A Habitable Zone for Complex Life (HZCL) is a range of distances from a star suitable for complex aerobic life. Different types of limitations preventing complex life give rise to different zones.^[1] Conventional habitable zones are based on compatibility with water.^[2] Most zones start at a distance from the host star and then end at a distance farther from the star. A planet or exoplanet (a planet outside the Solar System) would need to orbit inside the boundaries of this zone. With multiple zonal constraints, the zones would need to overlap for the planet to support complex life. The requirements for bacterial life produce much larger zones than those for complex life, which requires a very narrow zone.^[3][4][5]

Exoplanets

The first confirmed exoplanets was discovered in 1992, several planets orbiting the pulsar PSR B1257+12.^[6] Since then the list of exoplanets has grown to the thousands.^[7] Most exoplanets are hot Jupiter planets, that orbit very close the star.^[8] Many exoplanets are super-Earths, that could be a gas dwarf or large rocky planet, like Kepler-442b at a mass 2.36 times Earths.^[9]

Star

Unstable stars are young and old stars, or very large or small stars. Unstable stars have changing solar luminosity that changes the size of the life habitable zones. Unstable stars also produce extreme solar flares and coronal mass ejections. Solar flares and coronal mass ejections can strip away a planet's atmosphere that is not replaceable. Thus life habitable zones require and very stable star like the Sun, at ±0.1% solar luminosity change.^[10][11] Finding a stable star, like the Sun, is the search for a solar twin, with solar analogs that have been found.^[12] Stars with an age of 4.6 billion years are at the most stable state. Proper star metallicity, size, mass, age, color, and temperature are also very important to having low luminosity variations.^[13][14][15] The Sun is unique as it is metal rich for its age and type, a G2V star. The Sun is currently in its most stable stage and has the correct metallicity to make it very stable.^[16] Dwarf stars (red dwarf/orange dwarf/brown dwarf/subdwarf) are not only unstabe, but also emit low energy, so habitable zone is very close to the star and the planet is tidally locked. Planet very close also puts it out of almost all the Life habitable zones.^[17] Giant stars (subgiant/giant star/red giant/red supergiant) are unstable and emit high energy, so habitable zone is very far from star. Planet very far out also puts it out of almost all the Life habitable zones, in a place were there are no rocky planets.^[18]

Named habitable zones

A conventional habitable zone is defined by liquid water.

• Habitable zone (HZ) (also called the Circumstellar Habitable Zone), the orbit around a star that would allows liquid water to remain for a short period of time (a given period of time) on at least a small part of the planet's surface. Thus within the HZ, water, (H₂O) is between 0 °C (32 °F; 273 K) and 100 °C (212 °F; 373 K) temperature.^[19][20][21] This zone is a temperature zone, set by the star's radiation and distance from the star. In the Solar System the planet Mars is just at the outer boundary of the habitable zone. The planet Venus is at the inner edge of the habitable zone, but due to its thick atmosphere it has no water. The HZ includes planets with elliptic orbits; such planets might orbit into and out of the HZ. When a planet moves out of the HZ, all its water would freeze to ice on the outside of the HZ, and/or all water would become steam on the inner side. The HZ could be defined as the region where bacteria, a form of life, could possibly survive for a short period of time. The HZ is also sometimes called the "Goldilocks" zone.
• Optimistic habitable zone (OHZ): a zone where liquid surface water could have been on a planet at some time in its past history. This zone would be larger than the HZ. Mars is an example of a planet in the OHZ.: it is just beyond the HZ today, but had liquid water for a short time span before the Mars carbonate catastrophe, some 4 billion years ago.^[22][23]
• Continuously habitable zone (CHZ): a zone where liquid water persists on the surface of a planet for years. This requires a near-circular planetary orbit and a stable star. The zone may be much smaller than the habitable zone.^[24][22]
• Conservative habitable zone: a zone where liquid surface water remains on a planet over a long time span, as on Earth. This might also need a greenhouse effect provoded by gases such as CO₂ and water vapor to maintain the correct temperature. Rayleigh scattering would also be needed.^[22][11]

