用户:PhiLiP/太阳系的形成与演化假说史
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/Pierre-Simon_Laplace.jpg/250px-Pierre-Simon_Laplace.jpg)
从已知最早的文献开始,人类便一直在探索着世界的起源与结局;然而,在从古至今的绝大多数时间里,人们都没有试图让这样的理论与“太阳系”的存在产生联系,只因为今日我们所理解的太阳系的存在,在当时几乎没有人知晓或相信。迈向太阳系的形成与演化理论的第一步,是对日心说的广泛认同:这一模型将太阳放置在了整个系统的中心,而地球则在轨道上围绕之作转动。这一观念曾酝酿有千年之久,但直到17世纪末才为公众所广泛接受。“太阳系”一词有记录以来最早的一次使用,出现在1704年。[1]
现今被接受的假说
[编辑]![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/71/Protoplanetary-disk.jpg/200px-Protoplanetary-disk.jpg)
星云假说是至今以来最为人们所广泛接受的行星形成理论。这一假说认为,太阳系形成于46亿年前的一个横跨数光年的巨大分子云的引力坍缩。分子云的坍缩造就了几颗恒星,其中包括有太阳。构成太阳系的所有气体的质量要略低于太阳本身。绝大多数的质量都被太阳系核心所吸收,并从而形成了太阳;剩余的质量则形成了扁平的原行星盘,并最后形成了太阳系的各大行星与其他天体。
就像太阳与行星的诞生那样,他们总有一天会走向死亡。随着太阳年岁的增长,它将逐渐冷却,并膨胀成为一颗直径大过现在许多倍的红巨星,其后,它将会抛弃其外壳(并形成行星状星云),成为恒星末期阶段的白矮星。太阳系的行星将跟随太阳演化的过程而变化;一部分将会被毁灭,其余的则会被抛入星际空间;但是,经过足够的时间后,围绕太阳旋转的天体最终都将不复存在。
形成假说
[编辑]1734年,Emanuel Swedenborg第一次提出了星云假说[2],并由Immanuel Kant在1755年加以详述与扩展。1796年,Pierre-Simon Laplace亦独立地提出了相似的理论。[3]
1749年,Georges-Louis Leclerc, Comte de Buffon提出了行星形成的一种观点:他认为,太阳曾与一颗彗星发生过碰撞,碰撞中散射出了形成行星的物质。然而,Laplace于1796年证伪了这一观点,并得出结论认为,以该种方式形成的行星,最终都将会坠落到太阳表面。在今日,通常都认为,通过此种方式创造出太阳系所需的质量远大于彗星的质量。[4]Laplace认为,行星近圆形的轨道是得出其形成原因所必需要考虑的因素。[4]
1755年,Immanuel Kant推测,所观测到的星云有可能实际上是恒星与行星形成的场所。1796年,Laplace详细地描述到,星云会坍塌成为恒星,随后,剩下的物质会逐渐地向外部扩展形成一个扁平的盘,并最后形成行星。[4]
其他的理论
[编辑]星云假说首要面临的障碍是角动量的问题:如果太阳的确是通过星云的坍塌而形成,那么行星围绕它转动的速度将远比现实中的慢。而实际上在整个系统中,尽管太阳有着几乎99%的质量,但却只占有1%的角动量。[5]
Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories.[4] For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets.[4]</ref> However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely.[4] Objections to the hypothesis were also raised by the American astronomer Henry Norris Russell, who showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun.[6]
In 1944, the Soviet astronomer Otto Schmidt proposed that the Sun, in its present form, passed through a dense interstellar cloud, emerging enveloped in a cloud of dust and gas, from which the planets eventually formed. This solved the angular momentum problem by assuming that the Sun's slow rotation was peculiar to it, and that the planets did not form at the same time as the Sun.[4] However, this hypothesis was severely dented by Victor Safronov who showed that the amount of time required to form the planets from such a diffuse envelope would far exceed the Solar System's determined age.[4]
In 1960, W. H. McCrea proposed the protoplanet theory, in which the Sun and planets individually coalesced from matter within the same cloud, with the smaller planets later captured by the Sun's larger gravity.[4] This theory has a number of issues, such as explaining the fact that the planets all orbit the Sun in the same direction, which would appear highly unlikely if they were each individually captured.[4]
The capture theory, proposed by M. M. Woolfson in 1964, posits that the Solar System formed from tidal interactions between the Sun and a low-density protostar. The Sun's gravity would have drawn material from the diffuse atmosphere of the protostar, which would then have collapsed to form the planets.[7] However, the capture theory predicts a different age for the Sun than for the planets,[来源请求] whereas the similar ages of the Sun and the rest of the Solar System indicate that they formed at roughly the same time.[8]
Reemergence of the nebular hypothesis
[编辑]![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/HST_betaPictoris_comb.jpg/200px-HST_betaPictoris_comb.jpg)
In 1978, astronomer A. J. R. Prentice revived the Laplacian nebular model in his Modern Laplacian Theory by suggesting that the angular momentum problem could be resolved by drag created by dust grains in the original disc which slowed down the rotation in the centre.[4][9] Prentice also suggested that the young Sun transferred some angular momentum to the protoplanetary disc and planetesimals through supersonic ejections understood to occur in T Tauri stars.[4][10] However, his contention that such formation would occur in toruses or rings has been questioned, as any such rings would disperse before collapsing into planets.[4]
The birth of the modern widely accepted theory of planetary formation—the Solar Nebular Disk Model (SNDM)—can be traced to the works of Soviet astronomer Victor Safronov.[11] His book Evolution of the protoplanetary cloud and formation of the Earth and the planets,[12] which was translated to English in 1972, had a long-lasting effect on the way scientists thought about the formation of the planets.[13] In this book almost all major problems of the planetary formation process were formulated and some of them solved. The Safronov's ideas were further developed in the works of George Wetherill, who discovered runaway accretion.[4] By the early 1980s, the nebular hypothesis in the form of SNDM had come back into favour, led by two major discoveries in astronomy. First, a number of apparently young stars, such as Beta Pictoris, were found to be surrounded by discs of cool dust, much as was predicted by the nebular hypothesis. Second, the Infrared Astronomical Satellite, launched in 1983, observed that many stars had an excess of infrared radiation that could be explained if they were orbited by discs of cooler material. Most astronomers today believe that the Sun's loss of angular momentum can be explained by loss of its matter to space in a phenomenon similar to the solar wind.[11]
