Nebular hypothesis

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The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. It suggests that the Solar System formed from nebulous material in space. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg.^[1][2][3] Originally applied to our own Solar System, this process of planetary system formation is now thought to be at work throughout the universe.^[4] The widely accepted modern variant of the nebular hypothesis is the solar nebular disk model (SNDM) or simply solar nebular model.^[5] This nebular hypothesis offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the nebular hypothesis are echoed in modern theories of planetary formation, but most elements have been superseded.

According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.^[4]

The protoplanetary disk is an accretion disk which feeds the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains may eventually coagulate into kilometer-sized planetesimals. If the disk is massive enough the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes around 100 million to a billion years.^[4]

The formation of giant planets is a more complicated process. It is thought to occur beyond the so-called frost line, where planetary embryos are mainly made of various types of ice. As a result they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants like Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.^[4]

History

There is evidence that the nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg.^[1][2] Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further in 1755, when Kant published his Universal Natural History and Theory of the Heavens, wherein he argued that gaseous clouds—nebulae, which slowly rotate, gradually collapse and flatten due to gravity and eventually form stars and planets.^[5]
A similar model was developed independently and proposed in 1796 by Pierre-Simon Laplace.^[5] in his Exposition du systeme du monde. He envisioned that the Sun originally had an extended hot atmosphere throughout the volume of the Solar System. His theory featured a contracting and cooling protosolar cloud—the protosolar nebula. As this cooled and contracted, it flattened and spun more rapidly, throwing off (or shedding) a series of gaseous rings of material; and according to him, the planets condensed from this material. His model was similar to Kant's, except more detailed and on a smaller scale.^[5] While the Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties. The main problem was angular momentum distribution between the Sun and planets. The planets have 99% of the angular momentum, and this fact could not be explained by the nebular model.^[5] As a result this theory of planet formation was largely abandoned at the beginning of the 20th century.

The fall of the Laplacian model stimulated scientists to find a replacement for it. During the 20th century many theories were proposed including the planetesimal theory of Thomas Chamberlin and Forest Moulton (1901), tidal model of Jeans (1917), accretion model of Otto Schmidt (1944), protoplanet theory of William McCrea (1960) and finally capture theory of Michael Woolfson.^[5] In 1978 Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the modern Laplacian theory.^[5] None of these attempts was completely successful and many of the proposed theories were descriptive.

The birth of the modern widely accepted theory of planetary formation—the solar nebular disk model (SNDM)—can be traced to the Soviet astronomer Victor Safronov.^[6] His book Evolution of the protoplanetary cloud and formation of the Earth and the planets,^[7] which was translated to English in 1972, had a long lasting effect on the way scientists think about the formation of the planets.^[8] In this book almost all major problems of the planetary formation process were formulated and some of them solved. Safronov's ideas were further developed in the works of George Wetherill, who discovered runaway accretion.^[5] While originally applied only to our own Solar System, the SNDM was subsequently thought by theorists to be at work throughout the universe; as of 1 September 2014, 1821 extrasolar planets have since been discovered in our galaxy.^[9]

Solar nebular model: achievements and problems

Achievements

The star formation process naturally results in the appearance of accretion disks around young stellar objects.^[10] At the age of about 1 million years, 100% of stars may have such disks.^[11] This conclusion is supported by the discovery of the gaseous and dusty disks around protostars and T Tauri stars as well as by theoretical considerations.^[12] Observations of these disks show that the dust grains inside them grow in size on short (thousand-year) time scales, producing 1 centimeter sized particles.^[13]

The accretion process, by which 1 km planetesimals grow into 1,000 km sized bodies, is well understood now.^[14] This process develops inside any disk where the number density of planetesimals is sufficiently high, and proceeds in a runaway manner. Growth later slows and continues as oligarchic accretion. The end result is formation of planetary embryos of varying sizes, which depend on the distance from the star.^[14] Various simulations have demonstrated that the merger of embryos in the inner part of the protoplanetary disk leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.^[15]

Problems and criticism

The physics of accretion disks encounters some problems.^[16] The most important one is how the material, which is accreted by the protostar, loses its angular momentum. One possible explanation suggested by Hannes Alfvén was that angular momentum was shed by the solar wind during its T Tauri phase. The momentum is probably transported to the outer parts of the disk, but the precise mechanism of this transport is not well understood. Another possible process for shedding angular momentum is magnetic braking, where the spin of the star is transferred into the surrounding disk via that star's magnetic field.^[17] The process or processes responsible for the disappearance of the disks are also poorly known.^[18][19]

The formation of planetesimals is the biggest unsolved problem in the nebular disk model. How 1 cm sized particles coalesce into 1 km planetesimals is a mystery. This mechanism appears to be the key to the question as to why some stars have planets, while others have nothing around them, not even dust belts.^[20]

The formation of giant planets is another unsolved problem. Current theories are unable to explain how their cores can form fast enough to accumulate significant amounts of gas from the quickly disappearing protoplanetary disk.^[14][21] The mean lifetime of the disks, which are less than ten million (10⁷) years, appears to be shorter than the time necessary for the core formation.^[11]

Another problem of giant planet formation is their migration. Some calculations show that interaction with the disk can cause rapid inward migration, which, if not stopped, results in the planet reaching the "central regions still as a sub-Jovian object."^[22]

A major critique came during the 19th century from James Clerk Maxwell who maintained that different rotation between the inner and outer parts of a ring could not allow condensation of material.^[23] It was also rejected by astronomer Sir David Brewster who stated that "those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process." He argued that under such view, "the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earthy and must have an atmosphere."^[24] Brewster claimed that Sir Isaac Newton's religious beliefs had previously considered nebular ideas as tending to atheism, and quoted him saying that "the growth of new systems out of old ones, without the mediation of a Divine power, seemed to him apparently absurd."^[25]

