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Habitable zone

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Habitable_zone

In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

The habitable zone is also called the Goldilocks zone, a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".

Since the concept was first presented in 1953, many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope 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 in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the HZ might outnumber planets.

In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars, or ammoniates, due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.

History

An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4). The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry, "... we may presume, that, being susceptible of vegetation, it [the Earth] has been placed, in the series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate."

The concept of a circumstellar habitable zone was first introduced in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The concept was later discussed in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars, coined the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. A major early contributor to the habitable zone concept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.

The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of the circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million. At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization. The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase. In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone. Kasting was the first to present a detailed model for the habitable zone for exoplanets.

An update to habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez. The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.

Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry. In 2013, further developments in habitable zone concepts were made with the proposal of a circum planetary habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.

It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment. However, surface conditions are dependent on a host of different individual properties of that planet. This misunderstanding is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.

Determination

Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.

Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However, their atmospheric conditions vary substantially.

The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapor. The entire orbits of the Moon, Mars, and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes, no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossible for these bodies to sustain liquid water on their surface.

Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, Mars, Vesta and Ceres, suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.

According to the extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources.

For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.

Estimates of the circumstellar habitable zone boundaries of the Solar System
Inner edge (AU)	Outer edge (AU)	Year	Notes
0.725	1.24	1964, Dole	Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
	1.005–1.008	1969, Budyko	Based on studies of ice albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969 and North 1975.
0.92–0.96		1970, Rasool and De Bergh	Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
0.958	1.004	1979, Hart	Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
	3.0	1992, Fogg	Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
0.95	1.37	1993, Kasting et al.	Founded the most common working definition of the habitable zone used today. Assumes that CO₂ and H₂O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate–silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
	2.0	2010, Spiegel et al.	Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
0.75		2011, Abe et al.	Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
	10	2011, Pierrehumbert and Gaidos	Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in the Solar System.
0.77–0.87	1.02–1.18	2013, Vladilo et al.	Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar.
0.99	1.67	2013, Kopparapu et al.	Revised estimates of the Kasting et al. (1993) formulation using updated moist greenhouse and water loss algorithms. According to this measure, Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapor. Once the stratosphere becomes wet, water vapor photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a CO₂ dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in CO₂ will not create enough warming to prevent CO₂ catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.67 AU. This definition does not take into account possible radiative warming by CO₂ clouds.
0.38		2013, Zsom et al.	Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.
0.95		2013, Leconte et al.	Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System.
0.95	2.4	2017, Ramirez and Kaltenegger	An expansion of the classical carbon dioxide-water vapor habitable zone assuming a volcanic hydrogen atmospheric concentration of 50%.
0.93–0.91		2019, Gomez-Leal et al.	Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of the climate sensitivity in both cases.
0.99	1.004		Tightest bounded estimate from above
0.38	10		Most relaxed estimate from above

Extrasolar extrapolation

Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at $\sqrt{0.25}$ , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the HZ concept is more complex.

Spectral types and star-system characteristics

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.

With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions. Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones. Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment for life.

Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modeling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking. Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration. At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs. Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales.

Stellar evolution

Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years, would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve. Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now, and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase. In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes and huge starspots which can cover 20% of the star's surface area, have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux, so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously habitable Earth as a red giant. However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU. At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. Given that this new equilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. Moreover, as Ramirez and Kaltenegger (2016) showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant. Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage.

Desert planets

A planet's atmospheric conditions influence its ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.

Other considerations

Earth's hydrosphere. Water covers 71% of Earth's surface, with the global ocean accounting for 97.3% of the water distribution on Earth.

A planet cannot have a hydrosphere—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis. For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep such as GJ 1214 b or Kepler-22b may be.

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing.Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape. In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as a liquid.

Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth), a study led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos had eliminated the CO₂-H₂O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years.

In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between the two sides.

Planetary mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io, but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet. Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.

A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation (cryptobiosis) and/or hyperthermostability. Tardigrades, for example, can survive in a dehydrated state temperature between 0.150 K (−273 °C) and 424 K (151 °C). Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm.

Extrasolar discoveries

A 2015 review concluded that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. These are at a distance of 990, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e is 11.9 light-years away. It is in the inner edge of its planetary system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F).

Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put η_e, the fraction of stars with planets in the HZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their HZs.

Early findings

The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water. One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the HZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.

Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons. Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it is unclear whether such satellites could form in the first place.

HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth. The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A. Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.

Though, in theory, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.

Habitable super-Earths

The 2007 discovery of Gliese 581c, the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for a short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014, and astronomers are divided about its existence.

Discovered in August 2011, HD 85512 b was initially speculated to be habitable, but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.

Kepler-22 b, discovered in December 2011 by the Kepler space probe, is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. Gliese 667 Cc, discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planets known.

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163 is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C. HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307. In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone; however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable.

Near Earth-sized planets and Solar analogs

Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5 R_🜨 the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology.

A solar analog (or "solar twin") is a star that resembles the Sun. No solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation.

Using data collected by NASA's Kepler space telescope and the W. M. Keck Observatory, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone.

On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to the Sun, in the HZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.

With a radius estimated at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method though its mass remains unknown and its parent star is not a Solar analog.

Kapteyn b, discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii were found orbiting the habitable zone of the red subdwarf Kapteyn's Star, 12.8 light-years away.

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.^[166] However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth.

Kepler-452b, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452.

The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016. The discovery is considered significant because it dramatically increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.

Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf K2-72 around 227 light years from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.

Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs. The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.

Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away.

At 11 light-years away, the second closest planet, Ross 128 b, was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 times Earth's mass, is it roughly Earth-sized and likely rocky in composition.

Discovered in March 2018, K2-155d is about 1.64 times the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.

One of the earliest discoveries by the Transiting Exoplanet Survey Satellite (TESS) announced on July 31, 2019, is a Super-Earth planet GJ 357 d orbiting the outer edge of a red dwarf 31 light years away.

K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.

In September 2020, astronomers identified 24 superhabitable planet (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.

