# Habitable zone

The habitable zone (also life zone, habitable zone or obsolete ecosphere ) is the general term used to describe the distance between a planet and its central star so that water can be permanently present in liquid form as a prerequisite for earth-like life on the surface.

Occasionally the concept of an environment in which life in a known or similar form is possible is extended to parameters other than climate and liquid water. We speak of a UV habitable zone in which the ultraviolet radiation must correspond to that of the (early) earth, or of a habitable zone of a galaxy in which sufficiently heavy elements have already formed, but on the other hand not too many supernova explosions happen. Finally, there is the concept of cosmic habitable age.

In the English-speaking world, the habitable zone is also goldilocks zone 'and also partially lehnübersetzt Goldlöckchen- zone named after the fairy tale Goldilocks and the Three Bears ( Goldilocks and the Three Bears ), in which the right balance between two extremes plays a role.

## term

The term habitable zone goes back to the astronomer Su-Shu Huang and was coined in the late 1950s. The term literally means "habitable zone" in German. This is misleading and has led to criticism. In the actual sense of the word, “habitable” refers to a celestial body with a fully developed oxygen-carbon ecology suitable for humans. In the general current astrobiological understanding, however, a habitable zone means a parameter area in which a celestial body can, but does not have to, produce life.

### Ecosphere or habitable zone

A habitable zone has also been referred to as an ecosphere . The term ecosphere goes back to Hubertus Strughold (1953/1955). But today ecosphere is no longer used in this sense . This is due to the alternative term habitable zone, which has now established itself.

## Circumstellar habitable zones

### The classic habitable zone of liquid water

An example of a system based on stellar luminosity for predicting the location of the habitable zone around types of stars

The primary circumstellar habitable zone (depends circumstellar habitable zone, CHZ) from the temperature and luminosity from the star orbits around the planet. Only within a certain distance is the value of the energy per unit area that the planet receives in a range that allows liquid water via the resulting surface temperature.

In a very simple way, the habitable zone can be calculated from the luminosity of the star. The average radius of this zone of any star can be calculated using the following equation:

${\ displaystyle d = {\ sqrt {L _ {\ mathrm {star}} / L _ {\ mathrm {sun}}}}}$
in which
${\ displaystyle d}$is the average radius of the habitable zone in AE ,
${\ displaystyle L _ {\ mathrm {star}} \,}$is the bolometric luminosity of a star , and
${\ displaystyle L _ {\ mathrm {sun}} \,}$is the bolometric luminosity of the sun .

For a star with 25% solar brightness, the central area of ​​the habitable zone would be about 0.5  AU away from the star, with a star twice as bright as the sun the distance would be 1.4 AU. That is the result of the distance law of light brightness. The central area of ​​the habitable zone is defined in this simple model in such a way that an exoplanet with a comparable atmosphere of the earth (structure and density) corresponds roughly to the global average temperature of the earth, the edges correspond to the temperatures at which water freezes or boils.

In addition, the surface properties, in particular the albedo (the reflectivity) of the planet, also play a major role. Modern calculations also take into account the development of the planet's atmosphere, as caused by the atmospheric and partly purely chemical greenhouse effect .

In 1959, physicists Philip Morrison and Giuseppe Cocconi described this zone for the first time in a SETI research report. In 1961 Frank Drake published the Drake equation named after him .

As both the star and the planet change over time, the habitable zone also changes. The luminosity of a star increases in the course of its development. In order for life to develop on another planet in a form similar to that on earth, this planet not only has to be at the right distance, but the circumstances must not change on correspondingly long time scales. The planet must be within the habitable zone all the time, even if it slowly shifts to a greater distance from the central star. Normally one assumes a minimum period of 4 to 6 billion years for this time . If one wants to emphasize the temporal aspect, one also speaks of the continuous habitable zone; but mostly one also means in the short form "the continuous".

### Habitable zone taking into account the planetary climate

The concept of the CHZ has been significantly refined since the beginnings outlined above by including climate calculations, in particular the greenhouse effect caused by carbon dioxide and water .