Formation of stars and protoplanetary disks

Protostars

The visible-light (left) and infrared (right) views of the Trifid Nebula—a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius

Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds roughly 300,000 times the mass of the Sun and 20 parsecs in diameter.^[4][26] Over millions of years, giant molecular clouds are prone to collapse and fragmentation.^[27] These fragments then form small, dense cores, which in turn collapse into stars.^[26] The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae.^[4] They possess diameters of 0.01–0.1 pc (2,000–20,000 AU) and a particle number density of roughly 10,000 to 100,000 cm⁻³.^[a][26][28]

The initial collapse of a solar-mass protostellar nebula takes around 100,000 years.^[4][26] Every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot hydrostatic (not contracting) core containing a small fraction of the mass of the original nebula.^[29] This core forms the seed of what will become a star.^[4][29] As the collapse continues, conservation of angular momentum means that the rotation of the infalling envelop accelerates,^[19][30] which largely prevents the gas from directly accreting onto the central core. The gas is instead forced to spread outwards near its equatorial plane, forming a disk, which in turn accretes onto the core.^[4][19][30] The core gradually grows in mass until it becomes a young hot protostar.^[29] At this stage, the protostar and its disk are heavily obscured by the infalling envelope and are not directly observable.^[10] In fact the remaining envelope's opacity is so high that even millimeter-wave radiation has trouble escaping from inside it.^[4][10] Such objects are observed as very bright condensations, which emit mainly millimeter-wave and submillimeter-wave radiation.^[28] They are classified as spectral Class 0 protostars.^[10] The collapse is often accompanied by bipolar outflows—jets—that emanate along the rotational axis of the inferred disk. The jets are frequently observed in star-forming regions (see Herbig–Haro (HH) objects).^[31] The luminosity of the Class 0 protostars is high — a solar-mass protostar may radiate at up to 100 solar luminosities.^[10] The source of this energy is gravitational collapse, as their cores are not yet hot enough to begin nuclear fusion.^[29][32]

Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47

As the infall of its material onto the disk continues, the envelope eventually becomes thin and transparent and the young stellar object (YSO) becomes observable, initially in far-infrared light and later in the visible.^[28] Around this time the protostar begins to fuse deuterium. If the protostar is sufficiently massive (above 80 Jupiter masses), hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf.^[32] This birth of a new star occurs approximately 100,000 years after the collapse begins.^[4] Objects at this stage are known as Class I protostars,^[10] which are also called young T Tauri stars, evolved protostars, or young stellar objects.^[10] By this time the forming star has already accreted much of its mass: the total mass of the disk and remaining envelope does not exceed 10–20% of the mass of the central YSO.^[28]

At the next stage the envelope completely disappears, having been gathered up by the disk, and the protostar becomes a classical T Tauri star.^[b] This happens after about 1 million years.^[4] The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, and it is accreted at a rate of 10⁻⁷ to 10⁻⁹ solar masses per year.^[35] A pair of bipolar jets is usually present as well.^[36] The accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines (up to 100% of the intrinsic luminosity of the star), magnetic activity, photometric variability and jets.^[37] The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles.^[37] The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10 million years.^[4] The disk eventually disappears due to accretion onto the central star, planet formation, ejection by jets and photoevaporation by UV-radiation from the central star and nearby stars.^[38] As a result the young star becomes a weakly lined T Tauri star, which slowly, over hundreds of millions of years, evolves into an ordinary sun-like star.^[29]

Protoplanetary disks

Debris disks detected in HST archival images of young stars, HD 141943 and HD 191089, using improved imaging processes (24 April 2014).^[39]

Under certain circumstances the disk, which can now be called protoplanetary, may give birth to a planetary system.^[4] Protoplanetary disks have been observed around a very high fraction of stars in young star clusters.^[11][40] They exist from the beginning of a star's formation, but at the earliest stages are unobservable due to the opacity of the surrounding envelope.^[10] The disk of a Class 0 protostar is thought to be massive and hot. It is an accretion disk, which feeds the central protostar.^[19][30] The temperature can easily exceed 400 K inside 5 AU and 1,000 K inside 1 AU.^[41] The heating of the disk is primarily caused by the viscous dissipation of turbulence in it and by the infall of the gas from the nebula.^[19][30] The high temperature in the inner disk causes most of the volatile material—water, organics, and some rocks to evaporate, leaving only the most refractory elements like iron. The ice can survive only in the outer part of the disk.^[41]

A protoplanetary disk forming in the Orion Nebula

The main problem in the physics of accretion disks is the generation of turbulence and the mechanism responsible for the high effective viscosity.^[4] The turbulent viscosity is thought to be responsible for the transport of the mass to the central protostar and momentum to the periphery of the disk. This is vital for accretion, because the gas can be accreted by the central protostar only if it loses most of its angular momentum, which must be carried away by the small part of the gas drifting outwards.^[18][19] The result of this process is the growth of both the protostar and of the disk radius, which can reach 1,000 AU if the initial angular momentum of the nebula is large enough.^[30] Large disks are routinely observed in many star-forming regions such as the Orion nebula.^[12]

Artist's impression of the disc and gas streams around young star HD 142527.^[42]

The lifespan of the accretion disks is about 10 million years.^[11] By the time the star reaches the classical T-Tauri stage, the disk becomes thinner and cools.^[35] Less volatile materials start to condense close to its center, forming 0.1–1 μm dust grains that contain crystalline silicates.^[13] The transport of the material from the outer disk can mix these newly formed dust grains with primordial ones, which contain organic matter and other volatiles. This mixing can explain some peculiarities in the composition of solar system bodies such as the presence of interstellar grains in the primitive meteorites and refractory inclusions in comets.^[41]

Various planet formation processes, including exocomets and other planetesimals, around Beta Pictoris, a very young type A V star (NASA artist's conception).

