The habitable zone

The habitable zone (HZ), also called the circumstellar habitable zone or the Goldilocks zone, describes the band of orbits around a star where a planet’s surface might have liquid water. Many scientists consider liquid water essential for a planet to be habitable. The zone’s boundaries depend on the star’s brightness and how a planet’s atmosphere interacts with it. At the inner edge, starlight trapped by greenhouse gases heats the planet enough to boil away its water. At the outer edge, even with atmospheric help, starlight is too weak and the planet’s water freezes. Various models add many other factors. The habitable zone has become a key tool for finding habitable planets because exoplanet discoveries usually give approximate orbital distances.

The nickname “Goldilocks zone” comes from the fairy tale “Goldilocks and the Three Bears,” where a girl chooses from three items, rejecting extremes and picking the one that is “just right.” Since the concept was first introduced, many stars have been confirmed to have a planet in their habitable zone, including some systems with multiple such planets. Most of these planets are super-Earths or gas giants, more massive than Earth, because larger planets are easier to detect. On November 4, 2013, astronomers using Kepler space telescope data reported that the Milky Way might contain as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs. About 11 billion of these could orbit Sun-like stars. Proxima Centauri b, about 4.2 light-years away in the constellation Centaurus, is the nearest known exoplanet and orbits in its star’s habitable zone. The habitable zone also interests researchers studying the habitability of natural satellites, since moons in the zone might outnumber planets.

The classical habitable zone concept only applies to planetary surfaces where life depends on stellar energy. Deep biospheres exist on Earth, but they would not be detectable on exoplanets. Other circumstellar zones have been proposed where non-water solvents—such as those supporting hypothetical life with alternative biochemistries—could exist as liquids on the surface.

The idea of a habitable zone originated in geography in the late 19th century. In 1883, Alexander Winchell discussed planetary habitability with a definition close to what is now called the habitable zone for complex life. Possibly the earliest use of the term “habitable zone” was in 1913, by Edward Maunder in his book *Are The Planets Inhabited?*. Hubertus Strughold’s 1953 treatise *The Green and the Red Planet* used the term “ecosphere” and referred to various “zones” where life could emerge. That same year, Harlow Shapley wrote about the “Liquid Water Belt,” describing the same concept in more scientific detail. Both works stressed the importance of liquid water. American astrophysicist Su-Shu Huang developed the modern concept of circumstellar habitable zones in a series of papers during the 1950s and 1960s. In 1964, Stephen H. Dole, in a RAND Corporation study called *Habitable Planets for Man*, discussed what makes a planet habitable for humans and estimated about 600 million habitable planets in the Milky Way, with about 50 within 100 light-years. Dole later co-authored a less technical version with science-fiction writer Isaac Asimov, published at the height of the space race. The term “Goldilocks zone” emerged in the 1970s, referring to a region around a star where the temperature is “just right” for liquid water.

In 1993, James Kasting and colleagues developed a model for habitable zones around stars of different brightness using a simple climate model. Refinements to that model and extensions to smaller stars led to new zone estimates and the introduction of “conservative” and “optimistic” habitable zone concepts. Practical application of the concept came with improvements in telescope technology. In 2009, the Kepler space telescope was launched specifically to detect exoplanets in habitable zones. By the mission’s end, data suggested that at least 20%, and perhaps as many as 50%, of stars visible at night have Earth-sized planets in their habitable zone. In 2018, the Transiting Exoplanet Survey Satellite continued the search.

In 1999, Guillermo Gonzalez and others noted that metals strongly influence planet formation and the development of life. Together with paleontologist Peter Ward and astronomer Donald Brownlee, Gonzalez introduced the idea of the “galactic habitable zone.” This region, where life is most likely to emerge in a galaxy, includes areas close enough to the galactic center for stars to be enriched with heavier elements, but not so close that intense radiation and enormous gravitational forces frequently disrupt star systems, planetary orbits, and the emergence of life. The discovery of exoplanets unlike any in the Solar System led to suggestions that life might emerge even in conditions outside currently defined habitable zones. Subsequently, some astrobiologists have proposed extending the concept to other solvents, such as dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, which could support hypothetical life forms that use alternative biochemistries.