Goldilocks zonePerhaps 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 what we can capture with a simple calculation based on the distance of a planet from its 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 650 million years ago.
Even leaving aside questions of climate, not all exoplanets have a surface on which liquid water could exist. Many of the exoplanets found in the past 20 years are massive, Jupiter-sized planets. This is not surprising, as bigger planets are easier to find (even if they are not the most numerous). But a Jupiter-sized exoplanet in the habitable zone of a star is unlikely to have liquid water, much less prove a suitable habitat for life as we know it. Jupiter has an outer atmosphere of gaseous and liquid hydrogen overlaying a metallic hydrogen envelope that extends for thousands of kilometers. Any rocky surface is confined to a core buried under millions of atmospheres of pressure.
But if a Jupiter-like exoplanet orbits within a star’s habitable zone, it begs the question: might that exoplanet host habitable moons like the Ewok’s home? Jupiter has Europa, which is suspected to have liquid water buried under an ice crust, and Saturn has Enceladus, which definitely has water hidden underneath its coat of ice. So Earth-like exomoons are certainly not out of the question.
http://arstechnica.com/science/2014/04/move-over-exoplanets-exomoons-may-harbor-life-too/