In astronomy and astrobiology, the habitable zone (HZ), also known as the circumstellar habitable zone (CHZ), or as the Goldilocks zone, is the range of orbits around a star within which a planetary surface could potentially support liquid water. Liquid water is considered by many scientists as necessary for a planet to be habitable. The range depends upon the brightness of the star interacting with a planet's atmosphere. At the inner edge the star's light, trapped by greenhouse gases in a planet's atmosphere, boils off the planet's water. At the outer edge light from the star is insufficient even with help from atmospheric gas and the planet's water freezes. Many other factors are added in various habitable zone models. The habitable zone has become a key tool in the search for habitable planets because discoveries of exoplanets yield approximate orbital radii.
The alternative name, Goldilocks zone, is a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.
The classical habitable zone concept was defined only for planetary surfaces where habitat depends on stellar energy. Deep biospheres are known on Earth, but they would not be detectable on exoplanets. Other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.
History
The concept of habitable zone arose in geography in the late 19th century. Alexander Winchell discussed planetary habitability in 1883, using a definition closer to what is now called a 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: A Physiological Study of the Possibility of Life on Mars used the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist developed the modern concept of circumstellar habitable zones in a series of papers in the 1950s and 60s.
The concept of habitability was further developed in 1964 by Stephen H. Dole in a RAND Corporation study, Habitable Planets for Man, in which he discussed the determinants of planetary habitability for humans, eventually estimating the number of habitable planets in the Milky Way to be about 600 million and about 50 such planets within . Dole subsequently co-authored a less technical version of the study in a book with science-fiction author Isaac Asimov which appeared in print at the height of the space race. The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.
In 1993 James Kasting and colleagues developed a model for habitable zones across stars with different brightness using a simple climate model. Refinements of the climate model and extension to smaller stars led to new zone range estimates and the introduction of "conservative" and "optimistic" habitable zone concepts.
Practical application of the habitable zone concept arrived with improvements in telescope technology. In 2009, the Kepler space telescope was launched specifically to detect exoplanets in habitable zones. By the end of the mission the data suggested that at least 20% and perhaps as many as 50% of the stars visible at night have Earth-sized planets in their habitable zone. In 2018, the Transiting Exoplanet Survey Satellite continued the search.
Extensions of the concept
In 1999, Guillermo Gonzalez and other noted that metals have important impact on planet formation and the development of life. Together with paleontologist Peter Ward and fellow astronomer Donald Brownlee Gonzalez introduced the idea of the "galactic habitable zone". The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.
The discovery of exoplanets unlike any in the Solar System led to suggestions that life might emerge even in conditions outside of currently defined habitable zones. Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry. In 2013, further developments in habitable zone concepts were made with the proposal of a circum- planetary habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.
Definition
A 'circumstellar habitable zone' is the region around a star which allows planets with liquid water. Whether a planet is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit) and the radiative flux of the host star. These values are then used in climate models together with absorption coefficients for and to calculate the edges of the zone. Two different cutoffs may be applied. For the conservative habitable zone the inner edge represents water loss as the planet overheats and the outer edge is the maximum greenhouse effect limit where the atmospheric absorption is inadequate to keep water from freezing. The optimistic habitable zone (OHZ) expands the range to allow for changes in the host star luminosity with age. Based on analysis of possible water on Venus and Mars in the past, the inner edge of the OHZ is called the "recent Venus" edge and the outer edge is called "early Mars". Different climate models give different predictions for the size of the zone.
Habitable planets The habitable zone concept is central to the search for habitable planets. Thousands of exoplanets have been discovered but detailed study of these remote objects is very difficult. Each discovery includes an estimate of the planet's orbital radius and this radius can be compared to the edges of the habital zone.
A planet may be in a "habitable zone" and still be uninhabitable. While conventional habitable zones are defined by the possibility of water, whether a given planet in a habitable zone has water depends on surface conditions that are dependent on many of different individual properties of that planet beyond those considered in the definition of a habitable zone. This misunderstanding is reflected in excited reports of 'habitable planets'.No, the Exoplanet K2-18b Is Not Habitable. News outlets that said otherwise are just crying wolf—but they're not the only ones at fault. Laura Kreidberg, Scientific American. 23 September 2019. Some astronomers have suggested that the term "habitability" should not even be used. Ultimately only technology for remote determination of signs of life will lead to discovery of habitable planets.
Many additional criteria must be met for a habitable zone for complex life, but again a planet in such a zone may still not support complex life. Planets capable of supporting human life would face even more stringent requirements. These include surface temperatures between and light in the visible range with brightness lumens/cm2, gravity below about , an oxygen atmosphere without toxic gases, other life forms essential for food, tolerable wind velocity, low levels of dust, radioactivity, meteor impacts, earthquakes and lightning.
Solar System estimates
Estimates for the habitable zone within the Solar System vary widely. Zones based on the Sun's effective temperature () and the redistribution of heat via an atmosphere lead a zone between and (the Earth is defined to be at , an astronomical unit). These limits include Earth and Mars but not Venus. The inner edge of the zone could be small as for particularly favorable atmospheric and planet surface reflectivity properties.
For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.