Notable exoplanets – Kepler space telescope
Confirmed small exoplanets in habitable zones. (Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).

Habitability outside the HZ

Liquid-water environments have been found to exist in the absence of atmospheric pressure and at temperatures outside the HZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.

Outside the HZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.

With some theorising that life on Earth may have actually originated in stable, subsurface habitats, it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'. On Earth itself, living organisms may be found more than 6 km (3.7 mi) below the surface.

Another possibility is that outside the HZ organisms may use alternative biochemistries that do not require water at all. Astrobiologist Christopher McKay, has suggested that methane (CH
₄) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on 1,610,000,000 km (1.0×10⁹ mi; 11 AU) from the star. This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife. In addition, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.

Significance for complex and intelligent life

The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the HZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a HZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics) and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation). But others, such as Ian Stewart and Jack Cohen in their 2002 book Evolving the Alien argue that complex intelligent life may arise outside the HZ. Intelligent life outside the HZ may have evolved in subsurface environments, from alternative biochemistries or even from nuclear reactions.

On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvent ecosystems, supporting large complex life forms such as Riftia pachyptila. Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone. Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. In addition, the lichens Rhizocarpon geographicum and Xanthoria elegans have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.

Species, including humans, known to possess animal cognition require large amounts of energy, and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earth analogs in the HZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits or space habitat analogs on the surface may be eliminated, and complex Earth life can thrive.

Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe. The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter $n e$ , which is the average number of planetary-mass objects orbiting within the HZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe. A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, HZ candidates are the priority targets to narrow waterhole searches and the Allen Telescope Array now extends Project Phoenix to such candidates.

Because the HZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and 2003 Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the HZ. The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its HZ. A Message from Earth in 2008, and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the HZ—Gliese 581 c, d, and the unconfirmed g.

Superhabitable world

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Superhabitable_world

Artist's impression of one possible appearance of a superhabitable planet. The reddish hue is vegetation.

A superhabitable world is a hypothetical type of planet or moon that is better suited than Earth for the emergence and evolution of life. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search for habitable exoplanets and proposed clarifications. The authors argued that knowing whether a world is located within the star's habitable zone is insufficient to determine its habitability, that the principle of mediocrity cannot adequately explain why Earth should represent the archetypal habitable world, and that the prevailing model of characterization was geocentric or anthropocentric in nature. Instead, they proposed a biocentric approach that prioritized astrophysical characteristics affecting the abundance and variety of life on a world's surface.

If a world possesses more diverse flora and fauna than there are on Earth, then it would empirically show that its natural environment is more hospitable to life. To identify such a world, one should consider its geological processes, formation age, atmospheric composition, ocean coverage, and the type of star that it orbits. In other words, a superhabitable world would likely be larger, warmer, and older than Earth, with an evenly-distributed ocean, and orbiting a K-type main-sequence star. In 2020, astronomers, building on Heller and Armstrong's hypothesis, identified 24 potentially superhabitable exoplanets based on measured characteristics that fit these criteria.

Stellar characteristics

A star's characteristics is a key consideration for planetary habitability. The types of stars generally considered to be potential hosts for habitable worlds include F, G, K, and M-type main-sequence stars. The most massive stars—O, B, and A-type, respectively—have average lifespans on the main sequence that are considered too short for complex life to develop, ranging from a few hundred million years for A-type stars to only a few million years for O-type stars. Thus, F-type stars are described as the "hot limit" for stars that can potentially support life, as their lifespan of 2 to 4 billion years would be sufficient for habitability. However, F-type stars emit large amounts of ultraviolet radiation, and without the presence of a protective ozone layer, could disrupt nucleic acid-based life on a planet's surface.

On the opposite end, the less massive red dwarfs, which generally includes M-type stars, are by far the most common and long-lived stars in the universe, but ongoing research points to serious challenges to their ability to support life. Due to the low luminosity of red dwarfs, the circumstellar habitable zone (HZ) is in very close proximity to the star, which causes any planet to become tidally locked. The primary concern for researchers, however, is the star's propensity for frequent outbreaks of high-energy radiation, especially early in its life, that could strip away a planet's atmosphere. At the same time, red dwarfs do not emit enough quiescent UV radiation (i.e., UV radiation emitted during inactive periods) to support biological processes like photosynthesis.

Dismissing both ends, G and K-type stars—yellow and orange dwarfs, respectively—have been primary objects of interest for astronomers because they are seen to provide the best life-supporting characteristics. However, Heller and Armstrong argue that a limiting factor to the habitability of yellow dwarfs is their higher emissions of quiescent UV radiation compared to cooler orange dwarfs. For this reason, along with the shorter lifespan of yellow dwarfs, the authors are led to conclude that orange dwarfs offer the best conditions for a superhabitable world. Also nicknamed "Goldilocks stars," orange dwarfs emit low enough levels of ultraviolet radiation to eliminate the need for a protective ozone layer, but just enough to contribute to necessary biological processes. Moreover, the long average lifespan of an orange dwarf (18 to 34 billion years, compared to 10 billion for the Sun) provides stable habitable zones that do not move very much throughout the star's lifetime.

Planetary characteristics

Age

The age of a superhabitable world should be greater than Earth's age (~4.5 billion years). This is based on the belief that as a planet ages, it experiences increasing levels of biodiversity, since native species have had more time to evolve, adapt, and stabilize the environmental conditions suitable for life. As for the maximum age, research points to rocky planets existing as early as 12 billion years ago.

It was initially believed that since older stars contained little to no heavy elements (i.e., metallicity), they were incapable of forming rocky planets. Early exoplanet discoveries supported this hypothesis, as they were mostly gas giants orbiting in close proximity to stars with a heavy metal abundance. However, in 2012, the Kepler space telescope challenged this assumption when it discovered many rocky exoplanets orbiting stars with a relatively low metallicity. These findings suggested that the first Earth-sized planets likely appeared much earlier in the universe's lifetime at around 12 billion years ago.

Orbit and rotation

Habitable zone (HZ) position of some of the most similar and average surface temperature exoplanets.