Dust particles tend to stick to each other in the dense disk environment, leading to the formation of larger particles up to several centimeters in size.^[43] The signatures of the dust processing and coagulation are observed in the infrared spectra of the young disks.^[13] Further aggregation can lead to the formation of planetesimals measuring 1 km across or larger, which are the building blocks of planets.^[4][43] Planetesimal formation is another unsolved problem of disk physics, as simple sticking becomes ineffective as dust particles grow larger.^[20] The favorite hypothesis is formation by the gravitational instability. Particles several centimeters in size or larger slowly settle near the middle plane of the disk, forming a very thin—less than 100 km—and dense layer. This layer is gravitationally unstable and may fragment into numerous clumps, which in turn collapse into planetesimals.^[4][20]

Planetary formation can also be triggered by gravitational instability within the disk itself, which leads to its fragmentation into clumps. Some of them, if they are dense enough, will collapse,^[18] which can lead to rapid formation of gas giant planets and even brown dwarfs on the timescale of 1,000 years.^[44] However it is only possible in massive disks—more massive than 0.3 solar masses. In comparison typical disk masses are 0.01–0.03 solar masses. Because the massive disks are rare, this mechanism of the planet formation is thought to be infrequent.^[4][16] On the other hand, this mechanism may play a major role in the formation of brown dwarfs.^[45]

Asteroid collision—building planets (artist concept).

The ultimate dissipation of protoplanetary disks is triggered by a number of different mechanisms. The inner part of the disk is either accreted by the star or ejected by the bipolar jets,^[35][36] whereas the outer part can evaporate under the star's powerful UV radiation during the T Tauri stage^[46] or by nearby stars.^[38] The gas in the central part can either be accreted or ejected by the growing planets, while the small dust particles are ejected by the radiation pressure of the central star. What is finally left is either a planetary system, a remnant disk of dust without planets, or nothing, if planetesimals failed to form.^[4]

Because planetesimals are so numerous, and spread throughout the protoplanetary disk, some survive the formation of a planetary system. Asteroids are understood to be left-over planetesimals, gradually grinding each other down into smaller and smaller bits, while comets are typically planetesimals from the farther reaches of a planetary system. Meteorites are samples of planetesimals that reach a planetary surface, and provide a great deal of information about the formation of our Solar System. Primitive-type meteorites are chunks of shattered low-mass planetesimals, where no thermal differentiation took place, while processed-type meteorites are chunks from shattered massive planetesimals.^[47]

Formation of planets

Rocky planets

According to the solar nebular disk model, rocky planets form in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to prevent condensation of water ice and other substances into grains.^[48] This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals.^[c][48] Such conditions are thought to exist in the inner 3–4 AU part of the disk of a sun-like star.^[4]

After small planetesimals—about 1 km in diameter—have formed by one way or another, runaway accretion begins.^[14] It is called runaway because the mass growth rate is proportional to R⁴~M^4/3, where R and M are the radius and mass of the growing body, respectively.^[49] It is obvious that the specific (divided by mass) growth accelerates as the mass increases. This leads to the preferential growth of larger bodies at the expense of smaller ones.^[14] The runaway accretion lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter.^[14] Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals.^[14][49] In addition, the influence of larger bodies stops further growth of smaller bodies.^[14]

The next stage is called oligarchic accretion.^[14] It is characterized by the dominance of several hundred of the largest bodies—oligarchs, which continue to slowly accrete planetesimals.^[14] No body other than the oligarchs can grow.^[49] At this stage the rate of accretion is proportional to R², which is derived from the geometrical cross-section of an oligarch.^[49] The specific accretion rate is proportional to M^−1/3; and it declines with the mass of the body. This allows smaller oligarchs to catch up to larger ones. The oligarchs are kept at the distance of about 10·H_r (H_r=a(1-e)(M/3M_s)^1/3 is the Hill radius, where a is the semimajor axis, e is the orbital eccentricity, and M_s is the mass of the central star) from each other by the influence of the remaining planetesimals.^[14] Their orbital eccentricities and inclinations remain small. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them.^[14] Sometimes nearby oligarchs merge. The final mass of an oligarch depends on the distance from the star and surface density of planetesimals and is called the isolation mass.^[49] For the rocky planets it is up to 0.1 of the Earth mass, or one Mars mass.^[4] The final result of the oligarchic stage is the formation of about 100 Moon- to Mars-sized planetary embryos uniformly spaced at about 10·H_r.^[15] They are thought to reside inside gaps in the disk and to be separated by rings of remaining planetesimals. This stage is thought to last a few hundred thousand years.^[4][14]

The last stage of rocky planet formation is the merger stage.^[4] It begins when only a small number of planetesimals remains and embryos become massive enough to perturb each other, which causes their orbits to become chaotic.^[15] During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for 10 to 100 million years, is the formation of a limited number of Earth sized bodies. Simulations show that the number of surviving planets is on average from 2 to 5.^{[4][15][47][50]} In the Solar System they may be represented by Earth and Venus.^[15] Formation of both planets required merging of approximately 10–20 embryos, while an equal number of them were thrown out of the Solar System.^[47] Some of the embryos, which originated in the asteroid belt, are thought to have brought water to Earth.^[48] Mars and Mercury may be regarded as remaining embryos that survived that rivalry.^[47] Rocky planets, which have managed to coalesce, settle eventually into more or less stable orbits, explaining why planetary systems are generally packed to the limit; or, in other words, why they always appear to be at the brink of instability.^[15]

Giant planets

The dust disk around Fomalhaut—the brightest star in Piscis Austrinus constellation. Asymmetry of the disk may be caused by a giant planet (or planets) orbiting the star.

