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Planetary habitability

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.

As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life's flourishing. Of particular interest are those factors that have sustained complex, multicellular organisms on Earth and not just simpler, unicellular creatures. Research and theory in this regard is a component of a number of natural sciences, such as astronomy, planetary science and the emerging discipline of astrobiology.

An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism". In August 2018, researchers reported that water worlds could support life.

Habitability indicators and biosignatures must be interpreted within a planetary and environmental context. In determining the habitability potential of a body, studies focus on its bulk composition, orbital properties, atmosphere, and potential chemical interactions. Stellar characteristics of importance include mass and luminosity, stable variability, and high metallicity. Rocky, wet terrestrial-type planets and moons with the potential for Earth-like chemistry are a primary focus of astrobiological research, although more speculative habitability theories occasionally examine alternative biochemistries and other types of astronomical bodies.

The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science. The late 20th century saw two breakthroughs in the field. The observation and robotic spacecraft exploration of other planets and moons within the Solar System has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of exoplanets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. These findings confirm that the Sun is not unique among stars in hosting planets and expands the habitability research horizon beyond the Solar System.

Earth habitability comparison
While Earth is the only place in the Universe known to harbor life, estimates of habitable zones around other stars, along with the discovery of thousands of exoplanets and new insights into the extreme habitats on Earth where organisms known as extremophiles live, suggest that there may be many more habitable places in the Universe than considered possible until very recently. On 4 November 2013, astronomers reported, based on Kepler space mission 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 within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. As of June 2021, a total of 59 potentially habitable exoplanets have been found.

In August 2021, a new class of habitable planets, named ocean planets, which involves "hot, ocean-covered planets with hydrogen-rich atmospheres", has been reported. Hycean planets may soon be studied for biosignatures by terrestrial telescopes as well as space telescopes, such as the James Webb Space Telescope (JWST), which was launched on 25 December 2021.

Suitable star systems
An understanding of planetary habitability begins with the host star. The classical habitable zone (HZ) is defined for surface conditions only; but a metabolism that does not depend on the stellar light can still exist outside the HZ, thriving in the interior of the planet where liquid water is available.

Under the auspices of SETI's Project Phoenix, scientists Margaret Turnbull and Jill Tarter developed the "HabCat" (or Catalogue of Habitable Stellar Systems) in 2002. The catalogue was formed by winnowing the nearly 120,000 stars of the larger Hipparcos Catalogue into a core group of 17,000 potentially habitable stars, and the selection criteria that were used provide a good starting point for understanding which astrophysical factors are necessary for habitable planets. According to research published in August 2015, very large galaxies may be more favorable to the formation and development of habitable planets than smaller galaxies, like the Milky Way galaxy.

However, what makes a planet habitable is a much more complex question than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host star's plasma environment can influence the evolution of planets and life, if it originated. Liquid water is a necessary but not sufficient condition for life as we know it, as habitability is a function of a multitude of environmental parameters.

Spectral class
The spectral class of a star indicates its photospheric temperature, which (for main-sequence stars) correlates to overall mass. The appropriate spectral range for habitable stars is considered to be "late F" or "G", to "mid-K". This corresponds to temperatures of a little more than 7,000 K down to a little less than 4,000 K (6,700 °C to 3,700 °C); the Sun, a G2 star at 5,777 K, is well within these bounds. This spectral range probably accounts for between 5% and 10% of stars in the local Milky Way galaxy. "Middle-class" stars of this sort have a number of characteristics considered important to planetary habitability:
 * They live at least a few hundred million years, allowing life a chance to evolve. More luminous main-sequence stars of the "O" classes and many members of the "B" classes usually live less than 500 million years and in exceptional cases less than 10 million.
 * They emit enough high-frequency ultraviolet radiation to trigger important atmospheric dynamics such as ozone formation, but not so much that ionisation destroys incipient life.
 * They emit sufficient radiation at wavelengths conducive to photosynthesis.
 * Liquid water may exist on the surface of planets orbiting them at a distance that does not induce tidal locking.

K-type stars may be able to support life far longer than the Sun.

Whether fainter late K and M class red dwarf stars are also suitable hosts for habitable planets is perhaps the most important open question in the entire field of planetary habitability given their prevalence (habitability of red dwarf systems). Gliese 581 c, a "super-Earth", has been found orbiting in the "habitable zone" (HZ) of a red dwarf and may possess liquid water. However it is also possible that a greenhouse effect may render it too hot to support life, while its neighbor, Gliese 581 d, may be a more likely candidate for habitability. In September 2010, the discovery was announced of another planet, Gliese 581 g, in an orbit between these two planets. However, reviews of the discovery have placed the existence of this planet in doubt, and it is listed as "unconfirmed". In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.

A recent study suggests that cooler stars that emit more light in the infrared and near infrared may actually host warmer planets with less ice and incidence of snowball states. These wavelengths are absorbed by their planets' ice and greenhouse gases and remain warmer.

A 2020 study found that about half of Sun-like stars could host rocky, potentially habitable planets. Specifically, they estimated with that, on average, the nearest habitable zone planet around G and K-type stars is about 6 parsecs away, and there are about 4 rocky planets around G and K-type stars within 10 parsecs (32.6 light years) of the Sun.

A stable habitable zone
The habitable zone (HZ) is a shell-shaped region of space surrounding a star in which a planet could maintain liquid water on its surface. The concept was first proposed by astrophysicist Su-Shu Huang in 1959, based on climatic constraints imposed by the host star. After an energy source, liquid water is widely considered the most important ingredient for life, considering how integral it is to all life systems on Earth. However, if life is discovered in the absence of water, the definition of an HZ may have to be greatly expanded.

The inner edge of the HZ is the distance where runaway greenhouse effect vaporize the whole water reservoir and, as a second effect, induce the photodissociation of water vapor and the loss of hydrogen to space. The outer edge of the HZ is the distance from the star where a maximum greenhouse effect fails to keep the surface of the planet above the freezing point, and by (carbon dioxide) condensation.

A "stable" HZ implies two factors. First, the range of an HZ should not vary greatly over time. All stars increase in luminosity as they age, and a given HZ thus migrates outwards, but if this happens too quickly (for example, with a super-massive star) planets may only have a brief window inside the HZ and a correspondingly smaller chance of developing life. Calculating an HZ range and its long-term movement is never straightforward, as negative feedback loops such as the CNO cycle will tend to offset the increases in luminosity. Assumptions made about atmospheric conditions and geology thus have as great an impact on a putative HZ range as does stellar evolution: the proposed parameters of the Sun's HZ, for example, have fluctuated greatly.

Second, no large-mass body such as a gas giant should be present in or relatively close to the HZ, thus disrupting the formation of Earth-size bodies. The matter in the asteroid belt, for example, appears to have been unable to accrete into a planet due to orbital resonances with Jupiter; if the giant had appeared in the region that is now between the orbits of Venus and Mars, Earth would almost certainly not have developed in its present form. However a gas giant inside the HZ might have habitable moons under the right conditions.

Low stellar variation
Changes in luminosity are common to all stars, but the severity of such fluctuations covers a broad range. Most stars are relatively stable, but a significant minority of variable stars often undergo sudden and intense increases in luminosity and consequently in the amount of energy radiated toward bodies in orbit. These stars are considered poor candidates for hosting life-bearing planets, as their unpredictability and energy output changes would negatively impact organisms: living things adapted to a specific temperature range could not survive too great a temperature variation. Further, upswings in luminosity are generally accompanied by massive doses of gamma ray and X-ray radiation which might prove lethal. Atmospheres do mitigate such effects, but their atmosphere might not be retained by planets orbiting variables, because the high-frequency energy buffeting these planets would continually strip them of their protective covering.