During the main sequence phase, a star burns hydrogen in its core, producing energy through nuclear fusion. Over time, as the hydrogen fuel is consumed, the star's core contracts and heats up, leading to an increase in the rate of fusion. This causes the star to gradually become more luminous, and as its luminosity increases, the amount of energy it emits grows, pushing the habitable zone (HZ) outward. Because a main sequence star's luminosity gradually increases throughout its life, its HZ is not static but slowly moves outward. This means that any planet will experience a limited time within the HZ, known as its "habitable zone lifetime." Studies suggest that Earth's orbit lies near the inner edge of the Solar System's HZ, which could harm its long-term livability as it nears the end of its HZ lifetime.

Ideally, the orbit of a superhabitable world should be further out and closer to the center of the HZ relative to Earth's orbit, but knowing whether a world is in this region is insufficient on its own to determine habitability. Not all rocky planets in the HZ may be habitable, while tidal heating can render planets or moons habitable beyond this region. For example, Jupiter's moon Europa is well beyond the outer limits of the Solar System's HZ, yet as a result of its orbital interactions with the other Galilean moons, it is believed to have a subsurface ocean of liquid water beneath its icy surface.

There is no consensus on the optimal rotation rate for habitability, but a planet's rotation can affect the presence of geologically-active plate tectonics and the generation of a global magnetic field.

According to a 2023 paper by Jonathan Jernigan and colleagues, marine biological activity increases on planets with increasing obliquity and eccentricity. The authors suggest that planets with a high obliquity and/or eccentricity may be superhabitable, and that scientists should be keen to look for biosignatures on exoplanets with these orbital characteristics.

Mass and size

Assuming that a greater surface area would provide greater biodiversity, the size of a superhabitable world should generally be greater than 1 R_🜨, with the condition that its mass is not arbitrarily large. Studies of the mass-radius relationship indicate that there is a transition point between rocky planets and gaseous planets (i.e., mini-Neptunes) that occurs around 2 M_🜨 or 1.7 R_🜨. Another study argues that there is a natural radius limit, set at 1.6 R_🜨, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures.

Heller and Armstrong argue that the optimal mass and radius of a superhabitable world can be determined by geological activity; the more massive a planetary body, the longer time it will continuously generate internal heat—a major contributing factor to plate tectonics. Too much mass, however, can slow plate tectonics by increasing the pressure of the mantle. It is believed that plate tectonics peak in bodies between 1 and 5 M_🜨, and from this perspective, a planet can be considered superhabitable up to around 2 M_🜨. Assuming this planet has a density similar to Earth's, its radius should be between 1.2 and 1.3 R_🜨.

Geology

An important geological process is plate tectonics, which appears to be common in terrestrial planets with a significant rotation speed and an internal heat source. If large bodies of water are present on a planet, plate tectonics can maintain high levels of carbon dioxide (CO
₂) in its atmosphere and increase the global surface temperature through the greenhouse effect. However, if tectonic activity is not significant enough to increase temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by another energy source like tidal heating or stellar irradiation. On the other hand, if the effects of any of these processes are too strong, the amount of greenhouse gases in the atmosphere could cause a runaway greenhouse effect by trapping heat and preventing adequate cooling.

The presence of a magnetic field is important for the long-term survivability of life on the surface of a planet or moon. A sufficiently strong magnetic field effectively shields a world's surface and atmosphere against ionizing radiation emanating from the interstellar medium and its host star. A planet can generate an intrinsic magnetic field through a dynamo that involves an internal heat source, an electrically conductive fluid like molten iron, and a significant rotation speed, while a moon could be extrinsically protected by its host planet's magnetic field. Less massive bodies and those that are tidally locked are likely to have a weak to non-existent magnetic field, which over time can result in the loss of a significant portion of its atmosphere by hydrodynamic escape and become a desert planet. If a planet's rotation is too slow, such as with Venus, then it cannot generate an Earth-like magnetic field. A more massive planet could overcome this problem by hosting multiple moons, which through their combined gravitational effects, can boost the planet's magnetic field.

Surface features

Artistic impression of a possible Earth analog, Kepler-186f. Some superhabitable planets could have a similar appearance and may not have important differences with Earth.

The appearance of a superhabitable world should be similar to the conditions found in the tropical climates of Earth. Due to the denser atmosphere and less temperature variation across its surface, such a world would lack any major ice sheets and have a higher concentration of clouds, while plant life would potentially cover more of the planet's surface and be visible from space.

When considering the differences in the peak wavelength of visible light for K-type stars and the lower stellar flux of the planet, surface vegetation may exhibit colors different than the typical green color found on Earth. Instead, vegetation on these worlds could have a red, orange, or even purple appearance.

An ocean that covers a large portion of a world's surface with fractionate continents and archipelagos could provide a stable environment across its surface. In addition, the greater surface gravity of a superhabitable world could reduce the average ocean depth and create shallow ocean basins, providing the optimal environment for marine life to thrive. For example, marine ecosystems found in the shallow areas of Earth's oceans and seas, given the amount of light and heat they receive, are observed to have greater biodiversity and are generally seen as being more comfortable for aquatic species. This has led researchers to speculate that shallow water environments on exoplanets should be similarly suitable for life.

Climate

In general, the climate of a superhabitable planet would be warm, moist, and homogeneous, allowing life to extend across the surface without presenting large population differences. These characteristics are in contrast to those found on Earth, which has more variable and inhospitable regions that include frigid tundra and dry deserts. Deserts on superhabitable planets would be more limited in area and would likely support habitat-rich coastal environments.

The optimum surface temperature for Earth-like life is unknown, although it appears that on Earth, organism diversity has been greater in warmer periods. It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life. The denser atmosphere of a superhabitable planet would naturally provide a greater average temperature and less variability of the global climate. Ideally, the temperature should reach the optimal levels for plant life, which is 25 °C (77 °F). In addition, a large distributed ocean would have the ability to regulate a planet's surface temperature similar to Earth's ocean currents, and could allow it to maintain a moderate temperature within the habitable zone.