Outstanding issues
[编辑]While the broad picture of the nebular hypothesis is widely accepted,[14] many of the details are not well understood and continue to be refined.
The refined nebular model was developed entirely on the basis of observations of our own Solar System because it was the only one known until the mid 1990s. It was not confidently assumed to be widely applicable to other planetary systems, although scientists were anxious to test the nebular model by finding of protoplanetary discs or even planets around other stars.[15] As of March 2008, the discovery of nearly 280 extrasolar planets[16] has turned up many surprises, and the nebular model must be revised to account for these discovered planetary systems, or new models considered.
Among the extrasolar planets discovered to date are planets the size of Jupiter or larger but possessing very short orbital periods of only a few days. Such planets would have to orbit very closely to their stars; so closely that their atmospheres would be gradually stripped away by solar radiation.[17][18] There is no consensus on how to explain these so-called hot Jupiters, but one leading idea is that of planetary migration, similar to the process which is thought to have moved Uranus and Neptune to their current, distant orbit. Possible processes that cause the migration include orbital friction while the protoplanetary disc is still full of hydrogen and helium gas[19] and exchange of angular momentum between giant planets and the particles in the protoplanetary disc.[20][21][22]
The detailed features of the planets are another problem. The solar nebula hypothesis predicts that all planets will form exactly in the ecliptic plane. Instead, the orbits of the classical planets have various (but small) inclinations with respect to the ecliptic. Furthermore, for the gas giants it is predicted that their rotations and moon systems will also not be inclined with respect to the ecliptic plane. However most gas giants have substantial axial tilts with respect to the ecliptic, with Uranus having a 98° tilt.[23] The Moon being relatively large with respect to the Earth and other moons which are in irregular orbits with respect to their planet is yet another issue. It is now believed these observations are explained by events which happened after the initial formation of the Solar System.[24] For instance, the large axial tilts of many planets may be explained by giant impacts with massive planetesimals early in the Solar System's evolution.[25]
Solar evolution hypotheses
[编辑]Attempts to isolate the physical source of the Sun's energy, and thus determine when and how it might ultimately run out, began in the 19th century. At that time, the prevailing scientific view on the source of the Sun's heat was that it was generated by gravitational contraction. In the 1840s, astronomers J. R. Mayer and J. J. Waterson first proposed that the Sun's massive weight causes it to collapse in on itself, generating heat, an idea expounded upon in 1854 by both Hermann von Helmholtz and Lord Kelvin, who further elaborated on the idea by suggesting that heat may also be produced by the impact of meteors onto the Sun's surface.[26] However, the Sun only has enough gravitational potential energy to power its luminosity by this mechanism for about 30 million years—far less than the age of the Earth. (This collapse time is known as the Kelvin-Helmholtz timescale.)[27]
Albert Einstein's development of the theory of relativity in 1905 led to the understanding that nuclear reactions could create new elements from smaller precursors, with the loss of energy. In his treatise Stars and Atoms, Arthur Eddington suggested that pressures and temperatures within stars were great enough for hydrogen nucei to fuse into helium; a process which could produce the massive amounts of energy required to power the Sun.[26] In 1935, Eddington went further and suggested that other elements might also form within stars.[28] Spectral evidence collected after 1945 showed that the distribution of the commonest chemical elements, carbon, hydrogen, oxygen, nitrogen, neon, iron etc, was fairly uniform across the galaxy. This suggested that these elements had a common origin.[28] A number of anomalies in the proportions hinted at an underlying mechanism for creation. Lead is heavier than gold, but far more common. Hydrogen and helium (elements 1 and 2) are virtually ubiquitous yet lithium and beryllium (elements 3 and 4) are extremely rare.[28]
Red giants
[编辑]While the unusual spectra of red giant stars had been known since the 19th century,[29] it was George Gamow who, in the 1940s, first understood that they were stars of roughly solar mass that had run out of hydrogen in their cores and had resorted to burning the hydrogen in their outer shells.[来源请求] This allowed Martin Schwarzschild to drew the connection between red giants and the finite lifespans of stars. It is now understood that red giants are stars in the last stages of their life cycles.