The Sun, in this respect as in many others, is relatively benign: the variation between its maximum and minimum energy output is roughly 0.1% over its 11-year solar cycle. There is strong (though not undisputed) evidence that even minor changes in the Sun's luminosity have had significant effects on the Earth's climate well within the historical era: the Little Ice Age of the mid-second millennium, for instance, may have been caused by a relatively long-term decline in the Sun's luminosity. Thus, a star does not have to be a true variable for differences in luminosity to affect habitability. Of known solar analogs, one that closely resembles the Sun is considered to be 18 Scorpii; unfortunately for the prospects of life existing in its proximity, the only significant difference between the two bodies is the amplitude of the solar cycle, which appears to be much greater for 18 Scorpii.

High metallicity
While the bulk of material in any star is hydrogen and helium, there is a significant variation in the amount of heavier elements (metals). A high proportion of metals in a star correlates to the amount of heavy material initially available in the protoplanetary disk. A smaller amount of metal makes the formation of planets much less likely, under the solar nebula theory of planetary system formation. Any planets that did form around a metal-poor star would probably be low in mass, and thus unfavorable for life. Spectroscopic studies of systems where exoplanets have been found to date confirm the relationship between high metal content and planet formation: "Stars with planets, or at least with planets similar to the ones we are finding today, are clearly more metal rich than stars without planetary companions." This relationship between high metallicity and planet formation also means that habitable systems are more likely to be found around stars of younger generations, since stars that formed early in the universe's history have low metal content.

Planetary characteristics

Habitability indicators and biosignatures must be interpreted within a planetary and environmental context. Whether a planet will emerge as habitable depends on the sequence of events that led to its formation, which could include the production of organic molecules in molecular clouds and protoplanetary disks, delivery of materials during and after planetary accretion, and the orbital location in the planetary system. The chief assumption about habitable planets is that they are terrestrial. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks, and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. The possibility that life could evolve in the cloud tops of giant planets has not been decisively ruled out, though it is considered unlikely, as they have no surface and their gravity is enormous. The natural satellites of giant planets, meanwhile, remain valid candidates for hosting life.

In February 2011 the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the habitable zone. Six of the candidates in this zone are smaller than twice the size of Earth. A more recent study found that one of these candidates (KOI 326.01) is much larger and hotter than first reported. Based on the findings, the Kepler team estimated there to be "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.

In analyzing which environments are likely to support life, a distinction is usually made between simple, unicellular organisms such as bacteria and archaea and complex metazoans (animals). Unicellularity necessarily precedes multicellularity in any hypothetical tree of life, and where single-celled organisms do emerge there is no assurance that greater complexity will then develop. The planetary characteristics listed below are considered crucial for life generally, but in every case multicellular organisms are more picky than unicellular life.

Mass and size
Low-mass planets are poor candidates for life for two reasons. First, their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars, with its thin atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun), and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa) (0.18 inch Hg), does not occur. In addition, a lessened pressure reduces the range of temperatures at which water is liquid.

Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's magnetic field.

"Low mass" is partly a relative label: the Earth is low mass when compared to the Solar System's gas giants, but it is the largest, by diameter and mass, and the densest of all terrestrial bodies. It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface (the decay of radioactive elements within a planet's core is the other significant component of planetary heating). Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere. Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus: 0.3 Earth masses has been offered as a rough dividing line for habitable planets. However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher. Earth may in fact lie on the lower boundary of habitability: if it were any smaller, plate tectonics would be impossible. Venus, which has 85% of Earth's mass, shows no signs of tectonic activity. Conversely, "super-Earths", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.

Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.

Saturn's Titan, meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding.

A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets.

Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and to bombard living things with ionized particles. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core —but it is a significant component of the process.

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses. However it is possible for a habitable world to have a mass as low as 0.0268 Earth Masses. The radius of a potentially habitable exoplanet would range between 0.5 and 1.5 Earth radii.

Orbit and rotation
As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity. The Earth's orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury) have eccentricities that are similarly benign.

Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25. This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.

A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems.

If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt (see Snowball Earth).

The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature." Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.

Other orbital considerations include:
 * The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
 * The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
 * Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations; see Milankovitch cycles. Precession on Earth occurs over a 26,000-year cycle.

The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability. This position remains controversial.

In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to ocean tides, which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate.

Geology
Concentrations of radionuclides in rocky planet mantles may be critical for the habitability of Earth-like planets. Such planets with higher abundances likely lack a persistent dynamo for a significant fraction of their lifetimes, and those with lower concentrations may often be geologically inert. Planetary dynamos create strong magnetic fields which may often be necessary for life to develop or persist as they shield planets from solar winds and cosmic radiation. The electromagnetic emission spectra of stars could be used to identify those which are more likely to host habitable Earth-like planets. As of 2020, radionuclides are thought to be produced by rare stellar processes such as neutron star mergers.

Additional geological characteristics may be essential or major factors in the habitability of natural celestial bodies – including some that may shape the body's heat and magnetic field. Some of these are unknown or not well understood and being investigated by planetary scientists, geochemists and others.

Geochemistry
It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple amino acids such as glycine, have been found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth's collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur (required for the building of proteins) nor phosphorus (needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism) are rare.

Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust. This can be partly explained by the fact that many of these elements, such as hydrogen and nitrogen, along with their simplest and most common compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as silica (a compound of silicon and oxygen, accounting for oxygen's relative abundance). Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller–Urey experiment showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize amino acids.

Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans. The vast majority of the water—and arguably carbon—necessary for life must have come from the outer Solar System, away from the Sun's heat, where it could remain solid. Comets impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires, onto the early Earth, providing a kick-start to the origin of life.

Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.

Microenvironments and extremophiles
One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life, a so-called Goldilocks Edge or Great Prebiotic Spot. Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as mutation, selection, and genetic drift, operate in micro-organisms that act on and respond to changing micro-environments." Extremophiles are Earth organisms that live in niche environments under severe conditions generally considered inimical to life. Usually (although not always) unicellular, extremophiles include acutely alkaliphilic and acidophilic organisms and others that can survive water temperatures above 100 °C in hydrothermal vents.

The discovery of life in extreme conditions has complicated definitions of habitability, but also generated much excitement amongst researchers in greatly broadening the known range of conditions under which life can persist. For example, a planet that might otherwise be unable to support an atmosphere given the solar conditions in its vicinity, might be able to do so within a deep shadowed rift or volcanic cave. Similarly, craterous terrain might offer a refuge for primitive life. The Lawn Hill crater has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of Mars.

Earth environments that cannot support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the Atacama desert, generally considered the driest place on Earth, appears unable to support life, and it has been subject to study by NASA and ESA for that reason: it provides a Mars analog and the moisture gradients along its edges are ideal for studying the boundary between sterility and habitability. The Atacama was the subject of study in 2003 that partly replicated experiments from the Viking landings on Mars in the 1970s; no DNA could be recovered from two soil samples, and incubation experiments were also negative for biosignatures.

Ecological factors
The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.

Classification terminology
The Habitable Exoplanets Catalog uses estimated surface temperature range to classify exoplanets:
 * hypopsychroplanets - very cold (<−50 °C)
 * psychroplanets - cold (<−50 to 0 °C)
 * mesoplanets - medium temperature (0–50 °C; not to be confused with the other definition of mesoplanets)
 * thermoplanets - hot (50-100 °C)
 * hyperthermoplanets - (> 100 °C)

Mesoplanets would be ideal for complex life, whereas hypopsychroplanets and hyperthermoplanets might only support extremophilic life.