Named habitable zones for complex life

Over time and with more research, astronomers, cosmologists and astrobiologist have discovered more parameters needed for life. Each parameter could have a corresponding zone. Some of the named zones include:^[25][26]

• Ultraviolet habitable zone: a zone where the ultraviolet (UV) radiation from a star is neither too weak nor too strong for life to exist.^[27] Life needs the correct amount of ultraviolet for synthesis of biochemicals. The extent of the zone depends on the amount of ultraviolet radiation from the star, the range of UV wavelengths, the age of the star, and the atmosphere of the planet. In humans UV is used to produce vitamin D.^[28][29] Extreme ultraviolet (EUV) can cause atmospheric loss.^[10]
• Photosynthetic habitable zone: a zone where both long-term liquid water and oxygenic photosynthesis can occur. This depends on the range of wavelengths of the star's radiation. Red dwarf stars have low solar radiation, with a lower range of wavelengths, so the photosynthetic habitable zone will be closer to the star. A close orbit planet would also be tidally locked. Red dwarf stars also have shorter lives. The light intensity, humidity, carbon dioxide level, soil PH, and soil minerals also must be in the range for photosynthesis.^{[30][31][32][33]}
• Tropospheric habitable zone, or Ozone habitable zone: a zone where the planet would have the correct amount of ozone needed for life. Too much ozone and life would have breathing problems. Inhaling too much ozone causes inflammation and irritation.^[34] Too little troposphere ozone would produce biochemical smog. On Earth the troposphere ozone is part of the ground-level ozone protection. Tropospheric ozone is formed by the interaction of ultraviolet light with hydrocarbons and nitrogen oxides.^[2][35][36]
• Planet rotation rate habitable zone: the zone where a planet's rotation rate is best for life. If rotation is too slow, the day/night temperature difference is too great. The rotation rate also changes the planet's reflectivity^{[clarification needed]} and thus temperature. A fast rotation rate increases wind speed on the planet. The rotation rate affects the planet's clouds and their reflectivity. Slowing the rotation rate changes cloud distributions, cloud altitudes, and cloud opacities. These changes in the clouds changes the temperature of the planet. High ratation rate also can cause continuousvery high wind on the surface.^[37][38][39]
• Planet rotation axis tilt habitable zone, or Obliquity habitable zone: the region where a stable rotation axial tilt is maintained.^[40] This is spin axis on the plane of the orbit around the star. Earth's spin axis is tilted 23.5°, this gives seasons, providing snow and ice that can melt to provide water run off in the summer.^[41][42] Obliquity has an major impact on a planet's temperature, thus its habitable zone.^{[43][44][45][46]}
• Tidal habitable zone. Planets too close to the star will be tidally locked. The mass of the star and the distance from the star set the tidal habitable zone. A planet tidally locked has one side of the planet facing the star, this side would be very hot. The face away from the star would be well below freezing. A planet too close to the star will also have tidal heating from the star. Tidal heating can vary the planet's orbital eccentricity. Too far form the star and the planet will not received enough solar heat.^[47][48][49]
• Astrosphere habitable zone is the zone which a planet's astrosphere will be strong enough to protect the planet from the solar wind and cosmic rays. The astrosphere must be long lasting to protect the planet. Mars lost its water and most of its atmosphere after the losing its magnetic field and Mars carbonate catastrophe event.^[50][51] Star-Sun's solar wind is made of charged particles, including plasma, electrons, protons and alpha particles. The solar wind is different for each star. Earth's magnetic field is very large and has protected Earth since its formation.^[52][53][54]
• Atmosphere electric field habitable zone is place in ambipolar electric field is correct for the planet's electric field to helps ions overcome gravity.^[55] The planet's ionosphere must be correct to protect against the lose of the atmosphere. This is addition to a strong magnetic field to protect against the solar wind stripping away the atmosphere and water into outer space.^[56][57][58]
• Orbital eccentricity habitable zone is zone in which planets with a near circular orbit. As orbits with eccentricity have the planets move in and out of the habitable zones.^[59] In the solar system the grand tack hypothesis gives the theory of the unique placement of the gas giants, the solar system belts and the planets near circular orbits.^[60][61][62]

