Habitable zone

In astronomy and astrobiology, the habitable zone, the circumstellar habitable zone, the Goldilocks zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence. As such, it is considered by many to be a major factor of planetary habitability, and the most likely place to find extraterrestrial liquid water and biosignatures elsewhere in the universe.
The habitable zone is also called the Goldilocks zone, 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, 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 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.
In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars, or ammoniates, due to its different colligative properties. In addition, 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

An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia. The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry, "... we may presume, that, being susceptible of vegetation, it has been placed, in the series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate."
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 argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.
The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of the circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million. At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization. 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. James Kasting was the first to present a detailed model for the habitable zone for exoplanets.
An update to the habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez. 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.
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.

Determination

A 'circumstellar habitable zone' is the region around a star which allows planets with liquid water. Whether a given planet in this zone has water depends on surface conditions that are dependent on a host of different individual properties of that planet. This misunderstanding is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.
Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit, the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets that can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.

Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However, their atmospheric conditions vary substantially.
The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and, while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to at which water can only exist as vapor. The entire orbits of the Moon, Mars, and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius for 70 days in the Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes, no confirmation has been made of the presence of liquid water at the surface. While other objects orbit partly within this zone, including comets, Ceres is the only one of planetary mass.
Despite this, studies indicate the strong possibility of past liquid water on the surface of Venus, the Moon, Mars, Vesta and Ceres, suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.
According to the extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth, that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources.
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.

Inner edge	The outer edge	Year	Notes
0.725	1.24	1964, Dole	Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
	1.005–1.008	1969, Budyko	Based on studies of ice-albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969 and North 1975.
0.92–0.96		1970, Rasool and De Bergh	Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
0.958	1.004	1979, Hart	Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
	3.0	1992, Fogg	Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
0.95	1.37	1993, Kasting et al.	Founded the most common working definition of the habitable zone used today. Assumes that CO₂ and H₂O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate–silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
	2.0	2010, Spiegel et al.	Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
0.75		2011, Abe et al.	Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
	10	2011, Pierrehumbert and Gaidos	Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in the Solar System.
0.77–0.87	1.02–1.18	2013, Vladilo et al.	Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar.
0.99	1.67	2013, Kopparapu et al.	Revised estimates of the Kasting et al. formulation using updated moist greenhouse and water loss algorithms. According to this measure, Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al., this applies to an Earth-like planet where the "water loss" limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapor. Once the stratosphere becomes wet, water vapor photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in will not create enough warming to prevent catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.67 AU. This definition does not take into account possible radiative warming by clouds.
0.38		2013, Zsom et al.	Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.
0.95		2013, Leconte et al.	Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System.
0.95	2.4	2017, Ramirez and Kaltenegger	An expansion of the classical carbon dioxide-water vapor habitable zone assuming a volcanic hydrogen atmospheric concentration of 50%.
0.93–0.91		2019, Gomez-Leal et al.	Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model. It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of climate sensitivity in both cases.
0.99	1.004		The tightest bounded estimate from above
0.38	10		The most relaxed estimate from above