The HEC uses the following terms to classify exoplanets in terms of mass, from least to greatest: asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian.

Alternative star systems
In determining the feasibility of extraterrestrial life, astronomers had long focused their attention on stars like the Sun. However, since planetary systems that resemble the Solar System are proving to be rare, they have begun to explore the possibility that life might form in systems very unlike the Sun's.

It is believed that F, G, K and M-type stars could host habitable exoplanets. About half of the stars similar in temperature to the Sun could have a rocky planet able to support liquid water on its surface, according to research using data from NASA's Kepler Space Telescope.

Binary systems
Typical estimates often suggest that 50% or more of all stellar systems are binary systems. This may be partly sample bias, as massive and bright stars tend to be in binaries and these are most easily observed and catalogued; a more precise analysis has suggested that the more common fainter stars are usually singular, and that up to two thirds of all stellar systems are therefore solitary.

The separation between stars in a binary may range from less than one astronomical unit (AU, the average Earth–Sun distance) to several hundred. In latter instances, the gravitational effects will be negligible on a planet orbiting an otherwise suitable star and habitability potential will not be disrupted unless the orbit is highly eccentric (see Nemesis, for example). However, where the separation is significantly less, a stable orbit may be impossible. If a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed. Whether planets might form in binaries at all had long been unclear, given that gravitational forces might interfere with planet formation. Theoretical work by Alan Boss at the Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars.

One study of Alpha Centauri, the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 AU distance at closest approach (23 AU mean), and both should have stable habitable zones. A study of long-term orbital stability for simulated planets within the system shows that planets within approximately three AU of either star may remain rather stable (i.e. the semi-major axis deviating by less than 5% during 32 000 binary periods). The continuous habitable zone (CHZ for 4.5 billion years) for Centauri A is conservatively estimated at 1.2 to 1.3 AU and Centauri B at 0.73 to 0.74—well within the stable region in both cases.

Red dwarf systems
M-type stars also considered possible hosts of habitable exoplanets, even those with flares such as Proxima b. Determining the habitability of red dwarf stars could help determine how common life in the universe might be, as red dwarfs make up between 70 and 90% of all the stars in the galaxy. However, it is important to bear in mind that flare stars could greatly reduce the habitability of exoplanets by eroding their atmosphere.

Size
Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal locking. One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side, or if there was a gas giant in the habitable zone, with a habitable moon, which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the moon. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis.

This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO 2 and H2O) need only be 100 mb, for the star's heat to be effectively carried to the night side. This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.

Other factors limiting habitability
Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on the Earth the process depends on visible light. There are potential positives to this scenario. Numerous terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, for instance, which would be possible in a red dwarf system. A static primary star position removes the need for plants to steer leaves toward the sun, deal with changing shade/sun patterns, or change from photosynthesis to stored energy during night. Because of the lack of a day-night cycle, including the weak light of morning and evening, far more energy would be available at a given radiation level.

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time, while at other times they emit gigantic flares that can double their brightness in a matter of minutes. Such variation would be very damaging for life, as it would not only destroy any complex organic molecules that could possibly form biological precursors, but also because it would blow off sizeable portions of the planet's atmosphere.

For a planet around a red dwarf star to support life, it would require a rapidly rotating magnetic field to protect it from the flares. A tidally locked planet rotates only very slowly, and so cannot produce a geodynamo at its core. The violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidal locking, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop. However, observations of the 7 to 12-billion year old Barnard's Star showcase that even old red dwarfs can have significant flare activity. Barnard's Star was long assumed to have little activity, but in 1998 astronomers observed an intense stellar flare, showing that it is a flare star.

Longevity and ubiquity
Red dwarfs have one advantage over other stars as abodes for life: far greater longevity. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for 1 to 2.3 billion years more. Red dwarfs, by contrast, could live for trillions of years because their nuclear reactions are far slower than those of larger stars, meaning that life would have longer to evolve and survive.

While the likelihood of finding a planet in the habitable zone around any specific red dwarf is slight, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity. Furthermore, this total amount of habitable zone will last longer, because red dwarf stars live for hundreds of billions of years or even longer on the main sequence. However, combined with the above disadvantages, it is more likely that red dwarf stars would remain habitable longer to microbes, while the shorter-lived yellow dwarf stars, like the Sun, would remain habitable longer to animals.

G-type star systems
G-type stars would allow to host the exoplanets most similar to Earth, that is, Earth-like planets.

K-type star systems
K-type stars would provide the necessary conditions for super habitable exoplanets, which are exoplanets that could be more habitable than Earth.

Massive stars
Recent research suggests that very large stars, greater than ~100 solar masses, could have planetary systems consisting of hundreds of Mercury-sized planets within the habitable zone. Such systems could also contain brown dwarfs and low-mass stars (~0.1–0.3 solar masses). However the very short lifespans of stars of more than a few solar masses would scarcely allow time for a planet to cool, let alone the time needed for a stable biosphere to develop. Massive stars are thus eliminated as possible abodes for life.

However, a massive-star system could be a progenitor of life in another way – the supernova explosion of the massive star in the central part of the system. This supernova will disperse heavier elements throughout its vicinity, created during the phase when the massive star has moved off of the main sequence, and the systems of the potential low-mass stars (which are still on the main sequence) within the former massive-star system may be enriched with the relatively large supply of the heavy elements so close to a supernova explosion. However, this states nothing about what types of planets would form as a result of the supernova material, or what their habitability potential would be.

Four classes of habitable planets based on water
In a review of the factors which are important for the evolution of habitable Earth-sized planets, Lammer et al. proposed a classification of four water-dependent habitat types:

Class I habitats are planetary bodies on which stellar and geophysical conditions allow liquid water to be available at the surface, along with sunlight, so that complex multicellular organisms may originate.

Class II habitats include bodies which initially enjoy Earth-like conditions, but do not keep their ability to sustain liquid water on their surface due to stellar or geophysical conditions. Mars, and possibly Venus are examples of this class where complex life forms may not develop.

Class III habitats are planetary bodies where liquid water oceans exist below the surface, where they can interact directly with a silicate-rich core.
 * Such a situation can be expected on water-rich planets located too far from their star to allow surface liquid water, but on which subsurface water is in liquid form because of the geothermal heat. Two examples of such an environment are Europa and Enceladus. In such worlds, not only is light not available as an energy source, but the organic material brought by meteorites (thought to have been necessary to start life in some scenarios) may not easily reach the liquid water. If a planet can only harbor life below its surface, the biosphere would not likely modify the whole planetary environment in an observable way, thus, detecting its presence on an exoplanet would be extremely difficult.

Class IV habitats have liquid water layers between two ice layers, or liquids above ice.
 * If the water layer is thick enough, water at its base will be in solid phase (ice polymorphs) because of the high pressure. Ganymede and Callisto are likely examples of this class. Their oceans are thought to be enclosed between thick ice layers. In such conditions, the emergence of even simple life forms may be very difficult because the necessary ingredients for life will likely be completely diluted.

The galactic neighborhood
Along with the characteristics of planets and their star systems, the wider galactic environment may also impact habitability. Scientists considered the possibility that particular areas of galaxies (galactic habitable zones) are better suited to life than others; the Solar System, in the Orion Arm, on the Milky Way galaxy's edge is considered to be in a life-favorable spot:
 * It is not in a globular cluster where immense star densities are inimical to life, given excessive radiation and gravitational disturbance. Globular clusters are also primarily composed of older, probably metal-poor, stars. Furthermore, in globular clusters, the great ages of the stars would mean a large amount of stellar evolution by the host or other nearby stars, which due to their proximity may cause extreme harm to life on any planets, provided that they can form.
 * It is not near an active gamma ray source.
 * It is not near the galactic center where once again star densities increase the likelihood of ionizing radiation (e.g., from magnetars and supernovae). The supermassive black holes at the centers of galaxies may also prove a danger to any nearby bodies.
 * The circular orbit of the Sun around the galactic center keeps it out of the way of the galaxy's spiral arms where intense radiation and gravitation may again lead to disruption.