Fred Hoyle noted that, even while the distribution of elements was fairly uniform, different stars had varying amounts of each element. To Hoyle, this indicated that they must have originated within the stars themselves. The abundance of elements peaked around the atomic number for iron, an element that could only have been formed under intense pressures and temperatures. Hoyle concluded that iron must have formed within giant stars.[28] From this, in 1945 and 1946, Hoyle constructed the final stages of a star's life cycle. As the star dies, it collapses under its own weight, leading to a stratified chain of fusion reactions: carbon-12 fuses with helium to form oxygen-16; oxygen-16 fuses with helium to produce neon-20, and so on up to iron.[30] There was, however, no known method by which carbon-12 could be produced. Isotopes of beryllium produced via fusion were too unstable to form carbon, and for three helium atoms to form carbon-12 was so unlikely as to have been impossible over the age of the universe. However, in 1952 the physicist Ed Salpeter showed that a short enough time existed between the formation and the decay of the beryllium isotope that another helium had a small chance to form carbon, but only if their combined mass/energy amounts were equal to that of carbon-12. Hoyle, employing the anthropic principle, showed that it must be so, since he himself was made of carbon, and he existed. When the matter/energy level of carbon-12 was finally determined, it was found to be within a few percent of Hoyle's prediction.[31]
White dwarfs
[编辑]The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C. The pair 40 Eridani B/C was discovered by Friedrich Wilhelm Herschel on January 31, 1783;[32], p. 73 it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851.[33][34] In 1910, it was discovered by Henry Norris Russell, Edward Charles Pickering and Williamina Fleming that despite being a dim star, 40 Eridani B was of spectral type A, or white.[35]
White dwarfs were found to be extremely dense soon after their discovery. If a star is in a binary system, as is the case for Sirius B and 40 Eridani B, it is possible to estimate its mass from observations of the binary orbit. This was done for Sirius B by 1910,[36] yielding a mass estimate of 0.94 solar mass. (A more modern estimate is 1.00 solar mass.)[37] Since hotter bodies radiate more than colder ones, a star's surface brightness can be estimated from its effective surface temperature, and hence from its spectrum. If the star's distance is known, its overall luminosity can also be estimated. Comparison of the two figures yields the star's radius. Reasoning of this sort led to the realization, puzzling to astronomers at the time, that Sirius B and 40 Eridani B must be very dense. For example, when Ernst Öpik estimated the density of a number of visual binary stars in 1916, he found that 40 Eridani B had a density of over 25,000 times the Sun's, which was so high that he called it "impossible".[38]
Such densities are possible because white dwarf material is not composed of atoms bound by chemical bonds, but rather consists of a plasma of unbound nuclei and electrons. There is therefore no obstacle to placing nuclei closer to each other than electron orbitals—the regions occupied by electrons bound to an atom—would normally allow.[39] Eddington, however, wondered what would happen when this plasma cooled and the energy which kept the atoms ionized was no longer present.[40] This paradox was resolved by R. H. Fowler in 1926 by an application of the newly devised quantum mechanics. Since electrons obey the Pauli exclusion principle, no two electrons can occupy the same state, and they must obey Fermi-Dirac statistics, also introduced in 1926 to determine the statistical distribution of particles which satisfy the Pauli exclusion principle.[41] At zero temperature, therefore, electrons could not all occupy the lowest-energy, or ground, state; some of them had to occupy higher-energy states, forming a band of lowest-available energy states, the Fermi sea. This state of the electrons, called degenerate, meant that a white dwarf could cool to zero temperature and still possess high energy.