The formation of giant planets is an outstanding problem in the planetary sciences.^[16] In the framework of the solar nebular model two theories for their formation exist. The first one is the disk instability model, where giant planets form in the massive protoplanetary disks as a result of its gravitational fragmentation (see above).^[44] The second possibility is the core accretion model, which is also known as the nucleated instability model.^[16] The latter scenario is thought to be the most promising one, because it can explain the formation of the giant planets in relatively low mass disks (less than 0.1 solar masses). In this model giant planet formation is divided into two stages: a) accretion of a core of approximately 10 Earth masses and b) accretion of gas from the protoplanetary disk.^[4][16] Either method may also lead to the creation of brown dwarfs.^[51] Searches as of 2011 have found that core accretion is likely the dominant formation mechanism.^[51]

Giant planet core formation is thought to proceed roughly along the lines of the terrestrial planet formation.^[14] It starts with planetesimals that undergo runaway growth, followed by the slower oligarchic stage.^[49] Hypotheses do not predict a merger stage, due to the low probability of collisions between planetary embryos in the outer part of planetary systems.^[49] An additional difference is the composition of the planetesimals, which in the case of giant planets form beyond the so-called snow line and consist mainly of ice—the ice to rock ratio is about 4 to 1.^[21] This enhances the mass of planetesimals fourfold. However, the minimum mass nebula capable of terrestrial planet formation can only form 1–2 Earth-mass cores at the distance of Jupiter (5 AU) within 10 million years.^[49] The latter number represents the average lifetime of gaseous disks around sun-like stars.^[11] The proposed solutions include enhanced mass of the disk—a tenfold increase would suffice;^[49] protoplanet migration, which allows the embryo to accrete more planetesimals;^[21] and finally accretion enhancement due to gas drag in the gaseous envelopes of the embryos.^[21][52] Some combination of the above-mentioned ideas may explain the formation of the cores of gas giant planets such as Jupiter and perhaps even Saturn.^[16] The formation of planets like Uranus and Neptune is more problematic, since no theory has been capable of providing for the in situ formation of their cores at the distance of 20–30 AU from the central star.^[4] One hypothesis is that they initially accreted in the Jupiter-Saturn region, then were scattered and migrated to their present location.^[53]

Once the cores are of sufficient mass (5–10 Earth masses), they begin to gather gas from the surrounding disk.^[4] Initially it is a slow process, increasing the core masses up to 30 Earth masses in a few million years.^[21][52] After that, the accretion rates increase dramatically and the remaining 90% of the mass is accumulated in approximately 10,000 years.^[52] The accretion of gas stops when it is exhausted. This happens when a gap opens in the protoplanetary disk.^[54] In this model ice giants—Uranus and Neptune—are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post-runaway-gas-accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion.^[54] Migration is caused by the interaction of the planet sitting in the gap with the remaining disk. It stops when the protoplanetary disk disappears or when the end of the disk is attained. The latter case corresponds to the so-called hot Jupiters, which are likely to have stopped their migration when they reached the inner hole in the protoplanetary disk.^[54]

In this artist's conception, a planet spins through a clearing (gap) in a nearby star's dusty, planet-forming disc.

Giant planets can significantly influence terrestrial planet formation. The presence of giants tends to increase eccentricities and inclinations (see Kozai mechanism) of planetesimals and embryos in the terrestrial planet region (inside 4 AU in the Solar System).^[47][50] If giant planets form too early, they can slow or prevent inner planet accretion. If they form near the end of the oligarchic stage, as is thought to have happened in the Solar System, they will influence the merges of planetary embryos, making them more violent.^[47] As a result, the number of terrestrial planets will decrease and they will be more massive.^[55] In addition, the size of the system will shrink, because terrestrial planets will form closer to the central star. The influence of giant planets in the Solar System, particularly that of Jupiter, is thought to have been limited because they are relatively remote from the terrestrial planets.^[55]

The region of a planetary system adjacent to the giant planets will be influenced in a different way.^[50] In such a region, eccentricities of embryos may become so large that the embryos pass close to a giant planet, which may cause them to be ejected from the system.^{[d][47][50][50]} If all embryos are removed, then no planets will form in this region.^[50] An additional consequence is that a huge number of small planetesimals will remain, because giant planets are incapable of clearing them all out without the help of embryos. The total mass of remaining planetesimals will be small, because cumulative action of the embryos before their ejection and giant planets is still strong enough to remove 99% of the small bodies.^[47] Such a region will eventually evolve into an asteroid belt, which is a full analog of the asteroid belt in the Solar System, located from 2 to 4 AU from the Sun.^[47][50]

Meaning of accretion

Use of the term accretion disk for the protoplanetary disk leads to confusion over the planetary accretion process. The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge.^[30]

However, that meaning should not be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high-energy collisions between sizable planetesimals.^[14]

In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word. The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted into the giant planet's regular moons.^[56]

Primate cognition

From Wikipedia, the free encyclopedia

Primate cognition is the study of the intellectual and behavioral skills of non-human primates, particularly in the fields of psychology, behavioral biology, primatology, and anthropology.^[1]