Thus, relative isolation is ultimately what a life-bearing system needs. If the Sun were crowded amongst other systems, the chance of being fatally close to dangerous radiation sources would increase significantly. Further, close neighbors might disrupt the stability of various orbiting bodies such as Oort cloud and Kuiper belt objects, which can bring catastrophe if knocked into the inner Solar System.

While stellar crowding proves disadvantageous to habitability, so too does extreme isolation. A star as metal-rich as the Sun would probably not have formed in the very outermost regions of the Milky Way given a decline in the relative abundance of metals and a general lack of star formation. Thus, a "suburban" location, such as the Solar System enjoys, is preferable to a Galaxy's center or farthest reaches.

Alternative biochemistries
While most investigations of extraterrestrial life start with the assumption that advanced life-forms must have similar requirements for life as on Earth, the hypothesis of other types of biochemistry suggests the possibility of lifeforms evolving around a different metabolic mechanism. In Evolving the Alien, biologist Jack Cohen and mathematician Ian Stewart argue astrobiology, based on the Rare Earth hypothesis, is restrictive and unimaginative. They suggest that Earth-like planets may be very rare, but non-carbon-based complex life could possibly emerge in other environments. The most frequently mentioned alternative to carbon is silicon-based life, while ammonia and hydrocarbons are sometimes suggested as alternative solvents to water. The astrobiologist Dirk Schulze-Makuch and other scientists have proposed a Planet Habitability Index whose criteria include "potential for holding a liquid solvent" that is not necessarily restricted to water.

More speculative ideas have focused on bodies altogether different from Earth-like planets. Astronomer Frank Drake, a well-known proponent of the search for extraterrestrial life, imagined life on a neutron star: submicroscopic "nuclear molecules" combining to form creatures with a life cycle millions of times quicker than Earth life. Called "imaginative and tongue-in-cheek", the idea gave rise to science fiction depictions. Carl Sagan, another optimist with regards to extraterrestrial life, considered the possibility of organisms that are always airborne within the high atmosphere of Jupiter in a 1976 paper. Cohen and Stewart also envisioned life in both a solar environment and in the atmosphere of a gas giant.

"Good Jupiters"
"Good Jupiters" are gas giants, like the Solar System's Jupiter, that orbit their stars in circular orbits far enough away from the habitable zone not to disturb it but close enough to "protect" terrestrial planets in closer orbit in two critical ways. First, they help to stabilize the orbits, and thereby the climates of the inner planets. Second, they keep the inner stellar system relatively free of comets and asteroids that could cause devastating impacts. Jupiter orbits the Sun at about five times the distance between the Earth and the Sun. This is the rough distance we should expect to find good Jupiters elsewhere. Jupiter's "caretaker" role was dramatically illustrated in 1994 when Comet Shoemaker–Levy 9 impacted the giant.

However, the evidence is not quite so clear. Research has shown that Jupiter's role in determining the rate at which objects hit Earth is significantly more complicated than once thought.

The role of Jupiter in the early history of the Solar System is somewhat better established, and the source of significantly less debate. Early in the Solar System's history, Jupiter is accepted as having played an important role in the hydration of our planet: it increased the eccentricity of asteroid belt orbits and enabled many to cross Earth's orbit and supply the planet with important volatiles such as water and carbon dioxide. Before Earth reached half its present mass, icy bodies from the Jupiter–Saturn region and small bodies from the primordial asteroid belt supplied water to the Earth due to the gravitational scattering of Jupiter and, to a lesser extent, Saturn. Thus, while the gas giants are now helpful protectors, they were once suppliers of critical habitability material.

In contrast, Jupiter-sized bodies that orbit too close to the habitable zone but not in it (as in 47 Ursae Majoris), or have a highly elliptical orbit that crosses the habitable zone (like 16 Cygni B) make it very difficult for an independent Earth-like planet to exist in the system. See the discussion of a stable habitable zone above. However, during the process of migrating into a habitable zone, a Jupiter-size planet may capture a terrestrial planet as a moon. Even if such a planet is initially loosely bound and following a strongly inclined orbit, gravitational interactions with the star can stabilize the new moon into a close, circular orbit that is coplanar with the planet's orbit around the star.

Life's impact on habitability
A supplement to the factors that support life's emergence is the notion that life itself, once formed, becomes a habitability factor in its own right. An important Earth example was the production of molecular oxygen gas by ancient cyanobacteria, and eventually photosynthesizing plants, leading to a radical change in the composition of Earth's atmosphere. This environmental change is called the Great Oxidation Event. This oxygen proved fundamental to the respiration of later animal species. The Gaia hypothesis, a scientific model of the geo-biosphere pioneered by James Lovelock in 1975, argues that life as a whole fosters and maintains suitable conditions for itself by helping to create a planetary environment suitable for its continuity. Similarly, David Grinspoon has suggested a "living worlds hypothesis" in which our understanding of what constitutes habitability cannot be separated from life already extant on a planet. Planets that are geologically and meteorologically alive are much more likely to be biologically alive as well and "a planet and its life will co-evolve." This is the basis of Earth system science.

The role of chance
In 2020, a computer simulation of the evolution of planetary climates over 3 billion years suggested that feedbacks are a necessary but not a sufficient condition for preventing planets from ever becoming too hot or cold for life, and that chance also plays a crucial role. Related considerations include yet unknown factors influencing the thermal habitability of planets such as "feedback mechanism (or mechanisms) that prevents the climate ever wandering to fatal temperatures".

title: Planetary habitability


text:
Talk:Titan Rain

Untitled
This is a hot news item that no doubt will be further reported on in coming weeks. I think it should remain.

Good call. Check out the front page of the Washington Times online today, and you will see more stories about Titan Rain. --Chitown2pa

Mil-Stub
All reputable research into the subject indicates that these attacks originate from within the Chinese military and are aimed at the networks of defense contractors and the U.S. military. Thus, I'm mil-stubbing the article. Tzaquiel 18:09, 5 December 2006 (UTC)
 * What reputable research? The only "source" of information right now are from the main stream media which know as much about this issue as you or me.  You can't blame someone until you have proof or it's just fear mongering and sensationalistic headlines.  I have modified the article to reflect this.  —Preceding unsigned comment added by 24.222.16.170 (talk) 20:55, 12 February 2008 (UTC)

The attacks are believed to be the result of actions by People's Liberation Army Unit 61398. [3]

Adding on User:Tzaquiel point: 'believed' by whom? The ref[3] only links to a webpage introducing Unit 61398 (which has been already referenced to the wiki page), instead of any supporting evidence. The only piece of information I can direct to is a Twitter page. Not solid enough in my opinion. -- by by July 19, 2020

January 2015 pruning
Given the writing style and lack of inline sourcing, my guess is that there was a high school assignment to expand this article. I've removed most of that material. The article should be expanded, but with better writing and proper sourcing. OhNo itsJamie Talk 15:11, 8 January 2015 (UTC)

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title: Talk:Titan Rain


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User:Joshuajohaneman

— Joshua Johaneman 19:50, 9 June 2015 (UTC)

title: User:Joshuajohaneman


text:
Talk:Massimo Morsello

Revision
Is this article a translation from Italian? The English version needs reworking to bring it up to Wikipedia's standards. Simon
 * No, but I'm Italian, so my English probably is not perfect. I have seen people already corrected the worst blunders. I'm sorry. Cyclopia 20:01, 1 September 2005 (UTC)

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Talk:Akashvani (radio broadcaster)

Untitled
In South-Asia it is still claimed that Radio is a powerful medium and whatever expansion it has achieved is not sufficient. This view point is gathered when signal availability of other channels is abundantly available and it is estimated that the listeners of foreign radio services can be in millions. This is also true for those countries which are directing signal towards this region that their penetration is satisfactory and they get consolation that the target countries are taking necessary steps to counter their influence. This may be not their optimistic goals and they may consider that information flow should be one-sided and if they will not engage then it is possible other will engage their people.