The greenhouse effect on an inanimate rock planet or moon in the habitable zone is mainly regulated by the carbonate-silicate cycle :

1. Atmospheric CO 2 rains in the form of carbonic acid on the rock's surface, where the acid erodes silicate rocks and the carbon is bound in calcium silicate minerals.
2. The carbon-containing rock is transported by tectonic processes into the planetary lithosphere , where it melts into magma .
3. Volcanism releases the carbon again as CO 2 .

The cycle is self-regulating, as the amount of rain falls when the temperature drops, which is why less carbon is removed from the atmosphere than volcanism delivers in the long term, i.e. due to the earlier climate. As a result, the atmospheric carbon is enriched, the greenhouse effect increases and counteracts the cooling. When the temperature rises, the cycle regulates itself through a larger amount of rain to a lower greenhouse effect. The duration of the carbonate-silicate cycle on earth is several hundred thousand years.

The inner limit is now defined by a self-reinforcing greenhouse effect, in the course of which the planet's water escapes into interplanetary space, thus overriding the regulation of the carbonate-silicate cycle. This limit is around 0.95  AU in the solar system . At the outer limit, even clouds of frozen carbon dioxide can no longer produce a sufficient greenhouse effect. The outer limit of the CHZ of the solar system is 1.37 to 2.4 AU, depending on the model.

In the solar system only the earth is clearly within this belt around the sun . The Venus is, like that of the Sun Mercury , too close. The Mars depending on the model still just inside the CHZ and therefore could have had an adequate greenhouse effect. However, the planet is too small to keep plate tectonics going for billions of years. After the Martian lithosphere solidified, an important element of the non-biological climatic equilibrium, volcanism within the carbonate-silicate cycle, was lost, and so the climate on Mars could not stabilize in the long term. A planet of earth mass could therefore still harbor life at a distance from Mars, depending on the model parameters. At a distance from Jupiter , a planet would under no circumstances receive enough radiant energy to melt water.

Rasool and De Bergh (1970) were able to calculate that a galloping greenhouse effect would occur on earth if it were approx. 10 million km closer to the sun. It is controversial whether such a greenhouse effect could generally occur in the context of climate change on earth, which, similar to what is assumed for Venus, would lead to complete evaporation of all water oceans. What is possible, however, is the occurrence of a state called greenhouse earth , possibly even if the two-degree target agreed in the Paris Agreement is not met . Even this scenario would lead the earth's climate system within a few centuries to a condition that is physiologically unsustainable for the survival of the human species. The extent to which the position of a planet in a habilitable zone really leads to a theoretical habitability therefore depends on the current state of the planet's climate system and can change over time due to a change in the system's regime.

#### Estimates for the solar system

Two-dimensional inner solar system with the planetary orbits superimposed with the estimated minimum (dark green) and maximum (light green) extent of the habitable zone predicted for the solar system.

Estimates for the habitable zone in the solar system range from 0.725 to 3.0 astronomical units based on various scientific models:

 Inner limit Outer limit reference annotation 0.725 AU 1.24 AU Dole 1964 The study used optically thinned atmospheres and fixed albedos. 0.95 AU 1.01 AU Hart u. a. 1978, 1979 Class K0 or later stars cannot have a habitable zone. 0.95 AU 3.0 AU Fogg 1992 Fogg used carbon cycles. 0.95 AU 1.37 AU Kasting et al. a. 1993 - 1%… 2% further outside Budyko 1969 ... and the earth would be glaciated worldwide. - 1%… 2% further outside Sellers 1969 ... and the earth would be glaciated worldwide. - 1%… 2% further outside North 1975 ... and the earth would be glaciated worldwide. 4%… 7% closer - Rasool & DeBurgh 1970 ... and the oceans would never have condensed. - - Schneider and Thompson 1980 Hart disagreed with this. - - Kasting 1991 - - Kasting 1988 Water clouds can reduce the habitable zone as long as they counteract the greenhouse effect with higher albedos. - - Ramanathan and Collins 1991 Greenhouse effect : The inclusion of infrared radiation is greater than the cooling effect of water and albedo, and Venus should have started “dry”. - - Lovelock 1991 - - Whitemire et al. a. 1991