There are no solid arguments to explain if Earth's atmosphere has the optimal composition, but relative atmospheric oxygen levels is required to meet the high-energy demands of complex life (O
₂). Therefore, it is hypothesized that oxygen abundance in the atmosphere is essential for complex life on other worlds.

Emotivism

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Emotivism

Emotivism is a meta-ethical view that claims that ethical sentences do not express propositions but emotional attitudes. Hence, it is colloquially known as the hurrah/boo theory. Influenced by the growth of analytic philosophy and logical positivism in the 20th century, the theory was stated vividly by A. J. Ayer in his 1936 book Language, Truth and Logic, but its development owes more to C. L. Stevenson.

Emotivism can be considered a form of non-cognitivism or expressivism. It stands in opposition to other forms of non-cognitivism (such as quasi-realismand universal prescriptivism), as well as to all forms of cognitivism (including both moral realism and ethical subjectivism).

In the 1950s, emotivism appeared in a modified form in the universal prescriptivism of R. M. Hare.

History

Emotivism reached prominence in the early 20th century, but it was born centuries earlier. In 1710, George Berkeley wrote that language in general often serves to inspire feelings as well as communicate ideas. Decades later, David Hume espoused ideas similar to Stevenson's later ones. In his 1751 book An Enquiry Concerning the Principles of Morals, Hume considered morality not to be related to fact but "determined by sentiment":

In moral deliberations we must be acquainted beforehand with all the objects, and all their relations to each other; and from a comparison of the whole, fix our choice or approbation. … While we are ignorant whether a man were aggressor or not, how can we determine whether the person who killed him be criminal or innocent? But after every circumstance, every relation is known, the understanding has no further room to operate, nor any object on which it could employ itself. The approbation or blame which then ensues, cannot be the work of the judgement, but of the heart; and is not a speculative proposition or affirmation, but an active feeling or sentiment.

G. E. Moore published his Principia Ethica in 1903 and argued that the attempts of ethical naturalists to translate ethical terms (like good and bad) into non-ethical ones (like pleasing and displeasing) committed the "naturalistic fallacy". Moore was a cognitivist, but his case against ethical naturalism steered other philosophers toward noncognitivism, particularly emotivism.

The emergence of logical positivism and its verifiability criterion of meaning early in the 20th century led some philosophers to conclude that ethical statements, being incapable of empirical verification, were cognitively meaningless. This criterion was fundamental to A. J. Ayer's defense of positivism in Language, Truth and Logic, which contains his statement of emotivism. However, positivism is not essential to emotivism itself, perhaps not even in Ayer's form, and some positivists in the Vienna Circle, which had great influence on Ayer, held non-emotivist views.

R. M. Hare unfolded his ethical theory of universal prescriptivism in 1952's The Language of Morals, intending to defend the importance of rational moral argumentation against the "propaganda" he saw encouraged by Stevenson, who thought moral argumentation was sometimes psychological and not rational. But Hare's disagreement was not universal, and the similarities between his noncognitive theory and the emotive one — especially his claim, and Stevenson's, that moral judgments contain commands and are thus not purely descriptive — caused some to regard him as an emotivist, a classification he denied:

I did, and do, follow the emotivists in their rejection of descriptivism. But I was never an emotivist, though I have often been called one. But unlike most of their opponents I saw that it was their irrationalism, not their non-descriptivism, which was mistaken. So my main task was to find a rationalist kind of non-descriptivism, and this led me to establish that imperatives, the simplest kinds of prescriptions, could be subject to logical constraints while not [being] descriptive.

Proponents

Influential statements of emotivism were made by C. K. Ogden and I. A. Richards in their 1923 book on language, The Meaning of Meaning, and by W. H. F. Barnes and A. Duncan-Jones in independent works on ethics in 1934. However, it is the later works of Ayer and especially Stevenson that are the most developed and discussed defenses of the theory.

A. J. Ayer

A. J. Ayer's version of emotivism is given in chapter six, "Critique of Ethics and Theology", of Language, Truth and Logic. In that chapter, Ayer divides "the ordinary system of ethics" into four classes:

"Propositions that express definitions of ethical terms, or judgements about the legitimacy or possibility of certain definitions"
"Propositions describing the phenomena of moral experience, and their causes"
"Exhortations to moral virtue"
"Actual ethical judgments"

He focuses on propositions of the first class—moral judgments—saying that those of the second class belong to science, those of the third are mere commands, and those of the fourth (which are considered in normative ethics as opposed to meta-ethics) are too concrete for ethical philosophy. While class three statements were irrelevant to Ayer's brand of emotivism, they would later play a significant role in Stevenson's.

Ayer argues that moral judgments cannot be translated into non-ethical, empirical terms and thus cannot be verified; in this he agrees with ethical intuitionists. But he differs from intuitionists by discarding appeals to intuition as "worthless" for determining moral truths, since the intuition of one person often contradicts that of another. Instead, Ayer concludes that ethical concepts are "mere pseudo-concepts":

The presence of an ethical symbol in a proposition adds nothing to its factual content. Thus if I say to someone, "You acted wrongly in stealing that money," I am not stating anything more than if I had simply said, "You stole that money." In adding that this action is wrong I am not making any further statement about it. I am simply evincing my moral disapproval of it. It is as if I had said, "You stole that money," in a peculiar tone of horror, or written it with the addition of some special exclamation marks. … If now I generalise my previous statement and say, "Stealing money is wrong," I produce a sentence that has no factual meaning—that is, expresses no proposition that can be either true or false. … I am merely expressing certain moral sentiments.

Ayer agrees with subjectivists in saying that ethical statements are necessarily related to individual attitudes, but he says they lack truth value because they cannot be properly understood as propositions about those attitudes; Ayer thinks ethical sentences are expressions, not assertions, of approval. While an assertion of approval may always be accompanied by an expression of approval, expressions can be made without making assertions; Ayer's example is boredom, which can be expressed through the stated assertion "I am bored" or through non-assertions including tone of voice, body language, and various other verbal statements. He sees ethical statements as expressions of the latter sort, so the phrase "Theft is wrong" is a non-propositional sentence that is an expression of disapproval but is not equivalent to the proposition "I disapprove of theft".