That is why, these countries are beaming programs to this region and :Contributions/134.117.200.232|134.117.200.232]] (talk) 19:06, 30 September 2008 (UTC)

All India Radio Signature Tune
Who composed All India Radio Signature Tune which we hear everyday morning when AIR starts its transmission. It is widely believed that well known violin Artist Shri V.G.Jog did that. But it is also in circulation that Walter Kaufmann a Czech Composer who worked for AIR during 1930s had given the original tune which later on was improvised. Please write about the history of All India Radio Signature Tune.

-SIVA —Preceding unsigned comment added by 119.82.108.194 (talk) 02:43, 20 September 2010 (UTC)

Clean Up
It is a very notable and important article and it's interesting to see this in this state. It requires clean up. Anybody? It is not written like an advertisement though. I've removed the advert template. Anuandraj (talk) 22:50, 29 May 2011 (UTC)

I agree it needs a lot of clean up especially the controversy part.Its written as a blog :)  — Preceding unsigned comment added by Djds4rce (talk • contribs) 14:17, 31 May 2011 (UTC)

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Aakash wani
Can any one tell who suggested aakash wani name ?? Unnati saxena (talk) 14:03, 17 February 2016 (UTC)

Assessment comment
Substituted at 18:07, 17 July 2016 (UTC)

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Carson's Law

Carson's Law is an Australian television series made by Crawford Productions for the Ten Network between 1982-1984. The series was a period piece set in the 1920s and starred Lorraine Bayly as progressive solicitor Jennifer Carson. The episodes revolved around the cases taken on by Jennifer, and the various personal intrigues of her family.

Brief Synopsis
The series' premiere was billed as a 90-minute "movie-length" episode on 24 January 1983, with another two-hour episode in the same timeslot the following night, before settling into its twice-weekly 60-minute format the following week. Carson's Law was noted for its quality scripts and period production values. Despite being popular in Melbourne, where the series was based and filmed, it did not succeed in Sydney. Attempts to revamp the series to make it appeal more to Sydney audiences eventually alienated the programme's core audience, and it was cancelled in 1984 after 184 episodes. The final episode aired on ATV-10 on 1 December 1984; after being shuffled around the TEN-10 schedule, then reduced to one episode per week, then pushed back from 8:30pm to 9:30pm and, finally, to the even later 10:40pm slot, the final episode eventually aired in Sydney on 18 April 1985. '

International Broadcasts
Carson's Law was bought by several members of the UK ITV network. TVS aired edited half-hour episodes on Thursdays and Fridays at 3.30pm in 1987 and 1988. At the same time, Yorkshire Television showed a considerable number of episodes on Thursdays at 1.30pm between October 1984 and March 1988. Television South West screened a slightly lower number in their 6.35pm slot on Tuesdays during 1984. Ulster Television aired episodes on Monday nights at 11.10pm up until at least October 1992.

Carson's Law was also shown in Portuguese television, and is being currently re-played at RTP memoria (trans. "RTP Memory").

Carson's Law was shown in Catalan television (TV3) as well. The Catalan name for the series is Carson i Carson advocats.

Carson's Law was also shown in the Netherlands for its entirety by the NCRV broadcasting company. The company adapted the title to simply 'The Carsons'. It ran in the afternoon, and was quite popular.

Carson's Law was also shown in Israeli television (Channel 2, experimental broadcasting). It was called The race to the top of Jennifer Carson ("Ha-Merotz La-Tzameret Shel Jennifer Carson"). It ran on a weekly basis for about two years.

Carson's Law was also shown in Germany in the early 90s. It ran daily in the afternoon and was called "Carson & Carson".

DVD Releases
The first 13 episodes of the series was released onto Region 2 DVD in the Netherlands in September 2009. The disc set has removable Dutch subtitles with all audio in English. In Australia seven volumes covering the entire series of 184 episodes are available to purchase from the Crawford Australia website (Region 4). From November 2015 all seven volumes can be purchased in the rest of the world through Crawford's UK distributor, Eaton Films Ltd.

Filming
The series was filmed at the Global Television Studios in Nunawading, Melbourne in the set next to where Prisoner was filmed with only a partition separating the two programmes.

Main

 * Kevin Miles as Godfrey Carson
 * Lorraine Bayly as Jennifer Carson
 * Jon Sidney as William Carson
 * Gregg Cave as Billy Carson
 * Melanie Oppenheimer as Sarah Carson
 * Edward Upjohn as Sam Carson
 * Christine Amor as Felicity Carson
 * Ross Thompson as Robert Carson
 * Louise Pajo as Margery Carson
 * Chris Orchard as Thomas Carson
 * Christine Harris as Amy Carson (later Garrick)
 * Anna Maria Monticelli as Eva Tarrant
 * Irene Inescort as Eileen Brennan
 * Gordon Glenwright as Sgt. Vic Brown
 * Noel Trevarthen as Gerard Kent
 * Christian Horsley as Carson's love

Guests / recurring

 * Anthony Hawkins as Charlie Miller (1 episode)
 * Bruce Spence as Vernon (2 episodes)
 * Dennis Miller as Frank Stirling
 * Frank Gallacher as Dr Hellman (2 episodes)
 * Gerard Kennedy as James Dolman (8 episodes)
 * Janet Andrewartha as Phoebe King (1 episode)
 * Lesley Baker as Nurse (1 episode)
 * Lorna Lesley as Maureen (2 episodes)
 * Miles Buchanan as Findlay Nigel (1 episode)
 * Peta Toppano as Judith Taylor
 * Peter Kowitz as Phil (2 episodes)
 * Reg Gorman as Sergeant / Tate (2 episodes)
 * Roger Ward as Algernon 'Slammer' McQuade (3 episodes)
 * Sally McKenzie as Emily Forrest
 * Wynn Roberts as Andrew Riley KC / Andrews (4 episodes)

Episode list
Note: the first episode "The Clan" has been designated as episodes 1&2, as per the numbering on the Crawford Productions DVD release.

