Natural shielding againstspace weather andsolar wind, such as themagnetosphere depicted in this artistic rendition, is required for planets to sustain life for prolonged periods.
AHabitable Zone for Complex Life (HZCL) is a range of distances from a star suitable for complex aerobiclife. Different types of limitations preventingcomplex life give rise to different zones.[1] Conventionalhabitable zones are based on compatibility with water.[2] Most zones start at a distance from the hoststar and then end at a distance farther from the star. Aplanet would need toorbit 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 forbacterial life produce much larger zones than those for complex life, which requires a very narrow zone.[3][4][5]
Unstable stars are young and old stars, or very large or small stars. Unstable stars have changingsolar luminosity that changes the size of the life habitable zones. Unstable stars also produce extremesolar flares andcoronal mass ejections. Solar flares and coronal mass ejections can strip away a planet'satmosphere that is not replaceable. Thus life habitable zones require and very stable star like theSun, at ±0.1% solar luminosity change.[10][11] Finding a stable star, like the Sun, is the search for asolar twin, withsolar analogs that have been found.[12] Starmetallicity,mass,age,color, andtemperature all effect luminosity variations.[13][14][15] The Sun, aG2V star, has a mid-range metallicity optimal for the formation of rocky planets.[16] Dwarf stars (red dwarf/orange dwarf/brown dwarf/subdwarf) are not only unstable, but also emit low energy, so the habitable zone is very close to the star and planets becometidally locked on the timescales needed for the development of life.[17]Giant stars (subgiant/giant star/red giant/red supergiant) are unstable and emit high energy, so the habitable zone is very far from the star.[18]Multiple-star systems are also very common and are not suitable for complex life, as the planet orbit would be unstable due to multiple gravitational forces and solar radiation. Liquid water is possible in Multiple-star systems.[19][20][21][22]
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 allow 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.[23][24][25] This zone is atemperature zone, set by the star's radiation and distance from the star. In theSolar System the planetMars is just at the outer boundary of the habitable zone. The planetVenus is at the inner edge of the habitable zone, but due to its thick atmosphere it has no water. The HZ includes planets withelliptic 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 becomesteam on the inner side. The HZ could be defined as the region wherebacteria, 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 theMars carbonate catastrophe, some 4 billion years ago.[26][27]
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.[28][26]
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 agreenhouse effect provided by gases such as CO2 andwater vapor to maintain the correct temperature.Rayleigh scattering would also be needed.[26][11]
Over time and with moreresearch,astronomers,cosmologists andastrobiologist have discovered more parameters needed for life. Each parameter could have a corresponding zone. Some of the named zones include:[29][30]
Ultraviolet habitable zone: a zone where theultraviolet (UV) radiation from a star is neither too weak nor too strong for life to exist.[31] Life needs the correct amount of ultraviolet for synthesis ofbiochemicals. The extent of the zone depends on the amount of ultraviolet radiation from the star, the range of UVwavelengths, the age of the star, and the atmosphere of the planet. In humans UV is used to producevitamin D.[32][33]Extreme ultraviolet (EUV) can cause atmospheric loss.[10]
Photosynthetic habitable zone: a zone where both long-term liquid water and oxygenicphotosynthesis can occur.[34]
Tropospheric habitable zone, orozone habitable zone: a zone where the planet would have the correct amount ofozone needed for life.Inhaling too much ozone causes inflammation and irritation,[35] whereas too littletroposphere ozone would produce biochemical smog. On Earth, the troposphere ozone is part of theground-level ozone protection. Tropospheric ozone is formed by the interaction of ultraviolet light withhydrocarbons andnitrogen oxides.[2][36][37]
Planet rotation rate habitable zone: the zone where a planet'srotation 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'sreflectivity[clarification needed] and thus temperature. A fast rotation rate increaseswind speed on the planet. The rotation rate affects the planet'sclouds and their reflectivity. Slowing the rotation rate changes cloud distributions, cloud altitudes, and cloudopacities. These changes in the clouds changes the temperature of the planet. A high rotation rate also can cause continuous, very fast winds[quantify] on the surface.[38][39][40]
Planet rotation axis tilt habitable zone, orobliquity habitable zone: the region where a stableaxial tilt for a planet's rotation is maintained.[41] Earth's axis is tilted 23.5°; this gives seasons, providingsnow and ice that can melt to provide water run off in the summer.[42][43] Obliquity has a major impact on a planet's temperature, thus its habitable zone.[44][45][46][47]
Tidal habitable zone. Planets too close to the star becometidally 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 havetidal heating from the star. Tidal heating can vary the planet's orbital eccentricity. Too far from the star and the planet will not receive enough solar heat.[48][49][50]
Atmosphere electric field habitable zone: the place in which theambipolar electric field is correct for theplanet's electric field to helpions overcomegravity.[56][failed verification] The planet'sionosphere must be correct to protect against the loss of the atmosphere. This is addition to a strong magnetic field to protect against the solar wind stripping away the atmosphere and water intoouter space.[57][58][59]