Having argued that his theory of ethics is noncognitive and not subjective, he accepts that his position and subjectivism are equally confronted by G. E. Moore's argument that ethical disputes are clearly genuine disputes and not just expressions of contrary feelings. Ayer's defense is that all ethical disputes are about facts regarding the proper application of a value system to a specific case, not about the value systems themselves, because any dispute about values can only be resolved by judging that one value system is superior to another, and this judgment itself presupposes a shared value system. If Moore is wrong in saying that there are actual disagreements of value, we are left with the claim that there are actual disagreements of fact, and Ayer accepts this without hesitation:

If our opponent concurs with us in expressing moral disapproval of a given type t, then we may get him to condemn a particular action A, by bringing forward arguments to show that A is of type t. For the question whether A does or does not belong to that type is a plain question of fact.

C. L. Stevenson

Stevenson's work has been seen both as an elaboration upon Ayer's views and as a representation of one of "two broad types of ethical emotivism." An analytic philosopher, Stevenson suggested in his 1937 essay "The Emotive Meaning of Ethical Terms" that any ethical theory should explain three things: that intelligent disagreement can occur over moral questions, that moral terms like good are "magnetic" in encouraging action, and that the scientific method is insufficient for verifying moral claims. Stevenson's own theory was fully developed in his 1944 book Ethics and Language. In it, he agrees with Ayer that ethical sentences express the speaker's feelings, but he adds that they also have an imperative component intended to change the listener's feelings and that this component is of greater importance. Where Ayer spoke of values, or fundamental psychological inclinations, Stevenson speaks of attitudes, and where Ayer spoke of disagreement of fact, or rational disputes over the application of certain values to a particular case, Stevenson speaks of differences in belief; the concepts are the same. Terminology aside, Stevenson interprets ethical statements according to two patterns of analysis.

First pattern analysis

Under his first pattern of analysis an ethical statement has two parts: a declaration of the speaker's attitude and an imperative to mirror it, so "'This is good' means I approve of this; do so as well." The first half of the sentence is a proposition, but the imperative half is not, so Stevenson's translation of an ethical sentence remains a noncognitive one.

Imperatives cannot be proved, but they can still be supported so that the listener understands that they are not wholly arbitrary:

If told to close the door, one may ask "Why?" and receive some such reason as "It is too drafty," or "The noise is distracting." … These reasons cannot be called "proofs" in any but a dangerously extended sense, nor are they demonstratively or inductively related to an imperative; but they manifestly do support an imperative. They "back it up," or "establish it," or "base it on concrete references to fact."

The purpose of these supports is to make the listener understand the consequences of the action they are being commanded to do. Once they understand the command's consequences, they can determine whether or not obedience to the command will have desirable results.

The imperative is used to alter the hearer's attitudes or actions. … The supporting reason then describes the situation the imperative seeks to alter, or the new situation the imperative seeks to bring about; and if these facts disclose that the new situation will satisfy a preponderance of the hearer's desires, he will hesitate to obey no longer. More generally, reasons support imperatives by altering such beliefs as may in turn alter an unwillingness to obey.

Second pattern analysis

Stevenson's second pattern of analysis is used for statements about types of actions, not specific actions. Under this pattern,

'This is good' has the meaning of 'This has qualities or relations X, Y, Z … ,' except that 'good' has as well a laudatory meaning, which permits it to express the speaker's approval, and tends to evoke the approval of the hearer.

In second-pattern analysis, rather than judge an action directly, the speaker is evaluating it according to a general principle. For instance, someone who says "Murder is wrong" might mean "Murder decreases happiness overall"; this is a second-pattern statement that leads to a first-pattern one: "I disapprove of anything that decreases happiness overall. Do so as well."

Methods of argumentation

For Stevenson, moral disagreements may arise from different fundamental attitudes, different moral beliefs about specific cases, or both. The methods of moral argumentation he proposed have been divided into three groups, known as logical, rational psychological and nonrational psychological forms of argumentation.

Logical methods involve efforts to show inconsistencies between a person's fundamental attitudes and their particular moral beliefs. For example, someone who says "Edward is a good person" who has previously said "Edward is a thief" and "No thieves are good people" is guilty of inconsistency until he retracts one of his statements. Similarly, a person who says "Lying is always wrong" might consider lies in some situations to be morally permissible, and if examples of these situations can be given, his view can be shown to be logically inconsistent.

Rational psychological methods examine facts that relate fundamental attitudes to particular moral beliefs; the goal is not to show that someone has been inconsistent, as with logical methods, but only that they are wrong about the facts that connect their attitudes to their beliefs. To modify the former example, consider the person who holds that all thieves are bad people. If she sees Edward pocket a wallet found in a public place, she may conclude that he is a thief, and there would be no inconsistency between her attitude (that thieves are bad people) and her belief (that Edward is a bad person because he is a thief). However, it may be that Edward recognized the wallet as belonging to a friend, to whom he promptly returned it. Such a revelation would likely change the observer's belief about Edward, and even if it did not, the attempt to reveal such facts would count as a rational psychological form of moral argumentation.

Non-rational psychological methods revolve around language with psychological influence but no necessarily logical connection to the listener's attitudes. Stevenson called the primary such method "'persuasive,' in a somewhat broadened sense", and wrote:

[Persuasion] depends on the sheer, direct emotional impact of words—on emotive meaning, rhetorical cadence, apt metaphor, stentorian, stimulating, or pleading tones of voice, dramatic gestures, care in establishing rapport with the hearer or audience, and so on. … A redirection of the hearer's attitudes is sought not by the mediating step of altering his beliefs, but by exhortation, whether obvious or subtle, crude or refined.

Persuasion may involve the use of particular emotion-laden words, like "democracy" or "dictator", or hypothetical questions like "What if everyone thought the way you do?" or "How would you feel if you were in their shoes?"