1.	The Clan, Part 1

2.	The Clan, Part 2

3.	The Witness

4.	Requiem

5.	Case Deferred

6.	The Royston Trial

7.	An English Gentleman

8.	Foxtrot

9.	The Medicine Man

10.	Which Doctor

11.	Swan Song, Part 1

12.	Swan Song, Part 2

13.	Happy Families

14.	To The Future

15.	Lost Soul

16.	Pre-Selection

17.	On The Fence

18.	A Family Concern

19.	Mistress

20.	Reputations

21.	Cause

22.	Trial And Error

23.	Dead Men's Tales

24.	Dead End

25.	Candidate

26.	Strange Bedfellows

27.	Donnegan's Wake

28.	Confession

29.	Aftermath

30.	Waywards From The Bush

31.	All God's Children

32.	Blessed Are They

33.	A Useful Life

34.	Greater Good

35.	Chopsticks

36.	In A Market Garden

37.	The Prodigal

38.	Razorman

39.	Christmas Cheer

40.	Wedding

41.	Honeymoon

42.	Private Life

43.	Birthright

44.	Heritage

45.	Policeman's Lot

46.	True Colours

47.	Pound Of Flesh

48.	In Service

49.	Soiree

50.	Best Laid Plans

51.	The Sins Of The Sons

52.	The Perfect Gentleman

53.	Future Horizons

54.	Happy Events

55.	Scent Of Battle

56.	The Fortune Teller

57.	Fortune's Child

58.	Fire And Misfire

59.	St. Patrick's Day

60.	The Masquerade

61.	All That Glitters

62.	Laugh, Clown, Laugh

63.	Guardian Angels

64.	Lost Opportunities

65.	A Country Dance

66.	Faith, Hope And Fencing Wire

67.	A Matter Of Confidence

68.	The Tender Trap

69.	A Case For Compassion

70.	From Small Acorns

71.	Street Games, Night Moves

72.	Deceptions

73.	Orphans Of The Storm

74.	Falling Star

75.	Means To An End

76.	Teapots And Tears

77.	Seeing Stars

78.	Mirrors And Dreams

79.	The Americans Are Coming

80.	Picking Up The Pieces

81.	The Bolshevik

82.	The Comeback

83.	A Scent Of Lavender

84.	Changes

85.	Preferential Treatment

86.	Growing Pains

87.	A Weak Resolve

88.	The Catch

89.	Butcher's Picnic

90.	Child Of The Storm

91.	A Cat Among The Carsons

92.	A Change Of Heart

93.	Lunatic

94.	The Sisters

95.	A Chinese Puzzle

96.	Humanitarian Grounds

97.	A Case To Answer

98.	Scarlet Fever

99.	The Duel

100.	Vale

101.	Prelude

102.	Power Games

103.	Verdict: Not Innocent

104.	The Portrait

105.	A Small Misunderstanding

106.	The Outsider

107.	Lady Luck

108.	Win Some, Lose Some

109.	The Innocent

110.	A Crime Of Passion

111.	Virtue Undefiled

112.	The Lost Weekend

113.	The Tender Past

114.	A Question Of Circumstance

115.	White Knights

116.	New Friends

117.	Infatuation

118.	Snakes In The Grass

119.	A Man's Pride

120.	Gentlemen's Agreement

121.	Self Defence

122.	Flash Point

123.	Blood And Fire

124.	Wedding Vows

125.	An Anxious Advocate

126.	The Devil's Work

127.	The Last Goodbye

128.	A Fond Concept

129.	Luck Of The Draw

130.	A Lucky Investment

131.	Strike Me Lucky

132.	The Performance

133.	A Hold On Life

134.	The Old People's Friend

135.	The Last Enemy

136.	A Small Favour

137.	Throw A Seven, Part 1

138.	Throw A Seven, Part 2

139.	The Comeback

140.	Wild Oats

141.	Misalliance

142.	The Arrangement

143.	Hideous Night

144.	Gambits

145.	Smile For The Camera

146.	Malpractice

147.	Fear Is The Key

148.	Fire And Ice

149.	Fallen Into Darkness

150.	Seeds Of Doubt

151.	The Ivory Tower

152.	To The Future

153.	Heroic Deeds

154.	Another Mrs. Carson

155.	Desperate Measures

156.	The Long Farewell

157.	A Bob Each Way

158.	The Lesson

159.	One Step Ahead

160.	The Wages Of Sin

161.	The Mis-Shapen Effigy

162.	Trial By Editorial

163.	Ploys

164.	Baying For Blood

165.	The Dominant Gender

166.	The Man's Rights

167.	La Cenicienta

168.	All Is Fair

169.	Hell Hath No Fury

170.	Beyond All Reason

171.	The Break

172.	End Of The Road

173.	Wild Violets

174.	Divide And Conquer

175.	Blood On The Straw

176.	Buying Time

177.	A Question Of Liberty

178.	The Scapegoat

179.	Hide And Seek

180.	My Father, My Son

181.	Requiescat

182.	The Heir Apparent

183.	The Dark Tunnel

184.	Daylight

title: Carson's Law


text:
Portal:Finland/Finland news

Wikinews Finland portal

title: Portal:Finland/Finland news


text:
Ewell Blackwell

Ewell Blackwell (October 23, 1922 – October 29, 1996) was an American right-handed starting pitcher in Major League Baseball. Nicknamed "The Whip" for his sidearm, snap-delivery, Blackwell played for the Cincinnati Reds for most of his career (1942; 1946–52). He also played with the New York Yankees (1952–53) and finished his career with the Kansas City Athletics (1955).

Baseball career
The 6 ft, 195 lb Blackwell is considered to have been one of the greatest pitchers of his era, and starred in a six-year streak in the All-Star Game from 1946 through 1951. He was the winning pitcher of the 1950 All-Star Game, getting Joe DiMaggio to ground into a game-ending double play in the 14th inning.

On June 18, 1947, Blackwell pitched a 6–0 no-hitter against the Boston Braves. In his next start, June 22, against the Brooklyn Dodgers, he took a no-hitter into the ninth inning, trying to tie the achievement of his veteran Reds teammate Johnny Vander Meer from nine years earlier, of throwing consecutive no-hitters. However, the no-hit attempt was broken up by Eddie Stanky. The Reds won the game 4–0.

In a 10-season career, Blackwell posted an 82–78 record with 839 strikeouts and a 3.30 ERA in 1,321 innings pitched. In 1960, he was just the eighth player ever to be inducted into the Cincinnati Reds Hall of Fame. During a 2007 New York Mets broadcast, Blackwell was referred to as the best right-handed pitcher ever by Hall of Famer Ralph Kiner. Both Kiner and Hall of Fame catcher Roy Campanella called Blackwell the toughest pitcher they ever faced. Dodgers broadcaster Vin Scully also reported that batters were genuinely afraid to face him.

Blackwell's best year was 1947, when he recorded 22 wins against 8 losses, including 16 consecutive complete game victories for a weak-hitting team. At a slender 6 ft 6 inches, he was one of the first very tall pitchers, and a fearsome sight to hitters of that era. His bizarre sidearm delivery, described by a leading sports pundit as "looking like a man falling out of a tree", put unusual strain on his arm, abbreviating his success and, ultimately, his career. Along with arm problems, Blackwell had his right kidney removed in January 1949 after it became infected, and then had an emergency appendectomy in September 1950.

Alvin Dark thought Blackwell was the best pitcher in the NL for a brief interval. "It was difficult to pick up the ball in his windup–he'd curl his arm behind his back, and you'd get a glimpse of it there, and that's the last you'd see until it was coming at you."

In 1953 with the Yankees, Blackwell won both of his decisions but retired on July 6 because his arm "hurt too much." He said that he would rest the arm, then try to pitch again the next season, and he wound up playing one more year in 1955 for the Kansas City Athletics.

In 1948, Ziff-Davis Publishing Company published The Secrets of Pitching, By Ewell Blackwell, a short book giving advice for young pitchers.

Military service
From 1943 to 1946 during World War II, Blackwell served with the United States Army in Europe where he worked as a mess sergeant. When not cooking, he had time to play baseball and conduct instructional camps with European youth prior to his March 1946 discharge.

title: Ewell Blackwell


text:
User:Brego

see Brego

de:Benutzer:Brego fr:Utilisateur:Brego nl:Gebruiker:Brego ja:利用者:Brego

title: User:Brego


text:
Wikipedia:Articles for deletion/Arts Towns

 This page is an archive of the proposed deletion of the article below. Further comments should be made on the appropriate discussion page (such as the article's talk page or on a Votes for Undeletion nomination). No further edits should be made to this page.

