Habitable zone (noun, “HAB-it-uh-bull ZOHN”)
The habitable zone is the region around a star where planets could have liquid water on their surfaces. Here, planets are not so close to their stars that all their water would boil away. Nor are they so far from their stars that all their water would freeze. This not too hot, not too cold region is also called the Goldilocks zone. It has long been considered the best place to look for alien life. Why? All life as we know it needs liquid water.
But being in the habitable zone does not necessarily make a planet habitable. Take our own solar system, for example. The habitable zone extends from around the orbit of Venus to around the orbit of Mars. But Earth is the only planet that appears to have life.
Around other stars, some habitable zone worlds are gas giants like Jupiter. Those planets are not thought to be suitable for life. And small, rocky planets in this region may not have any water to start with. Even rocky worlds with liquid water may have other features that make them unlivable. For instance, to be in the habitable zone around a small, dim star, a planet must be very close to its sun. That close up, the planet can get blasted with extreme X-ray and ultraviolet radiation that stop life from arising.
But some worlds outside the habitable zone could be perfectly good for life. Massive planets with hydrogen-rich atmospheres could be warm enough to host liquid water almost any distance from their suns. Plus, alien life that is very different from Earth life may not need liquid water at all. In fact, some of the best places to search for life in our own solar system lie outside the habitable zone. Saturn’s moon Enceladus and Jupiter’s moon Europa are both too far from the sun to have liquid water on their surfaces. But these moons may have liquid oceans beneath their icy shells. Saturn’s moon Titan, meanwhile, might host exotic life in its ethane and methane lakes.
In short, the habitable zone may be a good place to start searching for alien life. But it’s definitely not the only place worth looking.
In a sentence
The nearest star to the sun, Proxima Centauri, may have a planet orbiting in its habitable zone.