Orbital eccentricity habitable zone: the zone in which planets maintain a nearly circular orbit. As orbits with eccentricity have the planets move in and out of the habitable zones.[60] In the Solar System, thegrand tack hypothesis proposes the theory of the unique placement of thegas giants, the Solar System belts and the planets near circular orbits.[61][62][63]
Coupled planet-moon - Magnetosphere habitable zone: the zone that planet'smoon and the planet's core produce a strongmagnetosphere, magnetic field to protect against the solar wind stripping away the planet's atmosphere and water into outer space. Just as Mars had amagnetic field for a short time. Earth's Moon had a large magnetosphere for several hundred million years after itsformation, as proposed in a 2020 study by Saied Mighani. The Moon's magnetosphere would have given added protection ofEarth'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 coupled planet–moon magnetosphere that would give planet the protection from stellar wind in the early Solar System. In the case of Earth, the Moon was closer to Earth in the early formation of the Solar System, giving added protection. This protection was needed then as the Sun was less stable.[10][64]
Pressure-dependent habitable zone: the zone in which planets may have the correctatmospheric pressure to have liquid surface water. With a low atmospheric pressure, the temperature at which water boils is much lower, and at pressures below that of thetriple point, liquid water cannot exist.[65][66] The average surface pressure on Mars today is close to that of the triple point of water; thus, liquid water cannot exist there.[67][68] Planets with high-pressure atmospheres may have liquid surface water, but life forms may experience difficulty withrespiratory systems in high-pressure[quantify] atmospheres.[69][70]
Galactic habitable zone (GHZ): The GHZ, also called theGalactic Goldilocks zone, is the place in agalaxy in whichheavy elements needed for a rocky planet and life are present, but also a place where strongcosmic rays will not kill life and strip the atmosphere off the planet. The termGoldilocks zone is used, as it is a fine balance between the two sites (heavy elements and strong cosmic rays). Not all galaxies are able to support life.[71] In many galaxies, life-killing events such asgamma-ray bursts can occur. About 90% of galaxies have long and frequent gamma ray bursts, thus no life. Cosmic rays pose athreat to life. Galaxies with many stars too close together or without any dust protection also are not hospitable for life.Irregular galaxies and other small galaxies do not have enough heavy elements.Elliptical galaxies are full of lethal radiation and lack heavy elements. Large spiral galaxies, like theMilky Way, have the heavy element needed for life at the center and out to about half distance from the center bar.[72] Not all large spiral galaxies are the same, as spiral galaxies with too much active star formation can be deadly life.[73][74] Too little star formation and thespiral arms will collapse.[75] Not all spiral galaxies have the correctgalactic ram pressure stripping parameters; too much ram pressure can deplete the galaxy of gas and thus end star formation. The Milky Way is abarred spiral galaxy, the bar is important to star formation and metallicity of the galaxy's stars and planets. A barred spiral galaxy must have stable arms with the just right star formation. Barred spiral galaxies make up about 65% of spiral galaxies, but most have too much star formation.[76]Peculiar galaxies lack stable spiral arms,[77] whileirregular galaxies contain too many new stars and lack heavy elements.[78][79]Unbarred spiral galaxies do not correct star formation and metallicity for a galactic Goldilocks zone.[76][80] 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 deadlyasteroid impacts and high radiation.[81][82][83] The planet must be in the correct place in the spiral galaxy: near the galactic center, radiation and gravitational forces are too great for life, whereas the outskirts of a spiral galaxy are metal-poor. 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.[84][16][85]
Supergalactic habitable zone: a place in asupercluster of galaxies that can provide for habitability of planets. The supergalactic habitable zone takes into account events in galaxies that can end habitability not only in a galaxy, but all galaxies nearby, such asgalaxies merging,active galactic nucleus,starburst galaxy,supermassive black holes andmerging black holes, all which output intense radiation. The supergalactic habitable zone also takes into account the abundance of various chemical elements in the galaxy, as not all galaxies or regions within have all the needed elements for life.[86][87][88][89]
Habitable zone for complex life (HZCL): the place that all the life habitable zones overlap for a long period of time, as in the Solar System.[90] The list of habitable zones for complex life has grown longer with increasing understanding of the Universe, galaxies, and the Solar System.[91][92][93][94] Complex life is normally defined aseukaryote life forms, including allanimals,plants,fungi, and mostunicellular organisms. Simple life forms are normally defined asprokaryotes.[95]
Some factors that depend on planetary distance and may limit complex aerobic life have not been given zone names. These include:
Milankovitch cycle TheMilankovitch cycle and ice age have been key is shaping Earth.[96][97] 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.[98][99][100]
Photosynthetic habitable zone has the need parameters for photosynthesis in plants. Thecarbohydrates produced are stored in or used by the plant. Photosynthesis is foundation of food on Earth
Troposphere habitable (Ozone habitable) zone as the correct atmospheric circulation andozone for life. The Three Cell Model of the circulation of the planetary atmosphere of the Earth, of which thetroposphere is the lowest layer.
Orbital eccentricity habitable zone is low enoughorbital eccentricity to support life. Elliptic orbit by eccentricity 0.0· 0.2· 0.4· 0.6· 0.8
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