Examples of habitable zones of stars in the main sequence :

Spectral class in AU
O6V 450-900
B5V 20-40
A5V 2.6-5.2
F5V 1.3-2.5
G5V 0.7-1.4
K5V 0.3-0.5
M5V 0.07-0.15

### Habitable zones around stars other than sun-like stars

#### Red dwarfs

After it was initially assumed that habitable zones are only possible around stars that are similar in size to the sun, red dwarfs are now also included in the considerations. In the case of stars with masses below 0.5 solar masses, the zone of sufficient energy would be so close to the star that the rotation of a planet there would normally be synchronized with its orbital time , i.e. That is, it always turns the same side to its central star (like the moon when revolving around the earth). However, a sufficiently dense atmosphere can redistribute the star's radiant energy with sufficient efficiency to enable liquid water over large parts of the planet.

#### Stars of greater mass than the sun

With stars much more massive than the sun, the lifespan is too short for a habitable zone to exist for several billion years. Stars 3–4 times the mass of the Sun have only been living for about a billion years.

#### White dwarfs

A habitable zone also exists at a distance of 0.02 to 0.1 AU around white dwarfs . They develop along a cooling sequence from extremely hot white dwarfs with surface temperatures of several 100,000 K within the Hubble time to temperatures of 3000 K with decreasing luminosity. Accordingly, the habitable zone migrates inward towards the star in the course of development. Although there is a habitable zone around these stars, it can be assumed that no life like on earth can develop, since in the early phase of the white dwarf, hard ultraviolet radiation split the molecules of existing water into hydrogen and oxygen, and the resulting molecular hydrogen contributed Earth-sized planet is gravitationally not bound.

### Other possible habitable areas with liquid water

The above concept of the habitable zone makes only limited assumptions about the conditions under which life can arise. The main requirement is liquid water. Water plays a central role in life as a solvent for biochemical reactions. The problem, however, is that the classic concept of the habitable zone is based on purely atmospheric assumptions.
With the Jupiter moons Ganymede and Europa , the Saturn moons Enceladus and Titan as well as other icy moons (see extraterrestrial ocean ), celestial bodies are now also seen as candidates for harboring extraterrestrial life that are far outside the orbit of Mars and thus the classic habitable zone.
This is taken into account in the following classification:

• A class 1 habitat corresponds to an Earth-like planet in the CHZ described above.
• A class 2 habitat is a planet that is also located in a zone as defined above, but develops differently than the earth due to other parameters, for example planets around M stars or a planet on the edge of a habitable one Zone such as early Mars, before volcanism came to a standstill.
• Class 3 habitats are moons or planets with oceans below the surface, but which are in contact with rock surfaces. Examples of such objects in the solar system are Jupiter's moons Ganymede and Europa. In them the frozen water of the oceans can e.g. B. be liquefied by tidal friction or radioactive nuclides .
• As a class 4 habitats are called pure water environments, either moons like Enceladus with a thick layer of ice that could be liquid only within the ice, or pure ocean planet.

### Known exoplanets in a habitable zone

Comparison of the size and orbital position of the planet Kepler-22b (with fantasy representation of a possible surface view) with planets of the solar system

In early 2011, NASA published preliminary observation data from the Kepler mission , according to which more than 50 of the 1235 planet candidates listed would be within a habitable zone. In December 2011, NASA confirmed the discovery of Kepler 22b , the first exoplanet to be found in a habitable zone. Another candidate was before the results of the Kepler mission of about 20 light years from the Earth distant Gliese 581 c , the second planet of the Red dwarf Gliese 581 , which is but now no longer considered a potentially habit profitable Planet, as it was too intense radiation gets from his star. However, these assumptions are not based on direct observations, but on model calculations and are dependent on numerous model parameters. Since April 2014, Kepler-186f , which orbits the red dwarf Kepler-186 , which is about 500 light-years away , has been considered the most Earth-like of the planets that have so far been recorded in a habitable zone. While Kepler-452b has also been considered habitable since July 2015, this assumption has been questioned since 2018. According to a message from NASA in April 2020, the exoplanet Kepler-1649c can also be classified in this category.