Planetary nebulae
[编辑]![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/NGC7293_%282004%29.jpg/200px-NGC7293_%282004%29.jpg)
Planetary nebulae are generally faint objects, and none are visible to the naked eye. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae somewhat resembled the gas giants, and William Herschel, discoverer of Uranus, eventually coined the term 'planetary nebula' for them, although, as we now know, they are very different from planets.
The central stars of planetary nebulae are very hot. Their luminosity, though, is very low, implying that they must be very small. Only once a star has exhausted all its nuclear fuel can it collapse to such a small size, and so planetary nebulae came to be understood as a final stage of stellar evolution. Spectroscopic observations show that all planetary nebulae are expanding, and so the idea arose that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.
Lunar origins hypotheses
[编辑]![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/dc/George_Darwin_sepia_tone.jpg/150px-George_Darwin_sepia_tone.jpg)
Over the centuries, many scientific hypotheses have been advanced concerning the origin of Earth's Moon. One of the earliest was the so-called binary accretion model, which concluded that the Moon accreted from material in orbit around the Earth left over from its formation. Another, the fission model, was developed by George Darwin, (son of Charles Darwin) who noted that, as the Moon is gradually receding from the Earth at a rate of about 4 cm per year, so at one point in the distant past it must have been part of the Earth, but was flung outward by the momentum of Earth's then–much faster rotation. This hypothesis is also supported by the fact that the Moon's density, while less than Earth's, is about equal to that of Earth's rocky mantle, suggesting that, unlike the Earth, it lacks a dense iron core. A third hypothesis, known as the capture model, suggested that the Moon was an independently orbiting body that had been snared into orbit by Earth's gravity.[42]
Apollo missions
[编辑]However, these hypotheses were all refuted by the Apollo lunar missions, which introduced a stream of new scientific evidence; specifically concerning the Moon's composition, its age, and its history. These lines of evidence contradict many predictions made by these earlier models.[42] The rocks brought back from the Moon showed a marked decrease in water relative to rocks elsewhere in the Solar System, and also evidence of an ocean of magma early in its history, indicating that its formation must have produced a great deal of energy. Also, oxygen isotopes in lunar rocks showed a marked similarity to those on Earth, suggesting that they formed at a similar location in the solar nebula. The capture model fails to explain the similarity in these isotopes (if the Moon had originated in another part of the Solar System, those isotopes would have been different), while the co-accretion model cannot adequately explain the loss of water (if the Moon formed in a similar fashion to the Earth, the amount of water trapped in its mineral structure would also be roughly similar). Conversely, the fission model, while it can account for the similarity in chemical composition and the lack of iron in the Moon, cannot adequately explain its high orbital inclination and, in particular, the large amount of angular momentum in the Earth/Moon system, more than any other planet–satellite pair in the Solar System.[42]
Giant impact hypothesis
[编辑]For many years after Apollo, the binary accretion model was settled on as the best hypothesis for explaining the Moon's origins, even though it was known to be flawed. Then, at a conference in Kona, Hawaii in 1984, a compromise model was composed that accounted for all of the observed discrepencies. Originally formulated by two independent research groups in 1976, the giant impact model supposed that a massive planetary object, the size of Mars, had collided with Earth early in its history. The impact would have melted Earth's crust, and the other planet's heavy core would have sunk inward and merged with Earth's. The superheated vapour produced by the impact would have risen into orbit around the planet, coalescing into the Moon. This explained the lack of water (the vapour cloud was too hot for water to condense), the similarity in composition (since the Moon had formed from part of the Earth), the lower density (since the Earth had formed from the Earth's crust and mantle, rather than its core, and the Moon's unusual orbit (since an oblique strike would have imparted a massive amount of angular momentum to the Earth/Moon system.[42]
Outstanding issues
[编辑]However, the giant impact model has been criticised for being too explanatory; it can be expanded to explain any future discoveries and as such, makes no predictions. Also, many claim that much of the material from the impactor would have ended up in the Moon, meaning that the isotope levels would be different, but they are not. Also, while water and many other volatile compounds are absent from the Moon's crust, many others, such as manganese, are not.[42]
Other natural satellites
[编辑]While the co-accretion and capture models are not currently accepted as valid explanations for the existence of the Moon, they have been employed to explain the formation of other natural satellites in the Solar System. Jupiter's Galilean satellites are believed to have formed via co-accretion,[43] while the Solar System's irregular satellites, such as Triton, are all believed to have been captured.[44]
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