1 AU from a Sun-like star, the results from the study show that the combined stellar radiation and thermal radiation on an Earth-type exomoon orbiting at 10 Jupiter-radii around a 13 Jupiter-mass giant planet would keep the Earth-type exomoon above the runaway greenhouse limit and uninhabitable for about 500 million years (Figure 2). For the Super-Ganymede, it would be in a runaway greenhouse state for about 600 million years. In fact, even in the absence of stellar radiation, thermal radiation from the giant planet alone can trigger a runaway greenhouse effect for the first ~200 million years.
Figure 2: The total illumination absorbed by an exomoon (thick black line) is composed of stellar radiation (black dashed line) and thermal radiation from the giant planet (red dashed line). The critical values for an Earth-type exomoon and a Super-Ganymede to enter the runaway greenhouse effect are indicated by dotted lines. Credit: Heller & Barnes (2013).
With the inclusion of tidal heating, the danger for an exomoon to undergo a runaway greenhouse effect increases. Heller & Barnes (2013) illustrate how the distance and orbital eccentricity of an Earth-type exomoon around a 13 Jupiter-mass giant planet determines whether the exomoon is in a Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) or Earth-like (green) state (Figure 3). Here, the giant planet is assumed to have an age of 500 million years. Furthermore, stellar radiation, thermal radiation from the giant planet and tidal heating are all included.
There is a minimum distance around the giant planet in which an Earth-type exomoon would be in a Tidal Venus or Tidal-Illumination Venus state, and hence uninhabitable. This minimum distance is referred to as the “habitable edge”. For a 13 Jupiter-mass giant planet at 1 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 20 and 12 Jupiter-radii respectively. For the same giant planet at 1.738 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 15 and 8 Jupiter-radii respectively. The habitable edge for an older giant planet would be smaller since thermal radiation from a giant planet is expected to decrease over time. As a means of comparison, Io, Europa, Ganymede, and Callisto orbit Jupiter at approximately 6.1, 9.7, 15.5, and 27.2 Jupiter-radii.
Figure 3: The four panels show the possible states for an Earth-type exomoon around a 13 Jupiter-mass host planet that has an age of 500 million years. Distances from the giant planet are shown on a logarithmic scale. In the left two panels, the giant planet orbits at a distance of 1 AU from a Sun-like star. In the right two panels, the giant planet orbits at a distance of 1.738 AU. In the upper two panels, the orbit of the exomoon around the giant planet has an eccentricity of 0.1. In the lower two panels, the eccentricity is 0.0001. Starting from the giant planet in the centre, the white circle visualizes the Roche radius (i.e. within this region, an Earth-type exomoon would be tidally disrupted), and the exomoon types correspond to Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) and Earth-like (green) states. Dark green depicts the extent of orbits for Earth-like exomoons in prograde orbits (i.e. orbits in the same direction as the giant planet’s spin) and light green depicts the extent of orbits for Earth-like exomoons in retrograde orbits (i.e. orbits in the opposite direction to the giant planet’s spin). Credit: Heller & Barnes (2013).
Heller & Barnes (2013), “Runaway greenhouse effect on exomoons due to irradiation from hot, young giant planets”, arXiv:1311.0292 [astro-ph.EP]