### Exoplanets crossing a habitable zone

A planet that is only temporarily in the habitable zone in its orbit

Planets that are only temporarily in the habitable zone on their eccentric orbit could also harbor life. Microorganisms that "sleep" at very high or low temperatures and "wake up" again when passing through the habitable zone could colonize such planets.

### Ultraviolet habitable zone

Analogous to a zone defined by the climate, a zone has been proposed in which the ultraviolet radiation from the central star has an intensity similar to that received by early Earth. This zone is based on the idea that chemical evolution not only requires energy, but also a source of negative entropy . On the other hand, the UV radiation must not be too intense, otherwise it will break down the molecules of early biochemistry too quickly.

### Ice moons

Several ice moons of the large gas planets of our solar system (especially Jupiter and Saturn ) are believed to have a hidden ocean under the ice layer, for example from Jupiter's moon Europa or Saturn's moon Enceladus . Two effects can cause such a heating and partial liquefaction of an ice sheet: internal radioactivity (as with the earth), but above all tidal forces (tidal heating), triggered by the planet they are orbiting. It is therefore assumed that there could be hydrothermal vents at the bottom of these oceans as on Earth . Since hydrothermal vents (as white and black smokers ) apparently played a decisive role in the origin and early evolution of life on earth, the possibility of at least primitive life forms for such ice moons cannot be ruled out, even if they are outside the regular habitable zone lie - not only for our solar system, but also for other planetary systems. Even for planemos without a central star, the possibility of hidden oceans on icy moons cannot be excluded from the outset.

## Galactic habitable zones

Possible galactic habitable zone of the Milky Way

The concept of a zone in which life can arise like on Earth was extended to galaxies in 2001 .

Originally, this concept (English galactic habitable zone, GHZ) only referred to the chemical development status of a galactic region, according to which there must be enough heavy elements in a region of a galaxy for life to arise. Most elements with atomic numbers greater than lithium are only created over time through nuclear fusion processes that take place inside the stars and are released into the interstellar medium when the stars die . In the inner regions of a galaxy, this nucleosynthesis takes place faster than in the outer regions, which is why a maximum radius of the galactic habitable zone can be defined.

Later, the star formation rate in the respective region of a galaxy was added as a further criterion. If a star with a planet is too close to a supernova explosion , which preferably takes place in regions with active star formation, the planet's atmosphere is too disturbed and the planet is exposed to too strong cosmic radiation for life to develop permanently. For spiral galaxies like the Milky Way , the supernovar rate increases towards the inner regions of a galaxy. Therefore one can also specify an inner radius of the galactic habitable zone.

This means that the galactic habitable zone of a spiral galaxy like the Milky Way forms a ring around the center of the galaxy. Inside this ring the star density is too high, outside the density is too low for enough stars to have already produced enough heavy elements. However, over time, the area will expand outward. On the other hand, many of these parameters are very uncertain, so it may well be possible that the entire Milky Way is “habitable” in this sense.

## Cosmic habitable age

The concept of the habitable age of the universe (English cosmic habitable age, CHA) is based on the chemical development of galaxies since the Big Bang and the knowledge about the structural development of galaxies and galaxy clusters. Based on the experiences of chemical evolution on earth, life can exist in the universe for at least 3.5 billion years and probably for a maximum of 5 billion years. On the other hand, in the future, nucleosynthesis by stars will slow down to such an extent that in an estimated 10 to 20 billion years, geologically important radioactive elements will no longer be present in sufficient quantities in the interstellar medium to keep plate tectonics going on a newly formed planet and so on through the carbonate-silicate cycle to make it suitable for the formation of life in the sense of the circumstellar habitable zone.

## Others

In order to better classify the properties and habitability of exoplanets, researchers proposed the Earth Similarity Index - ESI (German: Earth Similarity Index ) and the Planet Habitability Index - PHI (German: Planet Habitability Index ) in 2011 .

## literature

Commons : Habitable Zone  - collection of pictures, videos and audio files

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