Free Astronomy Magazine November-December 2019

47 NOVEMBER-DECEMBER 2019 EXOPLANETS ! A planet twice as massive as Earth, 40% larger by volume and placed at the right distance from its solar- type star, could offer life forms more hospitable conditions than those found on Earth. thermal contributions of the latter, com- bined with those of the star, can become significant enough to make the moon unin- habitable. If we now imagine pushing the system into increasingly distant regions of the stellar habitable zone, we find locations where the closely-placed moon will once again be habitable due to the decreasing supply of stellar heat. In extreme cases, even a moon-planet system located outside the habitable zone can offer habitable condi- tions if the two bodies are close enough to each other to keep water liquid on the sur- face of the moon. In a sense, gas giants away from the habitable stellar zone have their own planetary habitable zone. From the different scenarios so far pre- sented (and they are just some of those pos- sible), all the elasticity within the concept of habitability emerges, both when used to define a circumstellar region and to label a planet. The existence of superhabitable worlds and life forms far more advanced than terrestrial ones is likely. Despite this, the yearning to find something very similar to our reality is still irresistible. ering an extreme case under such condi- tions, showing clearly how the concept of “habitability” may be very loosely tied to that of “habitable zone.” Other borderline cases are represented by planets in remark- able elliptical orbits, which, although mov- ing mainly outside the habitable zone, could be heated by tidal forces exerted by their star. But the most interesting scenario is per- haps that of the great moons, which can be habitable regardless of their location in the habitable zone of a star thanks largely to the heat produced by the gravitational tides driven by their planet. Imagine a moon as big as Earth in orbit around a planet the size of Jupiter, both about 1 AU from a Sun-like star. If the moon moves in an orbit as wide as at least 20 times the planet’s radius, it is the star that provides it with all the energy needed to be habitable. The relevance of the planet as a source of reflected starlight, residual thermal emission, and even tidal heating for providing energy to drive sur- face processes on the moon is, in this sce- nario, marginal. However, if we move the moon to an orbit closer to the planet, the