• Carbon Dioxide habitable zone is the place the planets will have the correct levels of carbon dioxide. The range for carbon dioxide levels is very narrow for life. The gas, Carbon dioxide, is essential for life. Life forms use carbon dioxide to regulate respiration and control blood pH. Plants use carbon dioxide to create oxygen through photosynthesis. High carbon dioxide levels causes hypercapnia.^[63][64]
• Carbon Monoxide habitable zone is the zone where planets will have low levels of carbon monoxide. Carbon monoxide is poisonous, colorless, odorless, tasteless gas. Carbon monoxide is little less dense than air.^[65][66] A planet that has life given atmosphere, cannot have star any dimmer than the Sun, as this would have deadly levels of carbon monoxide. On Earth, carbon monoxide does not accumulate as the Sun has the correct parameters to destroy carbon monoxide in the atmosphere.^[67] Carbon stars are at the extreme side of deadly carbon monoxide levels.^[2][68][69]
• Coupled planet-moon - Magnetosphere habitable zone is the zone that planet's moon and the planet's core produce a strong magnetosphere, magnetic field to protect against the solar wind stripping away the planet's atmosphere and water into outer space. Just has Mars had a magnetic field for a sort time, Saied Mighani put forward in 2020, that Earth's Moon had a large magnetosphere for several hundred million years after its formation. The Moon's magnetospheres would have given added protection of Earth's atmosphere as the early Sun was not as stable as it today. In 2020 James Green modeled the coupled planet-moon-magnetosphere habitable zone. The modeling showed a planet–moon magnetospheres that are coupled together would give planet the protection from star stellar wind in the early solar system. In Earth case, the Moon was closer to Earth in the early formation of the solar system, giving added protection. This protection is needed then as the Sun was not as stable then.^[10][70]
• Pressure-dependent habitable zone is the zone to have the correct atmospheric pressure to have liquid surface water on the planet. With a thin atmospheric pressure the temperature at which water boils is much lower.^[71][72] As on Mars today, with its low atmospheric pressure the freezing, evaporation, and boiling point of water is all at the same temperature. Thus liquid water cannot exist on the surface of Mars.^[73][74] Planets with high-pressure atmospheres would have liquid surface water. But life forms would have difficulty with respiratory systems at high-pressure atmospheres.^[75][76]
• Galactic habitable zone (GHZ), also called the Galactic Goldilocks zone, is the place in a galaxy in which heavy elements needed for a rocky planet and life are present, but also a place where strong cosmic rays will not kill life and strip the atmosphere off the planet. The trem goldilocks is uses, as it is a fine balance between the two sites (heavy element and strong cosmic rays). Galactic habitable zone is the place a planet will have the needed parameters to support life. Not all galaxies are able to support life.^[77] Many galaxies has life killing events like gamma-ray burst. About 90% of galaxies have long and frequent gamma ray bursts, thus no life. Cosmic rays at a high levels will kill life, as there are health threat from cosmic rays. Galaxies with many stars too close together and.or without any dust protection, would not have a place for life. Irregular galaxies and other small galaxies do not have enough heavy elements. A Elliptical galaxy are full of lethal radiation and lack heavy elements. Large spiral galaxies, like the Milky Way, have the heavy element needs for life at its ceneter and out to about half distance from center bar.^[78] Not all Large spiral galaxies are the same, spiral galaxies with too much active star formation can kill the galaxy and life.^[79][80] Too little star formation and the spiral arms will collapse.^[81] Not all spiral galaxies have the correct Galactic ram pressure stripping parameters, too much ram pressure can deplete the galaxy of gas and thus end star formation. The Milky Way is a barred spiral galaxy, the bar is important to star formation and metallicity of the galaxy's stars and planets. Barred spiral galaxy, must have stable arms with the just right star formation. Bars galaxies are in about 65% of spiral galaxies, but most have too much star formation.^[82] A Peculiar galaxy lacks stable spiral arms.^[83] Irregular galaxy as too many new stars and lacks heavy elements.^[84][85] Unbarred spiral galaxy, do not correct star formation and metallicity for a galactic goldilocks zone.^[82][86] For long term life on a planet, the spiral arms must be stable for a long period of time, as in the Milky Way. The spiral arms must not be too close to each other, or there will be too much ultraviolet radiation. If the planet moves into or across a spiral arm the orbits of the planets could change, from gravitational disturbances. Movement across a spiral arms also would cause deadly asteroid impacts and high radiation.^[87][88][89] The planet must be in the correct place in the spiral galaxy, too close to the center and the center bar's radiation and gravitational force is too great for life. Too far from the center and there is not enough heavy elements for life. The Sun in 28,000 light years from the center bar in the galactic goldilocks zone. At this distance the Sun revolves in the galaxy at the same rate as the spiral-arm rotation, thus minimizing arm crossings.^[90][16][91]
• Supergalactic Habitable Zone is place in a supercluster of galaxies that can provide life habitability of planets. Supergalactic Habitable Zone takes into account events in galaxies that can end habitability not only in a galaxy, but all galaxies nearby. Events like: galaxies mergering, active galactic nucleus, starburst galaxy, supermassive black holes and merging black holes, all which output intense radiation. Supergalactic Habitable Zone also takes into account periodic table elements in the galaxy. As not all galaxies and places in a galaxy have all the needed elements for life.^{[92][93][94][95]}
• Habitable Zone for Complex Life (HZCL), would be the place that all the life habitable zones overlap for a long period of time, as in the Solar system.^[67] Other factors would included: very stable stars with correct color, stable asteroid belts; gas giant planets to protect the life on the planet from large asteroids and comets; all planets having near circular orbits; large moon to stabilize the planet, correct planet size; correct planet core; life forms that change with the star's brightness; plate tectonics; carbon cycle; stable water cycle; correct land to ocean area; correct atmospheric pressure and correct atmospheric gases; temperature range much smaller that for just bacteria life, where three phases of water can be present on the surface of the planet.^{[96][2][97][3][4]} Because research has not found other very stable stars (solar twin) or earth like exoplanets (Earth twin), the Rare Earth hypothesis as been gaining more evidence.^[98] The list of habitable zones for complex life has grown longer as more knowledge of the universe, galaxies and the solar system are gained.^{[99][100][101][102]} Complex life is normally defined as eukaryote life forms, this includes all animals, plants, fungi, and most unicellular organisms. Simple life forms are normally defined as prokaryote.^[103]