Criticism

Utilitarian philosopher Richard Brandt offered several criticisms of emotivism in his 1959 book Ethical Theory. His first is that "ethical utterances are not obviously the kind of thing the emotive theory says they are, and prima facie, at least, should be viewed as statements." He thinks that emotivism cannot explain why most people, historically speaking, have considered ethical sentences to be "fact-stating" and not just emotive. Furthermore, he argues that people who change their moral views see their prior views as mistaken, not just different, and that this does not make sense if their attitudes were all that changed:

Suppose, for instance, as a child a person disliked eating peas. When he recalls this as an adult he is amused and notes how preferences change with age. He does not say, however, that his former attitude was mistaken. If, on the other hand, he remembers regarding irreligion or divorce as wicked, and now does not, he regards his former view as erroneous and unfounded. … Ethical statements do not look like the kind of thing the emotive theory says they are.

James Urmson's 1968 book The Emotive Theory of Ethics also disagreed with many of Stevenson's points in Ethics and Language, "a work of great value" with "a few serious mistakes [that] led Stevenson consistently to distort his otherwise valuable insights".

Magnetic influence

Brandt criticized what he termed "the 'magnetic influence' thesis", the idea of Stevenson that ethical statements are meant to influence the listener's attitudes. Brandt contends that most ethical statements, including judgments of people who are not within listening range, are not made with the intention to alter the attitudes of others. Twenty years earlier, Sir William David Ross offered much the same criticism in his book Foundations of Ethics. Ross suggests that the emotivist theory seems to be coherent only when dealing with simple linguistic acts, such as recommending, commanding, or passing judgement on something happening at the same point of time as the utterance.

… There is no doubt that such words as 'you ought to do so-and-so' may be used as one's means of so inducing a person to behave a certain way. But if we are to do justice to the meaning of 'right' or 'ought', we must take account also of such modes of speech as 'he ought to do so-and-so', 'you ought to have done so-and-so', 'if this and that were the case, you ought to have done so-and-so', 'if this and that were the case, you ought to do so-and-so', 'I ought to do so-and-so.' Where the judgement of obligation has referenced either a third person, not the person addressed, or to the past, or to an unfulfilled past condition, or to a future treated as merely possible, or to the speaker himself, there is no plausibility in describing the judgement as command.

According to this view, it would make little sense to translate a statement such as "Galileo should not have been forced to recant on heliocentricism" into a command, imperative, or recommendation - to do so might require a radical change in the meaning of these ethical statements. Under this criticism, it would appear as if emotivist and prescriptivist theories are only capable of converting a relatively small subset of all ethical claims into imperatives.

Like Ross and Brandt, Urmson disagrees with Stevenson's "causal theory" of emotive meaning—the theory that moral statements only have emotive meaning when they are made to change in a listener's attitude—saying that is incorrect in explaining "evaluative force in purely causal terms". This is Urmson's fundamental criticism, and he suggests that Stevenson would have made a stronger case by explaining emotive meaning in terms of "commending and recommending attitudes", not in terms of "the power to evoke attitudes".

Stevenson's Ethics and Language, written after Ross's book but before Brandt's and Urmson's, states that emotive terms are "not always used for purposes of exhortation." For example, in the sentence "Slavery was good in Ancient Rome", Stevenson thinks one is speaking of past attitudes in an "almost purely descriptive" sense. And in some discussions of current attitudes, "agreement in attitude can be taken for granted," so a judgment like "He was wrong to kill them" might describe one's attitudes yet be "emotively inactive", with no real emotive (or imperative) meaning. Stevenson is doubtful that sentences in such contexts qualify as normative ethical sentences, maintaining that "for the contexts that are most typical of normative ethics, the ethical terms have a function that is both emotive and descriptive."

Philippa Foot's moral realism

Philippa Foot adopts a moral realist position, criticizing the idea that when evaluation is superposed on fact there has been a "committal in a new dimension." She introduces, by analogy, the practical implications of using the word injury. Not just anything counts as an injury. There must be some impairment. When we suppose a man wants the things the injury prevents him from obtaining, have not we fallen into the old naturalist fallacy?

It may seem that the only way to make a necessary connexion between 'injury' and the things that are to be avoided, is to say that it is only used in an 'action-guiding sense' when applied to something the speaker intends to avoid. But we should look carefully at the crucial move in that argument, and query the suggestion that someone might happen not to want anything for which he would need the use of hands or eyes. Hands and eyes, like ears and legs, play a part in so many operations that a man could only be said not to need them if he had no wants at all.

Foot argues that the virtues, like hands and eyes in the analogy, play so large a part in so many operations that it is implausible to suppose that a committal in a non-naturalist dimension is necessary to demonstrate their goodness.

Philosophers who have supposed that actual action was required if 'good' were to be used in a sincere evaluation have got into difficulties over weakness of will, and they should surely agree that enough has been done if we can show that any man has reason to aim at virtue and avoid vice. But is this impossibly difficult if we consider the kinds of things that count as virtue and vice? Consider, for instance, the cardinal virtues, prudence, temperance, courage and justice. Obviously any man needs prudence, but does he not also need to resist the temptation of pleasure when there is harm involved? And how could it be argued that he would never need to face what was fearful for the sake of some good? It is not obvious what someone would mean if he said that temperance or courage were not good qualities, and this not because of the 'praising' sense of these words, but because of the things that courage and temperance are.

Standard using and standard setting

As an offshoot of his fundamental criticism of Stevenson's magnetic influence thesis, Urmson wrote that ethical statements had two functions—"standard using", the application of accepted values to a particular case, and "standard setting", the act of proposing certain values as those that should be accepted—and that Stevenson confused them. According to Urmson, Stevenson's "I approve of this; do so as well" is a standard-setting statement, yet most moral statements are actually standard-using ones, so Stevenson's explanation of ethical sentences is unsatisfactory. Colin Wilks has responded that Stevenson's distinction between first-order and second-order statements resolves this problem: a person who says "Sharing is good" may be making a second-order statement like "Sharing is approved of by the community", the sort of standard-using statement Urmson says is most typical of moral discourse. At the same time, their statement can be reduced to a first-order, standard-setting sentence: "I approve of whatever is approved of by the community; do so as well."

