A nice diagram of what fits in the habitable zone, via NASA
As NASA’s Kepler mission began spotting planets outside our solar system, scientists were excited to announce the discovery of a planet orbiting within a star’s habitable zone. Also referred to as the “Goldilocks zone,” the habitable zone is the distance from the star that is neither too hot nor too cold for liquid water to exist on the planet’s surface—if it has water, so goes the reasoning, it could have life.
But a paper just published in the journal Astrobiology points out that the name is a bit of a misnomer. “To be habitable, a world (planet or moon) does not need to be located in the stellar habitable zone, and worlds in the habitable zone are not necessarily habitable,” the paper states. Without abandoning the water-indicates-life paradigm completely, the researchers take the habitable zone to task. They argue that outside the habitable zone, there could planets even more amenable to life. We just have to know what to look for.
While we don’t have an example of a planet supporting life outside of the habitable zone, we have empirical evidence that orbiting within the habitable zone means, necessarily, that liquid water exists on the planet.
Mars, for instance, is in the habitable zone around our Sun. And even though Mars may not be as totally frozen or arid as was once thought, if water on Mars is ever in a liquid form, it isn’t ever for very long. What’s more, Mars doesn’t have enough of an atmosphere, ozone layer, or adequate magnetic field for life as we know it to flourish on its surface.
What’s more, there are more sources of heat than just the star your planet is orbiting. Jupiter’s moon Europa, for example, may have a liquid ocean below its icy crust that is kept liquid via “tidal heating.” The pull from Jupiter varies thanks to Europa’s non-circular orbit around the gas giant, and the tidal bulge goes up and down as the moon is pulled on by Jupiter and its other large moons. This flexing, scientists think, could be heating Europa from within. As such it’s one of the likeliest places in the solar system that could have water, and thus life.
For the rest of the story: http://motherboard.vice.com/blog/alien-life-may-exist-beyond-the-habitable-zone