The Habitable Zone and the Search for Life

A star's habitable zone (sometimes called its CHZ where the C stands for 'continuously' or 'circumstellar') refers to the range of distances where liquid water might exist on (or near) the surface of a rocky or terrestrial planet. We define it this way since we believe that liquid water is essential for the development and existence of life. In our Solar System, this zone stretches from just outside Venus' orbit to just inside that of Mars (see Figure 1). Handily enough, this zone includes the Earth !

Figure 1: The habitable zones of our Solar System and that of the planets of Gliese 581. Note how the planets need to be much closer (the x-axis is logarithmic) to the faint Gliese 581 (it's about 1/100 as bright as our Sun) to be able to receive enough light for the possibility of life supporting conditions.
Credit: Wikipedia

Of course, few scientific definitions are ever that simple. Each planet may have cloud cover or atmospheric conditions that affect this, moons outside of this zone may have other sources of energy (such as the tidal forces that Jupiter's moons experience), low-mass stars can become tidally locked to nearby exoplanets and massive stars undergo dramatic changes which can dramatically affect both the size and lifetimes of their habitable zones. 

The habitable zone is often referred to as the 'Goldilocks Zone' where conditions are 'just right' ... so the tantalising question for planetary scientists and astrobiologists revolves around how unique we believe the Earth to be in being conducive to life. Further, how might we go about detecting life on other planets and exoplanets. 

Find out about the history of exoplanets.