In the Star Wars universe, everyone’s favourite furry aliens, the Ewoks, famously lived on the “forest moon of Endor”. In scientific terms, the Ewok’s home world would be referred to as an exomoon, which is simply a moon that orbits an exoplanet – any planet that orbits a star other than our sun.
Although more than 1,000 exoplanets have been discovered since the first one was found in 1995, only a handful of those are thought to be habitable, at least by life as we know it. New research shows that exomoons, too, could provide habitable environments. Although we are yet to find exomoons, we have good reasons to believe that there should be many, even more than exoplanets.
Perhaps the most habitable planet found to date is the recently announced Kepler-186f. This is one of five exoplanets discovered by NASA’s Kepler satellite, all orbiting a small, faint, red dwarf star, 500 light years away in the constellation of Cygnus.
Kepler-186f is an Earth-sized planet that orbits its star in only 130 days and is about as distant from its star as Mercury is from the Sun. But, because the red dwarf is much dimmer than the Sun, Kepler-186f receives only about one-third of the energy that the Earth does. As a result, Kepler-186f lies at the outer edge of its star’s “habitable zone”. This is the hypothetical region of space surrounding a star in which liquid water may conceivably exist on the surface of any exoplanets.
In our own solar system, Venus lies too close to the Sun and is too hot. Mars lies too far from the Sun and is too cold. But Earth, of course, lies within the critical “Goldilocks zone”, where the temperature is just right.
Simply residing in the habitable zone, though, is no guarantee that an exoplanet has water oceans. The climate of a planet is much more complicated than we can capture with a simple calculation based on the distance of a planet from a star. We know that Mars probably had running water on its surface in the past, but now it is a frozen desert. Earth, meanwhile, was probably in a completely frozen “snowball” state about 650m years ago.