Primates are capable of high levels of cognition; some make tools and use them to acquire foods and for social displays;^[2][3] some have sophisticated hunting strategies requiring cooperation, influence and rank;^[4] they are status conscious, manipulative and capable of deception;^[5] they can recognise kin and conspecifics;^[6][7] they can learn to use symbols and understand aspects of human language including some relational syntax, concepts of number and numerical sequence.^[8][9][10]

Studies in primate cognition

Theory of mind

Premack and Woodruff's 1978 article "Does the chimpanzee have a theory of mind?" was a contentious issue because of the problem of inferring from animal behavior the existence of thinking, of the existence of a concept of self or self-awareness, or of particular thoughts.
Non-human research still has a major place in this field, however, and is especially useful in illuminating which nonverbal behaviors signify components of theory of mind, and in pointing to possible stepping points in the evolution of what many claim to be a uniquely human aspect of social cognition. While it is difficult to study human-like theory of mind and mental states in species which we do not yet describe as "minded" at all, and about whose potential mental states we have an incomplete understanding, researchers can focus on simpler components of more complex capabilities.

For example, many researchers focus on animals' understanding of intention, gaze, perspective, or knowledge (or rather, what another being has seen). Part of the difficulty in this line of research is that observed phenomena can often be explained as simple stimulus-response learning, as it is in the nature of any theorizers of mind to have to extrapolate internal mental states from observable behavior. Recently, most non-human theory of mind research has focused on monkeys and great apes, who are of most interest in the study of the evolution of human social cognition.

There has been some controversy over the interpretation of evidence purporting to show theory of mind ability—or inability—in animals. Two examples serve as demonstration: first, Povinelli et al. (1990)^[11] presented chimpanzees with the choice of two experimenters from which to request food: one who had seen where food was hidden, and one who, by virtue of one of a variety of mechanisms (having a bucket or bag over his head; a blindfold over his eyes; or being turned away from the baiting) does not know, and can only guess. They found that the animals failed in most cases to differentially request food from the "knower." By contrast, Hare, Call, and Tomasello (2001)^[12] found that subordinate chimpanzees were able to use the knowledge state of dominant rival chimpanzees to determine which container of hidden food they approached.

Tomasello and like-minded colleagues who originally argued that great apes did not have theory of mind have since reversed their position. Povinelli and his colleagues, however, maintain that Tomasello's group has misinterpreted the results of their experiments. They point out that most evidence in support of great ape theory of mind involves naturalistic settings to which the apes may have already adapted through past learning. Their "reinterpretation hypothesis" explains away all current evidence supporting attribution of mental states to others in chimpanzees as merely evidence of risk-based learning; that is, the chimpanzees learn through experience that certain behaviors in other chimpanzees have a probability of leading to certain responses, without necessarily attributing knowledge or other intentional states to those other chimpanzees. They therefore propose testing theory of mind abilities in great apes in novel, and not naturalistic settings. Kristin Andrews takes the reinterpretation hypothesis one step further, arguing that it implies that even the well-known false-belief test used to test children's theory of mind is susceptible to being interpreted as a result of learning.

Language

The modeling of human language in animals is known as animal language research. Nim Chimpsky, a chimpanzee, was successfully taught 125 signs during his life, though some disagree on whether this can be constituted as true language. There have been other, more successful animal language projects, such as Kanzi and Koko, as well as some parrots.^{[citation needed]}
It has been suggested that Nim Chimpsky's limited success was due to short training sessions, rather than true language immersion.^{[citation needed]}

Tool use

Tool use by a gorilla

There are many reports of primates making or using tools, both in the wild or when captive. Chimpanzees, orangutans, gorillas, capuchin monkeys, baboons, and mandrills have all been reported as using tools. The use of tools by primates is varied and includes hunting (mammals, invertebrates,^[13] fish), collecting honey,^[14] processing food (nuts, fruits, vegetables and seeds), collecting water, weapons and shelter.

Tool making is much rarer, but has been documented in orangutans,^[15] bonobos and bearded capuchin monkeys. Research in 2007 shows that chimpanzees in the Fongoli savannah sharpen sticks to use as spears when hunting, considered the first evidence of systematic use of weapons in a species other than humans.^[16][17]

In the wild, mandrills have been observed to clean their ears with modified tools. Scientists filmed a large male mandrill at Chester Zoo (UK) stripping down a twig, apparently to make it narrower, and then using the modified stick to scrape dirt from underneath its toenails.^[18][19]

Captive gorillas have made a variety of tools.^[20]

Problem solving

In 1913, Wolfgang Köhler started writing a book on problem solving titled The Mentality of Apes (1917). In this research, Köhler observed the manner in which chimpanzees solve problems, such as that of retrieving bananas when positioned out of reach. He found that they stacked wooden crates to use as makeshift ladders in order to retrieve the food. If the bananas were placed on the ground outside of the cage, they used sticks to lengthen the reach of their arms.

Köhler concluded that the chimps had not arrived at these methods through trial-and-error (which American psychologist Edward Thorndike had claimed to be the basis of all animal learning, through his law of effect), but rather that they had experienced an insight (sometimes known as the Eureka effect or an “aha experience”), in which, having realized the answer, they then proceeded to carry it out in a way that was, in Köhler’s words, “unwaveringly purposeful.”

