Children's illustration depicting the Sun's Habitable Zone (green). This is the region around a star where life is considered most likely to exist. It is neither too far from the star to be too cold, nor too close in to be too it. It's often called the 'Goldilocks Zone' - just right for life as we know it. However, other places in the Solar System are outside this zone but considered possibly inhabited - such as some of the moons of Jupiter and Saturn, like Titan, Enceladus, Europa and Ganymede. Mars is just within the zone but is not inhabited, and speculation is still ongoing about whether it once was.