Other orbital-distance related factors

Some factors that depend on planetary distance and may limit complex aerobic life have not been given zone names. These include:

• Milankovitch cycle The Milankovitch cycle and ice age have been key is shaping Earth.^[104][105] Life on Earth today is using water melting from the last ice age. The ice ages cannot be too long or too cold for life to survive. Milankovitch cycle has an impact on the planet's obliquity also.^{[106][107][108]}

Life

Life on Earth is carbon-based. However, some theories suggest that life could be based on other elements in the periodic table.^[109] Other elements proposed have been silicon, boron, arsenic, ammonia, methane and others. As more research as been done on life on Earth, it has been found that only carbon's organic molecules have the complexity and stability to form life.^{[110][111][112]} Carbon properties allows for complex chemical bonding that produces covalent bonds needed for organic chemistry. Carbon molecules are lightweight and relatively small in size. Carbon's ability to bond to oxygen, hydrogen, nitrogen, phosphorus, and sulfur (called CHNOPS) is key to life.^{[113] [114][115]}

Gallery

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  Photosynthetic habitable zone has the need parameters for photosynthesis in plants. The carbohydrates produced are stored in or used by the plant. Photosynthesis is foundation of food on Earth
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  Troposphere habitable (Ozone habitable) zone as the correct atmospheric circulation and ozone for life. The Three Cell Model of the circulation of the planetary atmosphere of the Earth, of which the troposphere is the lowest layer.
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  Orbital eccentricity habitable zone is low enough orbital eccentricity to support life. Elliptic orbit by eccentricity
    0.0 ·   0.2 ·   0.4 ·   0.6 ·   0.8
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  Planet rotation rate habitable zone, is a planet rotating rate on its axis, too slow causes high temperature variations, too fast causes high winds
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  Supergalactic Habitable Zone is place with low solar radiation and the periodic table elements needed for life. Chart show cosmogenic origin of elements in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae