Circumstellar habitable zone

As a habitable zone (also life zone, habitable zone or outdated ecosphere, English also Goldilocks zone after the fairy tale Goldilocks and the Three Bears ) is called in general the distance range in which a planet must be located from its central star to allow water permanently in liquid form Earth-like may be present as a prerequisite for life on the surface.

Occasionally, the concept of an environment in which life as we know or similar is possible, extended to other parameters as air and liquid water. So is spoken by a UV habitable zone in which the ultraviolet rays of the ( early ) must correspond to earth, or of a habitable zone of a galaxy in which already have formed enough heavy elements, but on the other hand, not too many supernova explosions occur. Finally, there is the concept of the cosmic habitable age.

• 2.1 The classical habitable zone of liquid water
• 2.2 Habitable zone taking into account the planetary climate 2.2.1 Estimates for the solar system

• 2.3.1 Red dwarfs
• 2.3.2 Stars with greater mass than the Sun
• 2.3.3 White Dwarfs

Term

The concept of the habitable zone goes back to the astronomer Su- Shu Huang and was coined in the late 1950s. The term literally means in German " habitable zone ". This is misleading and has led to criticism. In the true sense of the word refers to " habitable " a celestial body with a fully developed, suitable for human oxygen-carbon ecology. In general, today's astro biological understanding is by habit profitable zone, however, a range of parameters meant, in which a celestial body life can bring, but not necessarily.

Ecosphere or habitable zone

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

Circumstellar habitable zones

The classical habitable zone of liquid water

The basic parameters that the circumstellar habitable zone ( circumstellar habitable zone, CHZ ) on the temperature and luminosity of the star off to the orbits of the planet. Only within a certain distance range of the value of the energy per unit area, which receives the planet in a range, which allows on the resulting surface temperature of liquid water.

In a very simple consideration of the habitable zone can therefore be calculated from the luminosity of the star. The mean radius of this zone of any star can be calculated with the following equation:

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

Addition, however, also plays the finish, especially the albedo ( the albedo ) of the planet, a major role. Modern calculations also take into account the evolution of the planet's atmosphere, as caused by the atmospheric and partly purely chemical greenhouse effect.

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

Since change both the star and the planet in the course of time, also the habitable zone changes. The luminosity of a star increases its development in the course. Be set as develop our life on a planet in a form, it must be located not only at the correct distance, but the circumstances may be on appropriately long time scales not change. The planet needs all the time within the habitable zone are, even if they slowly shifts to a greater distance from the central star. Normally one assumes a minimum period 4-6 billion years for this time. If one wants to emphasize the aspect of time, one also speaks of the continuous habitable zone; but usually we think in the short form " continuous ".

Habitable zone taking into account the planetary climate

The concept of CHZ was substantially refined since the beginnings outlined above by incorporating climate calculations, in particular the greenhouse effect due to 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:

The cycle is self-regulating, because with decreasing temperatures the amount of rain falls, so less carbon is removed from the atmosphere than the volcanism in the long term, that due to the previous climate provides. Thus, the atmospheric carbon is enriched, the greenhouse effect is increasing and counteracts the cooling. With rising temperatures, the cycle over a larger amount of rain also regulated even at a lower greenhouse effect. The duration of the carbonate silicate cycle on the earth is several hundred thousand years.

The inner boundary is now defined by a self-reinforcing global warming, during which the water escapes the planet into interplanetary space, and therefore the regulation of the carbonate -silicate cycle overrides. This limit is in the solar system at about 0.95 AU. At the outer boundary itself clouds can not cause sufficient greenhouse effect more of frozen carbon dioxide. The outer boundary of the CHZ of the solar system is, depending on the model, at 1.37 to 2.4 AU.

In the solar system, only the Earth is well within this belt around the sun. Venus is the sun, like the Mercury, too close. The Mars depending on the model still just inside the CHZ and could therefore have had a sufficient greenhouse effect. However, the planet is too small to hold a plate tectonics over billions of years in transition. Thus, after solidification of the Martian lithosphere was an important element of the non-biological climate equilibrium, the volcanism within the carbonate -silicate cycle, walk, and as the climate on Mars could not stabilize the long term. A planet of Earth's mass could thus at a distance of Mars, depending on the model parameters, still harbor life. At the distance of Jupiter, a planet would receive enough radiation energy under any circumstances, to melt water.

Estimates for the solar system

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

Examples habit profitable zones of stars of the main sequence:

Habitable zones around other than sun-like stars

Red dwarfs

After it was initially assumed that only such stars habitable zones are possible which have a similar size as our sun, referring now also red dwarfs in the considerations. Although would be in low-mass stars, the zone of sufficient energy so close to the star that the rotation of a planet would be synchronized there a rule with its orbital period, ie it uses its central star is always the same page ( just as the moon in the orbit around the Earth ). However, a sufficiently dense atmosphere redistribute the radiant energy of the star sufficiently efficient to allow for large parts of the planet, liquid water.

Stars with larger mass than the Sun

At much more massive stars than the Sun, the life is too short to allow a habitable zone may consist of several billion years. To live with the 3- star 4 times the mass of the Sun have only about a billion years.

White dwarfs

A habitable zone exists at a distance of 0.02 to 0.1 AE white dwarfs. They develop along a cooling sequence of 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 moves to the course of development inside the star. Although at these stars there is a habitable zone, it is likely that no life can develop as on Earth, as in the early phase of the white dwarf hard ultraviolet radiation has split the molecules existing water into hydrogen and oxygen, and thereby formed molecular hydrogen at Earth-sized planet is gravitationally bound not.

Other possible habitable areas with liquid water

The above concept of the habitable zone only makes limited assumptions, the conditions under which life can flourish. The main requirement is liquid water. Water plays a central role for 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 and Saturn's moon Enceladus, but also celestial bodies are now being considered as a candidate for harboring extraterrestrial life that are far outside the orbit of Mars and thus the classical habitable zone. This is considered in the following classification:

• A Class 1 habitat corresponds to a terrestrial planets in the CHZ described above.
• A Class 2 habitat is a planet, although it is also located in a zone defined as above, but nevertheless different from the earth developed on the basis of other parameters, so for example planets around M stars, or a planet at the edge of a habitable zone such as early Mars before the volcanism came to a halt.
• Class 3 habitats are moons or planets with oceans under 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 be liquefied by tidal friction, for example.
• As a class 4 habitats pure water environments are referred to as either moons Enceladus with a thick layer of ice that could be liquid within the ice, or pure ocean planet.

Known exoplanets in the habitable zone

In early 2011, NASA had published preliminary observations of the Kepler mission, therefore, are more than 50 of the 1235 discovered planet candidates within a habitable zone. In December 2011, NASA confirmed the discovery of Kepler 22b, the first extrasolar planet like the Earth is in the habitable zone. Another candidate was before the results of the Kepler mission of about 20 light years away from Earth, Gliese 581 c, the second planet of the red dwarf Gliese 581, but who has now lost its status as a possible habit profitable Planet as it was too intense radiation receives from its star. Later, another planet in the system, Gliese 581 d, become the focus of attention. The planet at eight times the mass of Earth orbiting its star within 84 days once. The planet is in his system in a zone, the climatic conditions as that of the early Mars possible. Thus, he would be a candidate for a planet whose conditions could favor the evolution of life. However, these assumptions are based on model calculations, not on direct observations, and will depend on many model parameters.

Exoplanets that pass through a habitable zone

Even planets that only temporarily residing on its eccentric orbit in the habitable zone could harbor life. Microorganisms at very high or low temperatures, " sleep " and when passing the habitable zone again "wake up" could colonize those planets.

Ultraviolet habitable zone

Analogous to a plane defined by the climate zone, a zone has been proposed in which the ultraviolet radiation of the central star has a similar intensity as it has received the early Earth. This zone is based on the consideration that the chemical evolution not only energy, but also a source of negative entropy required. On the other hand, the UV radiation can not be too intense, otherwise it decomposes the molecules of early biochemistry too quickly.

Galactic habitable zones

The concept of a zone may arise as to the earth in the life, 2001 was extended to galaxies.

Originally, this concept was referring (English galactic habitable zone, GHZ ) only on the chemical state of development of a galactic region, after enough heavy elements in a region of a galaxy must be present so that life can arise. Most of the elements with atomic numbers greater than lithium only build up over time by nuclear fusion processes that occur inside stars, and will be delivered on the death of the stars into the interstellar medium. In the inner regions of a galaxy this nucleosynthesis is faster than in the outer regions, so you can define a maximum radius of the galactic habitable zone.

Later came as a further criterion to this, the rate of star formation in the region of a galaxy added. A star with a planet is at a supernova explosion, which occur preferentially dense in regions of active star formation, thus the planet's atmosphere becomes very disturbed and the planet exposed to strong cosmic radiation than that life could develop permanent. For spiral galaxies like our Milky Way, the supernova rate increases to the inner regions of a galaxy to go. Therefore, you 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. Within this ring, the stellar density is too high, outside the density is too low to allow enough stars have already produces enough heavy elements. Over time, however, enlarges the area to the outside. On the other hand, many of these parameters are very uncertain, so that it may be possible also quite possible that the entire Milky Way is " habitable " in this sense.

Cosmic habitables age

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

Others

In order to classify the properties and habitability of exoplanets better, researchers proposed in 2011 the Earth Similarity Index - ESI (Eng. about Erdähnlichkeits index) and the Planet Habitability Index - PHI (Eng. planetary habitability index) before.