The result of the debate was No consensus. Fernando Rizo T/C 00:05, 11 September 2005 (UTC)

Arts Towns
A mere definition of a self-defining neologism. Bishonen | talk 18:53, 1 September 2005 (UTC) ''Addendum: I nominated this some hours ago, when it was a mere definiton; it has now grown some flowing purple prose and a very ad-like style. It's hard to figure where it's intended to end up up—are some lists of current "arts towns" and their attractions on the way?—but the promotional intent is acknowledged on my talk page by the poster. This is wrong, no matter how worthy the cause. Wikipedia is not for supporting things. Please compare Geogre's vote below.'' Bishonen | talk 01:14, 2 September 2005 (UTC)
 * Keep Term is in regular usage, like "steel town" or "mining town" DV8 2XL 19:38, 1 September 2005 (UTC)
 * Keep/Clean up UniReb 21:34, 1 September 2005 (UTC)
 * Strong Keep. This can expand into something good, including how "steel towns" and "mining towns" become "arts towns". Tonywalton | Talk 22:14, 1 September 2005 (UTC)
 * Strong delete: Utter tosh.  "Arts" + "towns" will get loads of Google hits, just like "cockroach" + "towns" will.  "Arts towns" is a particular guide's terminology, and it's monstrously irregular in formation (see "steel town" and "mining town," above).  The "history" section is a POV fountain.  The "reference" section is a flat dump of tour guides.  Finally, this is duplicate material, and marginally literate duplicate material at that.  Wikipedia is not Froemer's.  Geogre 22:39, 1 September 2005 (UTC)
 * Keep, 713 google hits for "santa fe" + "art town" . Kappa 00:39, 2 September 2005 (UTC)
 * Delete, what Geogre said. And most of Kappa's Googles are repeats: . -Splash 01:09, 2 September 2005 (UTC)
 * Just like most of wikipedia's googles . Kappa 01:27, 2 September 2005 (UTC)
 * Kappa that is simply untrue; I have already explained this false reasoning to you [in this VfD]. Please read it and stop making this false assertion.  Also Delete: I agree with the nominator.  A steel town was so called b/c the economy revolved entirely around steel.  An arts town is modern day tourist brochure speak. Dottore So 02:13, 2 September 2005 (UTC)

 * Delete, adspeak. Zoe 05:23, September 2, 2005 (UTC)
 * Strong Keep. Term is in regular usage, agreed in North America, particularly in the USA. Definition and extension of article very useful for high school seniors, and those given essay assignments in this area at schools of the arts. Good idea. Urbane Planner 02:23, September 6, 2005 (UTC)
 * Delete. Judging from the Google hits, it seems just to be used in marketing by a few small towns in the US. Uppland 07:42, 6 September 2005 (UTC)
 * Delete. 713 is a truly pitiful Google count for a tourist phrase. Mention it in Santa Fe instead. / Peter Isotalo 09:05, 9 September 2005 (UTC)
 * Keep. Entries now include Canadian arts towns like Stratford, where I've been, and the arts town of Banff, home of the Banff Festival and an arts centre. The term is used throughout Canada in arts conferences, and in Quebec. European arts towns are needed. So it's valid. Keep, and expand. Shakespearean 02:01, September 10, 2005 (UTC)
 * The above discussion is preserved as an archive of the debate. Please do not modify it. Subsequent comments should be made on the appropriate discussion page (such as the article's talk page or in an undeletion request). No further edits should be made to this page.


text:
User talk:Jocosley

Welcome, from Journalist
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title: User talk:Jocosley


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User:Lambdageek

$$ \frac{\Gamma,x{:}\tau_1\vdash e : \tau_2}{\Gamma\vdash \lambda x.e : \tau_1\rightarrow\tau_2} $$

For more about me, see my homepage

title: User:Lambdageek


text:
Talk:Planetary habitability

Featured article review?
I've just chopped an unsourced section, but there is quite a lot of uncited text elsewhere, making the article non-compliant with Featured article criteria. Featured article review is currently working through featured articles that have not been reviewed in the last 8 years and this article is listed near the top. DrKay (talk) 13:49, 9 February 2015 (UTC)

Astrobiology missions
I wonder if it is appropriate to include a brief section on some of the space missions (Viking, Phoenix, Curiosity, ExoMars, 2020 Mars rover), that had an emphasis in habitability assessment. Cheers, BatteryIncluded (talk) 23:35, 15 March 2015 (UTC)

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Zones
The "habitable zone" is defined as water habitable zone, looks to be the only determinate for the habitability of a planet here. But, new measurements show there are other habitable zones. Planetary habitability for life must simultaneously reside in all of the 9 known habitable zones: water, ultraviolet, photosynthetic, ozone, rotation rate, obliquity, tidal, atmosphere, and atmospheric electric field.

 * this would need a source. Dudley Miles (talk) 09:30, 27 November 2016 (UTC)

 * Naturally: Maybe should be a new page Habitability zones? As there is lot of info on this topic.Telecine Guy (talk) 02:48, 2 December 2016 (UTC)

washington.edu, New definition could further limit habitable zones around distant suns, by Vince Stricherz, June 10, 2009

Universität Wien, Habitable Zone, Isabella Kraus, January 18, 2012

Texas U. Habitable Zones around Main Sequence Stars, by DP Whitmire - 1993

Ohio State, Habitable Zones around Stars, by Prof. Richard Pogge, 2012

Cornell University, Habitable Zones and UV Habitable Zones around Host Stars, by Jianpo Guo, Fenghui Zhang, Xianfei Zhang, Zhanwen Han, on 5 March 2010

Harvard, Tectonics and the photosynthetic habitable zone, by Sleep, N. H., Dec. of 2009

space.com, How to Find Aliens: Follow the Photosynthesis, By Charles Q. Choi, July 22, 2010

Washington U., The Feasibility of Photosynthesis on Extrasolar Planets, By Edward Schwieterman

[http://adsabs.harvard.edu/full/2004IAUS..213..309S Harvard, Photosynthetic Constraints on the Habitable Zone, By Storrie-Lombardi, M. C., Tsapin, A. I., McDonald, G. D., Coleman, M. L., & Meadows, V. S. Bioastronomy 2002: Life Among the Stars, Proceedings of IAU Symposium #213. Edited by R. Norris, and F. Stootman. San Francisco: Astronomical Society of the Pacific, 2003., p.309, 2001]

[http://www.astrobio.net/news-exclusive/could-alien-life-cope-with-a-hotter-brighter-star/ Astrobiology Magazine Contributor, Could Alien Life Cope with a Hotter, Brighter Star? By Adam Hadhazy, March 20, 2014]

How to Astrobiology Magazine, Find Habitable Planets in Our Galaxy's Danger Zone, By Gemma Lavender September 27, 2011

[http://www.iafe.uba.ar/u/pablo/Papers/Buccino_et_al_2006.pdf Universidad de Buenos Aires, Ultraviolet radiation constraints around the circumstellar habitable zones, by Andrea P. Buccino, Guillermo A. Lemarchand, Pablo J.D. Mauas. 9 December 2005]

Astrobiology Mag., Rotation of Planets Influences Habitability, By Amanda Doyle,  Aug. 7, 2014