Asking questions and giving negative answers

In the 1970s and the 1980s there had been suggestions that apes are unable to ask questions and to give negative answers. According to the numerous published studies ^{[21][22][23][24][25]} apes are able to answer human questions, and the vocabulary of the acculturated apes contains question words.
Despite these abilities, according to the published research literature, apes are not able to ask questions themselves, and in human-primate conversations questions are asked by the humans only. Ann and David Premacks designed a potentially promising methodology to teach apes to ask questions in the 1970s: “In principle interrogation can be taught either by removing an element from a familiar situation in the animal’s world or by removing the element from a language that maps the animal’s world. It is probable that one can induce questions by purposefully removing key elements from a familiar situation. Suppose a chimpanzee received its daily ration of food at a specific time and place, and then one day the food was not there. A chimpanzee trained in the interrogative might inquire ‘Where is my food?’ or, in Sarah’s case, ‘My food is ?’ Sarah was never put in a situation that might induce such interrogation because for our purposes it was easier to teach Sarah to answer questions”.^[26]

A decade later Premacks wrote: "Though she [Sarah] understood the question, she did not herself ask any questions -- unlike the child who asks interminable questions, such as What that? Who making noise? When Daddy come home? Me go Granny's house? Where puppy? Sarah never delayed the departure of her trainer after her lessons by asking where the trainer was going, when she was returning, or anything else".^[27]

Despite all their achievements, Kanzi and Panbanisha also have not demonstrated the ability to ask questions so far. Joseph Jordania suggested that the ability to ask questions could be the crucial cognitive threshold between human and ape mental abilities.^[28] Jordania suggested that asking questions is not a matter of the ability of using syntactic structures, that it is primarily a matter of cognitive ability. Questions can be (and are) asked without the use of syntactic structures, with the help of the questions intonation only (like this is the case in children's early pre-linguistic development).

Evolution of human intelligence

From Wikipedia, the free encyclopedia

 

The evolution of human intelligence refers to a set of theories that attempt to explain how human intelligence has evolved. These theories are closely tied to the evolution of the human brain and to the emergence of human language.

The timeline of human evolution spans approximately 7 million years,^[1] from the separation of the Pan genus until the emergence of behavioral modernity by 50,000 years ago. The first 3 million years of this timeline concern Sahelanthropus, the following 2 million concern Australopithecus and the final 2 million span the history of actual human species (the Paleolithic).
Many traits of human intelligence, such as empathy, theory of mind, mourning, ritual, and the use of symbols and tools, are apparent in great apes although in less sophisticated forms than found in humans.

History

Hominidae

Chimpanzee mother and baby

The great apes show considerable abilities for cognition and empathy.

Chimpanzees make tools and use them to acquire foods and for social displays; they have sophisticated hunting strategies requiring cooperation, influence and rank; they are status conscious, manipulative and capable of deception; they can learn to use symbols and understand aspects of human language including some relational syntax, concepts of number and numerical sequence.^[2]

In one study, young chimpanzees outperformed human college students in tasks requiring remembering numbers.^[3] This claim was refuted in a later study after it was noted that the chimpanzees had received extensive practice with the task while the students were evaluated on their first attempt. When human subjects were given time to practice, they substantially outperformed the young chimps.^[4] Chimpanzees are capable of empathy.

Homininae

Around 10 million years ago, the Earth's climate entered a cooler and drier phase, which led eventually to the Quaternary glaciation beginning some 2.6 million years ago. One consequence of this was that the north African tropical forest began to retreat, being replaced first by open grasslands and eventually by desert (the modern Sahara). As their environment changed from continuous forest to patches of forest separated by expanses of grassland, some primates adapted to a partly or fully ground-dwelling life. Here they were exposed to predators, such as the big cats, from whom they had previously been safe.

These environmental pressures caused selection to favor bipedalism: walking on hind legs. This gave the Homininae's eyes greater elevation, the ability to see approaching danger further off, and a more efficient means of locomotion (see main article for details).^{[citation needed]} It also freed the forelimbs (arms) from the task of walking and made the hands available for tasks such as gathering food. At some point the bipedal primates developed handedness, giving them the ability to pick up sticks, bones and stones and use them as weapons, or as tools for tasks such as killing smaller animals, cracking nuts, or cutting up carcasses. In other words, these primates developed the use of primitive technology. Bipedal tool-using primates form the Hominina subtribe, of which the earliest species, such as Sahelanthropus tchadensis, date to about 7 to 5 million years ago.

From about 5 million years ago, the Hominin brain began to develop rapidly in both size and differentiation of function.^[why?]

There has been a gradual increase in brain volume as humans progressed along the timeline of evolution (see Homininae), starting from about 600 cm³ in Homo habilis up to 1500 cm³ in Homo neanderthalensis. Thus, in general there's a correlation between brain volume and intelligence. However, modern Homo sapiens have a brain volume slightly smaller (1250 cm³) than neanderthals, and the Flores hominids (Homo floresiensis), nicknamed hobbits, had a cranial capacity of about 380 cm³ (considered small for a chimpanzee) about a third of that of H. erectus. It is proposed that they evolved from H. erectus as a case of insular dwarfism. With their three times smaller brain the Flores hominids apparently used fire and made tools as sophisticated as those of their ancestor H.erectus. In this case, it seems that for intelligence, the structure of the brain is more important than its volume.

Homo

By 2.4 million years ago Homo habilis had appeared in East Africa: the first known human species, and the first known to make stone tools.
The use of tools conferred a crucial evolutionary advantage, and required a larger and more sophisticated brain to co-ordinate the fine hand movements required for this task. The evolution of a larger brain created a problem for early humans, however. A larger brain requires a larger skull, and thus requires the female to have a wider birth canal for the newborn's larger skull to pass through. But if the female's birth canal grew too wide, her pelvis would be so wide that she would lose the ability to run: still a necessary skill in the dangerous world of 2 million years ago.