Atlantic history

From Wikipedia, the free encyclopedia

Atlantic history is a specialty field in history that studies the Atlantic World in the early modern period. The Atlantic World was created by the contact between Europeans and the Americas, and Atlantic History is the study of that world. It is premised on the idea that, following the rise of sustained European contact with the New World in the 16th century, the continents that bordered the Atlantic Ocean—the Americas, Europe, and Africa—constituted a regional system or common sphere of economic and cultural exchange that can be studied as a totality.

Its theme is the complex interaction between Europe (especially Great Britain, France, Spain, and Portugal) and their colonies in the Americas. It encompasses a wide range of demographic, social, economic, political, legal, military, intellectual and religious topics treated in comparative fashion by looking at both sides of the Atlantic. Religious revivals in Britain and Germany are studies, as well as the First Great Awakening in the Thirteen Colonies. Emigration, race and slavery are also important topics.

Researchers of Atlantic history typically focus on the interconnections and exchanges between these regions and the civilizations they harbored. In particular, they argue that the boundaries between nation states which traditionally determined the limits of older historiography should not be applied to such transnational phenomena as slavery, colonialism, missionary activity and economic expansion. Environmental history and the study of historical demography also play an important role, as many key questions in the field revolve around the ecological and epidemiological impact of the Columbian exchange.

Robert R. Palmer, an American historian of the French Revolution, pioneered the concept in the 1950s with a wide-ranging comparative history of how numerous nations experienced what he called The Age of the Democratic Revolution: A Political History of Europe and America, 1760–1800 (1959 and 1964). In this monumental work, he did not compare the French and the American Revolutions as successful models against other types of revolutions. Indeed, he developed a wider understanding of the changes that were led by revolutionary processes across the Western civilization. Such work followed in the footsteps of C. L. R. James who, in the 1930s, connected the French and Haitian Revolutions. Since the 1980s Atlantic history has emerged as an increasingly popular alternative to the older discipline of imperial history, although it could be argued that the field is simply a refinement and reorientation of traditional historiography dealing with the interaction between early modern Europeans and native peoples in the Atlantic sphere. The organization of Atlantic History as a recognized area of historiography began in the 1980s under the impetus of American historians Bernard Bailyn of Harvard University and Jack P. Greene of Johns Hopkins University, among others. The post-World War II integration of the European Union and the continuing importance of NATO played an indirect role in stimulating interest throughout the 1990s.

Development of the field

Bernard Bailyn's Seminar on the History of the Atlantic World promoted social and demographic studies, and especially regarding demographic flows of population into colonial America. As a leading advocate of the history of the Atlantic world, Bailyn has since 1995 organized an annual international seminar at Harvard designed to promote scholarship in this field. Professor Bailyn was the promoter of "The International Seminar on the History of the Atlantic World, 1500-1825" at Harvard University. This was one of the first, and most important, academic initiatives to launch the Atlantic perspective. From 1995-2010 the Atlantic History Seminar sponsored an annual meeting of young historians engaged in creative research on aspects of Atlantic History. In all, 366 young historians came through the Seminar program, 202 from universities in the US and 164 from universities abroad. Its purpose was to advance the scholarship of young historians of many nations interested in the common, comparative, and interactive aspects of the lives of the peoples in the lands that are part of the Atlantic basin, mainly in the early modern period in order to contribute to the study of this transnational historical subject.

Bailyn's Atlantic History: Concepts and Contours (2005) explores the borders and contents of the emerging field, which emphasizes cosmopolitan and multicultural elements that have tended to be neglected or considered in isolation by traditional historiography dealing with the Americas. Bailyn's reflections stem in part from his seminar at Harvard since the mid-1980s.

Other important scholars are Jack Greene, who directed a program at Johns Hopkins in Atlantic History from 1972 to 1992 that has now expanded to global concerns. Karen Ordahl Kupperman established the Atlantic Workshop at New York University in 1997.

Other scholars in the field include Ida Altman, Kenneth J. Andrien, David Armitage, Trevor Burnard, Jorge Canizares-Esguerra, Nicholas Canny, Philip D. Curtin, Laurent Dubois, J.H. Elliott, David Eltis, Alison Games, Eliga H. Gould, Anthony Grafton, Joseph C. Miller, Philip D. Morgan, Anthony Pagden, Jennifer L. Anderson, John Thornton, James D. Tracy, Carla G. Pestana, Isaac Land, Richard S. Dunn, and Ned C. Landsman.

Perspectives

Alison Games (2006) explores the convergence of the multiple strands of scholarly interest that have generated the new field of Atlantic history, which takes as its geographic unit of analysis the Atlantic Ocean and the four continents that surround it. She argues Atlantic history is best approached as a slice of world history. The Atlantic, moreover, is a region that has logic as a unit of historical analysis only within a limited chronology. An Atlantic perspective can help historians understand changes within the region that a more limited geographic framework might obscure. Attempts to write a Braudelian Atlantic history, one that includes and connects the entire region, remain elusive, driven in part by methodological impediments, by the real disjunction that characterized the Atlantic's historical and geographic components, by the disciplinary divisions that discourage historians from speaking to and writing for each other, and by the challenge of finding a vantage point that is not rooted in any single place.

Colonial studies

One impetus for Atlantic studies began in the 1960s with the historians of slavery who started tracking the routes of the transatlantic slave trade. A second source came from historians who studied the colonial history of the United States. Many were trained in early modern European history and were familiar with the historiography of the British Empire, which had been introduced a century before by George Louis Beer and Charles McLean Andrews. Historians studying colonialism have long been open to interdisciplinary perspectives, such as comparative approaches. In addition there was a frustration involved in writing about very few people in a small remote colony. Atlantic history opens the horizon to large forces at work over great distances.