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets, by J. C. Armstrong, Department of Physics, Weber State University & R. Barnes, University of Washington & S. Domagal-Goldman, NASA Goddard & J. Breiner, T. R. Quinn, and V. S. Meadows, University of Washington

astrobio.net, Tidal Habitable Zone, Jun 17, 2009

University of Washington, Tidal Constraints on Planetary Habitability, R. Barnes,1,2 B. Jackson,3 R. Greenberg,4 S. N. Raymond,2,5 and R., Heller6, 1Department of Astronomy, University of Washington, 2Virtual Planetary Lab, 3NASA Postdoctoral Program Fellow, Planetary Systems Laboratory, Goddard Space Flight Center, 4Lunar and Planetary Laboratory, University of Arizona, 5Center for Astrophysics and Space Astronomy, University of Colorado, 6Hamburger Sternwarte, University of Hamburg,

Harvard, Tidal Limits to planetary habitability, R. Barnes 2009

Surface composition determines temperature and therefore habitability of a planet, by Anthony Watts, July 15, 2016

[http://sci.esa.int/venus-express/57968-collinson-et-al-2016/ "The Electric Wind of Venus: A Global and Persistent 'Polar Wind'-Like Ambipolar Electric Field Sufficient for the Direct Escape of Heavy Ionospheric Ions: Venus Has Potential," Geophysical Research Letters, by Glyn Collinson. June 2016]

"Electric Mars: The First Direct Measurement of an Upper Limit for the Martian 'Polar Wind' Electric Potential," Geophysical Research Letters 42, by Glyn Collinson, November 2015

 * Zones move over time. Reviews of Geophysics, The faint young Sun problem, by Georg Feulner, Feb 11, 2014

planetary.org, The Habitable Zone of Inhabited Planets, by Jaime Green, July 7, 2014

ASTROBIOLOGY, Volume 16, Number 1, 2016, The Case for a Gaian Bottleneck: The Biology of Habitability, by Aditya Chopra and Charles H. Lineweaver

Explain the slang: habitable
I understand that specialists in a narrow field love to submerge in their own slang, but I think one role of the encyclopedia is to open the field to the public by translating the slang to a generally understandable language. To an average user it would be obvious that planet that is uninhabitable is unsuitable for colonization from the outside. That's the dictionary meaning of the word: a house is not habitable - you can travel there, but cannot live there. Reading this article, it implies that a planet that is not "habitable" (in the scientific slang sense) can be perfectly suitable for settlers. (Example? A huge star offers 100 thousand years stability, insufficient for "habitable", very much sufficient for colonization.) Is it possible to find a reliable source for an explanation that would clear this up? I volunteer to write the explanation, but I cannot find any source. --Kubanczyk (talk) 12:12, 11 March 2017 (UTC)
 * ♠While I'd take issue with "slang" ("jargon", perhaps, if that wasn't a bit pejorative for my liking), let me try & explain as I understand it. "Habitable" is "can be lived on without special equipment" (pressure suit, diving gear); "colonizable" (if that's a word...) includes planet (such as Mars) where either or both would be needed.
 * ♠In the scientific community, the usage is a bit different still: they tend to mean "suitable for the evolution & current existence of a life form like us" (which is a desperately anthropocentric kind of bias...).  TREKphiler   any time you're ready, Uhura  20:13, 15 March 2017 (UTC)

 * Habitability ONLY refers to the innert physical and chemical environment. Therefore it is possible to find a habitable planet that is sterile and uninhabited. 98.100.145.6 (talk) 20:28, 28 August 2017 (UTC)

 * "Habitability has been defined as the potential of an environment (past or present) to support life of any kind." NASA ASTROBIOLOGY STRATEGY 2015
 * BatteryIncluded (talk) 18:48, 21 September 2017 (UTC)

Is a sterile planet at all likely to have a human-breathable atmosphere? If not, habitability cannot include the scenario of a sterile planet that we could colonize. Dysamoria (talk) 16:42, 14 December 2021 (UTC)

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Mercury Past Habitability Reference
Planetary Characteristics: Orbit & Rotation: The two articles cited for a possible prior habitability of Mercury refer to a source paper that does not seem to mention life or habitability. Did I miss the relevant content behind a paywall? Dysamoria (talk) 16:45, 14 December 2021 (UTC)
 * One is a newspaper report and the other says "Metastable and potentially habitable conditions might have developed episodically or transiently within these crustal materials". This does not support the comment in the article so I will delete. Dudley Miles (talk) 17:14, 14 December 2021 (UTC)

title: Talk:Planetary habitability


text:
User:RingMaruf

I'm sick of wasting all of my time reverting people's blatantly incorrect shit.

title: User:RingMaruf


text:
Babcock University

Babcock University is a private Christian co-educational Nigerian university owned and operated by the Seventh-day Adventist Church in Nigeria. The university is located at Ilishan-Remo, Ogun State, Nigeria, equidistant between Ibadan and Lagos.

In 2017, the university had its first set of graduates from the Ben Carson School of Medicine.

It is a part of the Seventh-day Adventist education system, which is the world's second largest Christian school system.

History
Babcock University was named after an American missionary by the name of David C. Babcock, who pioneered the work of the Seventh-day Adventist Church in Nigeria in 1914. He was based in Erunmu in Oyo State, Nigeria.

The university was established as the Adventist College of West Africa (ACWA) in 1959, initially with seven students; who were hosted at the home of Chief Olufemi Okulaja. In 1975, it changed its name to Adventist Seminary of West Africa (ASWA). The university was officially inaugurated on 20 April 1999.

Academic divisions
From the initial four schools, Babcock University has added a postgraduate school in the third quarter of 2010, and a medical school in January 2012. The latest additions are the Music and Educational Foundations departments to the Joel Awoniyi School of Education & Humanities. As at 2013, Babcock hosts eight schools and two colleges. They are:

 * School of Social Sciences with departments like Economics, Social works etc.
 * School of Management Sciences with departments like Accounting, Business Administration and Marketing, Information Resources Management and Finance.
 * College of Health & Medical Sciences
 * School of Science & Technology
 * School of Computing & Engineering Sciences with departments like computer science, software engineering, etc.
 * School of Education and Humanities with departments like Education, etc.
 * School of Law & Security Studies with departments like Law, International law and diplomacy, Security Studies etc.
 * School of Nursing with departments like Nursing, etc.
 * School of Public & Applied Health with departments like Public Health, etc.
 * College of Post Graduate Studies.

Notable alumni

 * Adenike Akinsemolu, environmental sustainability advocate, educator, author, a social entrepreneur.
 * Davido, Musician & Performer.
 * Debo Ogundoyin, Speaker Oyo State house of Assembly.
 * Aisha Ochuwa, lawyer and entrepreneur
 * Beverly Osu, actress and model.
 * Olumide Oworu, Nigerian actor.
 * Olamide Samuel, International security expert
 * Ayotunde Fabamwo, Serial Entrepreneur

title: Babcock University


text:
User talk:Rasha Kanzaki

神咲羅紗（カンザキラシャ/Kanzaki Rasha） Rasha Kanzaki
日本人. 日本語は勿論母国語なのでまともに編集できる・する. まだ相当若い…と思う. 多分だが. 神咲が編集すると大変ぶっきらぼうな口調になる、日本語は. 日本語を読める方には私のホームページをご覧いただけます. 太陽無き世界という名前です. 意味は確か創設当時は色々あったのですけれど、今はもう忘れてしまいました. この前移転したばかりなので色々不具合があると思います. 創作サイトなんで、まぁつまり私の趣味はどうしても文学系に偏ると思われるでしょうけれど、音楽も好きです. とくに好きな歌手はBump of chickenなので、多分そっちのほうをよく見ます. 学校での選択科目は音楽です一応. Can speak English fluently and ofcourse can write nicely. May be a bit slangy because of my age. Please excuse me.
 * 日本語版
 * English version

title: User talk:Rasha Kanzaki