The solution to this was to give birth at an early stage of fetal development, before the skull grew too large to pass through the birth canal. This adaptation enabled the human brain to continue to grow, but it imposed a new discipline. The need to care for helpless infants for long periods of time forced humans to become less mobile^{[citation needed]}. Human bands increasingly stayed in one place for long periods, so that females could care for infants, while males hunted food and fought with other bands that competed for food sources^{[citation needed]}. As a result, humans became even more dependent on tool-making to compete with other animals and other humans, and relied less on body size and strength^{[citation needed]}.

About 200,000 years ago Europe and the Middle East were colonized by Neanderthal man, extinct by 20,000 following the appearance of modern humans in the region from 40,000 years ago.

Homo sapiens

Middle Stone Age bifacial points, engraved ochre and bone tools from the c. 75,000 year old M1 & M2 phases at Blombos cave.

"The Lion Man," found in the Hohlenstein-Stadel cave of Germany's Swabian Alb and dated to 32,000 years ago, is associated with the Aurignacian culture and is the oldest known anthropomorphic animal figurine in the world.

 

Dates approximate, consult articles for details

(From 2000000 BC till 2013 AD in (partial) exponential notation)

See also: Java Man (-1.75e+06), Yuanmou Man (-1.75e+06 : -0.73e+06),

Lantian Man (-1.7e+06), Nanjing Man (- 0.6e+06), Tautavel Man (- 0.5e+06),

Peking Man (- 0.4e+06), Solo Man (- 0.4e+06), and Peștera cu Oase (- 0.378e+05)

Homo sapiens intelligence

Around 200,000 years ago, Homo sapiens first appears in East Africa. It is unclear to what extent these early modern humans had developed language, music, religion etc. They spread throughout Africa over the following 50,000 years or so.
According to proponents of the Toba catastrophe theory, the climate in non-tropical regions of the earth experienced a sudden freezing about 70,000 years ago, because of a huge explosion of the Toba volcano that filled the atmosphere with volcanic ash for several years. This reduced the human population to less than 10,000 breeding pairs in equatorial Africa, from which all modern humans are descended. Being unprepared for the sudden change in climate, the survivors were those intelligent enough to invent new tools and ways of keeping warm and finding new sources of food (for example, adapting to ocean fishing based on prior fishing skills used in lakes and streams that became frozen).

Around 80–100,000 years ago, three main lines of Homo sapiens diverged, bearers of mitochondrial haplogroup L1 (mtDNA) / A (Y-DNA) colonizing Southern Africa (the ancestors of the Khoisan/Capoid peoples), bearers of haplogroup L2 (mtDNA) / B (Y-DNA) settling Central and West Africa (the ancestors of Niger–Congo and Nilo-Saharan speaking peoples), while the bearers of haplogroup L3 remained in East Africa.

The "Great Leap Forward" leading to full behavioral modernity sets in only after this separation. Rapidly increasing sophistication in tool-making and behaviour is apparent from about 80,000 years ago, and the migration out of Africa follows towards the very end of the Middle Paleolithic, some 60,000 years ago. Fully modern behaviour, including figurative art, music, self-ornamentation, trade, burial rites etc. is evident by 30,000 years ago. The oldest unequivocal examples of prehistoric art date to this period, the Aurignacian and the Gravettian periods of prehistoric Europe, such as the Venus figurines and cave painting (Chauvet Cave) and the earliest musical instruments (the bone pipe of Geissenklösterle, Germany, dated to about 36,000 years ago).^[5]

Models

Social brain hypothesis

The social brain hypothesis was proposed by British anthropologist Robin Dunbar, who argues that human intelligence did not evolve primarily as a means to solve ecological problems, but rather intelligence evolved as a means of surviving and reproducing in large and complex social groups.^[6][7] Some of the behaviors associated with living in large groups include reciprocal altruism, deception and coalition formation. These group dynamics relate to Theory of Mind or the ability to understand the thoughts and emotions of others, though Dunbar himself admits in the same book that it is not the flocking itself that causes intelligence to evolve (as shown by ruminants).^[8]

Dunbar argues that when the size of a social group increases, the number of different relationships in the group may increase by orders of magnitude. Chimpanzees live in groups of about 50 individuals whereas humans typically have a social circle of about 150 people, which is now referred to as Dunbar's number. According to the social brain hypothesis, when hominids started living in large groups, selection favored greater intelligence. As evidence, Dunbar cites a relationship between neocortex size and group size of various mammals.^[8] However, meerkats have far more social relationships than their small brain capacity would suggest. Another hypothesis is that it is actually intelligence that causes social relationships to become more complex, because intelligent individuals are more difficult to learn to know.^[9]

There are also studies that show that Dunbar's number is not the upper limit of the number of social relationships in humans either.^[10][11]

Sexual selection

This model is proposed by Geoffrey Miller who argues that human intelligence is unnecessarily sophisticated for the needs of hunter-gatherers to survive. He argues that the manifestations of intelligence such as language, music and art did not evolve because of their utilitarian value to the survival of ancient hominids. Rather, intelligence may have been a fitness indicator. Hominids would have been selected for greater intelligence as a proxy for healthy genes and a positive feedback loop of sexual selection would have led to the evolution of human intelligence in a relatively short period.^[12]
A sexual selection theory must explain why both sexes are intelligent. In many species, only males have impressive ornaments and show-off behavior. Sexual selection is also thought to be able to act on both males and females in species that are at least partially monogamous.^[13] With complete monogamy, there is assortative mating for sexually selected traits. This means that less attractive individuals will find other less attractive individuals to mate with. If attractive traits are good fitness indicators, this means that sexual selection increases the genetic load of the offspring of unattractive individuals. Without sexual selection, an unattractive individual might find a superior mate with few deleterious mutations, and have healthy children that are likely to survive. With sexual selection, an unattractive individual is more likely to have access only to an inferior mate who is likely to pass on many deleterious mutations to their joint offspring, who are then less likely to survive.^[12]

Sexual selection is often thought to be a likely explanation for other female-specific human traits, for example breasts and buttocks far larger in proportion to total body size than those found in related species of ape.^[12] It is often assumed that if breasts and buttocks of such large size were necessary for functions such as suckling infants, they would be found in other species. Growing human brains require more nutrition than brains of related species of ape. Human males find human female breasts attractive, in agreement with sexual selection acting on human females.