Criticism

Some critics have complained that Atlantic history is little more than imperial history under another name. It has been argued that it is too expansive in claiming to subsume both of the American continents, Africa, and Europe, without seriously engaging with them. According to Caroline Dodds Pennock, indigenous people are often seen as static recipients of transatlantic encounter, despite the fact that thousands of Native Americans crossed the ocean during the sixteenth century, some by choice.

Canadian scholar Ian K. Steele argued that Atlantic history will tend to draw students interested in exploring their country's historian beyond national myths, while offering historical support for such 21st century policies as the North American Free Trade Agreement (NAFTA), the Organization of American States (OAS), the North Atlantic Treaty Organization (NATO), the New Europe, Christendom, and even the United Nations (UN). He concludes, "The early modern Atlantic can even be read as a natural antechamber for American-led globalization of capitalism and serve as an historical challenge to the coalescing New Europe. No wonder that the academic reception of the new Atlantic history has been enthusiastic in the United States, and less so in Britain, France, Spain, and Portugal, where histories of national Atlantic empires continue to thrive."

Vaccine trial

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Vaccine_trial

A vaccine trial is a clinical trial that aims at establishing the safety and efficacy of a vaccine prior to it being licensed.

A vaccine candidate drug is first identified through preclinical evaluations that could involve high throughput screening and selecting the proper antigen to invoke an immune response.

Some vaccine trials may take months or years to complete, depending on the time required for the subjects to react to the vaccine and develop the required antibodies.

Preclinical stage

Preclinical development stages are necessary to determine the immunogenicity potential and safety profile for a vaccine candidate.

This is also the stage in which the drug candidate may be first tested in laboratory animals prior to moving to the Phase I trials. Vaccines such as the oral polio vaccine have been first tested for adverse effects and immunogenicity in monkeys as well as non-human primates and lab mice.

Recent scientific advances have helped to use transgenic animals as a part of vaccine preclinical protocol in hopes to more accurately determine drug reactions in humans. Understanding vaccine safety and the immunological response to the vaccine, such as toxicity, are necessary components of the preclinical stage. Other drug trials focus on the pharmacodynamics and pharmacokinetics; however, in vaccine studies it is essential to understand toxic effects at all possible dosage levels and the interactions with the immune system.

Phase I

The Phase I study consists of introducing the vaccine candidate to assess its safety in healthy people. A vaccine Phase I trial involves normal healthy subjects, each tested with either the candidate vaccine or a "control" treatment, typically a placebo or an adjuvant-containing cocktail, or an established vaccine (which might be intended to protect against a different pathogen). The primary observation is for detection of safety (absence of an adverse event) and evidence of an immune response.

After the administration of the vaccine or placebo, the researchers collect data on antibody production, on health outcomes (such as illness due to the targeted infection or to another infection). Following the trial protocol, the specified statistical test is performed to gauge the statistical significance of the observed differences in the outcomes between the treatment and control groups. Side effects of the vaccine are also noted, and these contribute to the decision on whether to advance the candidate vaccine to a Phase II trial.

One typical version of Phase I studies in vaccines involves an escalation study, which is used in mainly medicinal research trials. The drug is introduced into a small cohort of healthy volunteers. Vaccine escalation studies aim to minimize chances of serious adverse effects (SAE) by slowly increasing the drug dosage or frequency. The first level of an escalation study usually has two or three groups of around 10 healthy volunteers. Each subgroup receives the same vaccine dose, which is the expected lowest dose necessary to invoke an immune response (the main goal in a vaccine – to create immunity). New subgroups can be added to experiment with a different dosing regimen as long as the previous subgroup did not experience SAEs. There are variations in the vaccination order that can be used for different studies. For example, the first subgroup could complete the entire regimen before the second subgroup starts or the second can begin before the first ends as long as SAEs were not detected. The vaccination schedule will vary depending on the nature of the drug (i.e. the need for a booster or several doses over the course of short time period). Escalation studies are ideal for minimizing risks for SAEs that could occur with less controlled and divided protocols.

Phase II

The transition to Phase II relies on the immunogenic and toxicity results from Phase I in a small cohort of healthy volunteers. Phase II will consist of more healthy volunteers in the vaccine target population (~ hundreds of people) to determine reactions in a more diverse set of humans and test different schedules.

Phase III

Similarly. Phase III trials continue to monitor toxicity, immunogenicity, and SAEs on a much larger scale. The vaccine must be shown to be safe and effective in natural disease conditions before being submitted for approval and then general production. In the United States, the Food and Drug Administration (FDA) is responsible for approving vaccines.

Phase IV

Phase IV trials are typically monitor stages that collect information continuously on vaccine usage, adverse effects, and long-term immunity after the vaccine is licensed and marketed. Harmful effects, such as increased risk of liver failure or heart attacks, discovered by Phase IV trials may result in a drug being no longer sold, or restricted to certain uses; examples include cerivastatin (brand names Baycol and Lipobay), troglitazone (Rezulin) and rofecoxib (Vioxx). Further examples include the swine flu vaccine and the rotavirus vaccine, which increased the risk of Guillain-Barré syndrome (GBS) and intussusception respectively. Thus, the fourth phase of clinical trials is used to ensure long-term vaccine safety.

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Sunday, December 8, 2024

History

Determination

Solar System estimates

Extrasolar extrapolation

Spectral types and star-system characteristics

Stellar evolution

Desert planets

Other considerations

Extrasolar discoveries

Early findings

Habitable super-Earths

Near Earth-sized planets and Solar analogs

Habitability outside the HZ

Significance for complex and intelligent life

Stellar characteristics

Planetary characteristics

Age

Orbit and rotation

Mass and size

Geology

Surface features

Climate

History

Proponents

A. J. Ayer

C. L. Stevenson

First pattern analysis

Second pattern analysis

Methods of argumentation

Criticism

Magnetic influence

Philippa Foot's moral realism

Standard using and standard setting

Development of the field

Perspectives

Colonial studies

Criticism

Preclinical stage

Phase I

Phase II

Phase III

Phase IV