Sexual selection for intelligence and judging ability can act on indicators of success, such as highly visible displays of wealth (cattle, farmland, servants, etc.). It is possible that for females to successfully judge male intelligence, they must be intelligent themselves. This could explain why despite the absence of clear differences in intelligence between males and females on average, there are clear differences between male and female propensities to display their intelligence in ostentatious forms.^[12]

Ecological dominance-social competition model

A predominant model describing the evolution of human intelligence is ecological dominance-social competition (EDSC),^[14] explained by Mark V. Flinn, David C. Geary and Carol V. Ward based mainly on work by Richard D. Alexander. According to the model, human intelligence was able to evolve to significant levels because of the combination of increasing domination over habitat and increasing importance of social interactions. As a result the primary selective pressure for increasing human intelligence shifted from learning to master the natural world to competition for dominance among members or groups of its own species.

As advancement, survival and reproduction within an increasing complex social structure favored ever more advanced social skills, communication of concepts through increasingly complex language patterns ensued. Since competition had shifted bit by bit from controlling "nature" to influencing other humans, it became of relevance to outmaneuver other members of the group seeking leadership or acceptance, by means of more advanced social skills. A more social and communicative person would be more easily selected.

Intelligence dependent on brain size

Human intelligence is developed to an extreme level that is not necessarily adaptive in an evolutionary sense. Firstly, larger-headed babies are more difficult to give birth to and large brains are costly in terms of nutrient and oxygen requirements.^[15] Thus the direct adaptive benefit of human intelligence is questionable at least in modern societies, while it is difficult to study in prehistoric societies. Since 2005, scientists have been evaluating genomic data on gene variants thought to influence head size, and have found no evidence that those genes are under strong selective pressure in current human populations.^[16] The trait of head size has become generally fixed in modern human beings.^[17]

Intelligence as a disease resistance sign

A recent study^[18] argues that human cleverness is simply selected within the context of sexual selection as an honest signal of genetic resistance against parasites and pathogens. The number of people living with cognitive abilities seriously damaged by childhood infections is high; estimated in hundreds of millions. Even more people live with moderate mental damages, such as inability to complete difficult tasks, that are not classified as ‘diseases’ by medical standards, may still be considered as inferior mates by potential sexual partners. Pathogens currently playing a major role in this global challenge against human cognitive capabilities include viral infections like meningitis, protists like Toxoplasma and Plasmodium, and animal parasites like intestinal worms and schistosomes.^[19]

Thus, widespread, virulent, and archaic infections are greatly involved in natural selection for cognitive abilities. People infected with parasites may have brain damage and obvious maladaptive behavior in addition to visible signs of disease. Smarter people can more skillfully learn to distinguish safe non-polluted water and food from unsafe kinds and learn to distinguish mosquito infested areas from safe areas. Smarter people can more skillfully find and develop safe food sources and living environments. Given this situation, preference for smarter child-bearing/rearing partners increases the chance that their descendants will inherit the best resistance alleles, not only for immune system resistance to disease, but also smarter brains for learning skills in avoiding disease and selecting nutritious food. When people search for mates based on their success, wealth, reputation, disease-free body appearance, or psychological traits such as benevolence or confidence; the effect is to select for superior intelligence that results in superior disease resistance.

Group selection

Group selection theory contends that organism characteristics that provide benefits to a group (clan, tribe, or larger population) can evolve despite individual disadvantages such as those cited above. The group benefits of intelligence (including language, the ability to communicate between individuals, the ability to teach others, and other cooperative aspects) have apparent utility in increasing the survival potential of a group.

Nutritional status

Higher cognitive functioning develops better in an environment with adequate nutrition,^[20] and diets deficient in iron, zinc, protein, iodine, B vitamins, omega 3 fatty acids, magnesium and other nutrients can result in lower intelligence^[21][22] either in the mother during pregnancy or in the child during development. While these inputs did not have an effect on the evolution of intelligence they do govern its expression. A higher intelligence could be a signal that an individual comes from and lives in a physical and social environment where nutrition levels are high, whereas a lower intelligence could imply a child (and/or the child's mother) comes from a physical and social environment where nutritional levels are low. Previc^[23] emphasizes the contribution of nutritional factors, especially meat and shellfish consumption, to elevations of dopaminergic activity in the brain, which may have been responsible for the evolution of human intelligence since dopamine is crucial to working memory, cognitive shifting, abstract, distant concepts, and other hallmarks of advanced intelligence.

Flexible problem solving

The statement that such high intelligence "lack survival value", which is used by believers in social intelligence and sexual selection, invariably assumes a stable environment. If climate change is factored in, however, the evolution of human intelligence can be perfectly explained by flexible problem solving during those climate changes.^{[citation needed]} For example, solving the problem of catching fish when freshwater streams freeze and the only available fish are in nearby unfrozen seawater. However, such a theory must also account for the apparent uniqueness of human levels of intelligence, which can be explained by free hands that allows for efficient manipulation of the environment such as carrying things and inventing and using handmade tools, which gives intelligence a practical survival value that can be selected by evolution.^[24]