History of Solar System formation and evolution hypotheses
The history of scientific thought about the formation and evolution of the Solar System began with the Copernican Revolution. The first recorded use of the term "Solar System" dates from 1704. Since the seventeenth century, philosophers and scientists have been forming hypotheses concerning the origins of the Solar System and the Moon and attempting to predict how the Solar System would change in the future. René Descartes was the first to hypothesize on the beginning of the Solar System; however, more scientists joined the discussion in the eighteenth century, forming the groundwork for later hypotheses on the topic. Later, particularly in the twentieth century, a variety of hypotheses began to build up, including the now–commonly accepted nebular hypothesis.
Meanwhile, hypotheses explaining the evolution of the Sun originated in the nineteenth century, especially as scientists began to understand how stars in general functioned. In contrast, hypotheses attempting to explain the origin of the Moon have been circulating for centuries, although all of the widely accepted hypotheses were proven false by the Apollo missions in the mid-twentieth century. Following Apollo, in 1984, the giant impact hypothesis was composed, replacing the already-disproven binary accretion model as the most common explanation for the formation of the Moon.
Contemporary view
The most widely accepted model of planetary formation is known as the nebular hypothesis. This model posits that, 4.6billion years ago, the Solar System was formed by the gravitational collapse of a giant molecular cloud spanning several light-years. Many stars, including the Sun, were formed within this collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass concentrated in the center, forming the Sun, and the rest of the mass flattened into a protoplanetary disk, out of which all of the current planets, moons, asteroids, and other celestial bodies in the Solar System formed.Formation hypothesis
French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his book The World, written from 1629 to 1633. In his view, the universe was filled with vortices of swirling particles, and both the Sun and planets had condensed from a large vortex that had contracted, which he thought could explain the circular motion of the planets. However, this was before the knowledge of Newton's theory of gravity, which explains that matter does not behave in this way.The vortex model of 1944, formulated by the German physicist and philosopher Carl Friedrich von Weizsäcker, hearkens back to the Cartesian model by involving a pattern of turbulence-induced eddies in a Laplacian nebular disc. In Weizsäcker's model, a combination of the clockwise rotation of each vortex and the anti-clockwise rotation of the whole system could lead to individual elements moving around the central mass in Keplerian orbits, reducing energy dissipation due to overall motion. However, material would be colliding at a high relative velocity in the inter-vortex boundaries and, in these regions, small roller-bearing eddies would coalesce to give annular condensations. This hypothesis was much criticized, as turbulence is a phenomenon associated with disorder and would not spontaneously produce the highly ordered structure required by the hypothesis. It also does not provide a solution to the angular momentum problem or explain lunar formation and other very basic characteristics of the Solar System.
This model was modified in 1948 by Dutch theoretical physicist Dirk Ter Haar, who hypothesized that regular eddies were discarded and replaced by random turbulence, which would lead to a very thick nebula where gravitational instability would not occur. He concluded the planets must have formed by accretion, and explained the compositional difference between the planets as resulting from the temperature difference between the inner and outer regions, the former being hotter and the latter being cooler, so only refractories condensed in the inner region. A major difficulty was that, in this supposition, turbulent dissipation took place over the course of a single millennium, which did not give enough time for planets to form.
The nebular hypothesis was first proposed in 1734 by Swedish scientist Emanuel Swedenborg and later expanded upon by Prussian philosopher Immanuel Kant in 1755. A similar hypothesis was independently formulated by the Frenchman Pierre-Simon Laplace in 1796.
In 1749, Georges-Louis Leclerc, Comte de Buffon conceived the idea that the planets were formed when a comet collided with the Sun, sending matter out to form the planets. However, Pierre-Simon Laplace refuted this idea in 1796, stating that any planets formed in such a way would eventually crash into the Sun. Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation. Today, comets are known to be far too small to have created the Solar System in this way.
In 1755, Immanuel Kant speculated that observed nebulae could be regions of star and planet formation. In 1796, Laplace elaborated by arguing that the nebula collapsed into a star, and, as it did so, the remaining material gradually spun outward into a flat disc, which then formed planets.
Alternative hypotheses
However plausible it may appear at first sight, the nebular hypothesis still faces the obstacle of angular momentum; if the Sun had indeed formed from the collapse of such a cloud, the planets should be rotating far more slowly. The Sun, though it contains almost 99.9 percent of the system's mass, contains just 1 percent of its angular momentum, meaning that the Sun should be spinning much more rapidly.Tidal hypothesis
Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favor of a return to "two-body" hypothesis. For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the approach of some other star to the Sun ultimately formed the solar system. 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. In 1929, astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. American astronomer Henry Norris Russell also objected to the hypothesis by showing that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun.Chamberlin–Moulton model
In 1900, Forest Moulton showed that the nebular hypothesis was inconsistent with observations because of the angular momentum. Moulton and Chamberlin in 1904 originated the planetesimal hypothesis. Along with many astronomers of the time, they came to believe the pictures of "spiral nebulas" from the Lick Observatory were direct evidence of the formation of planetary systems, which later turned out to be galaxies.Moulton and Chamberlin suggested that a star had passed close to the Sun early in its life, causing tidal bulges, and that this, along with the internal process that leads to solar prominences, resulted in the ejection of filaments of matter from both stars. While most of the material would have fallen back, part of it would remain in orbit. The filaments cooled into numerous, tiny, solid planetesimals and a few larger protoplanets. This model received favorable support for about 3 decades, but passed out of favor by the late '30s and was discarded in the '40s due to the realization it was incompatible with the angular momentum of Jupiter. A part of the hypothesis, planetesimal accretion, was retained.
Lyttleton's scenario
In 1937 and 1940, Raymond Lyttleton postulated that a companion star to the Sun collided with a passing star. Such a scenario had already been suggested and rejected by Henry Russell in 1935, though it may have been more likely assuming the Sun was born in an open cluster, where stellar collisions are common. Lyttleton showed that terrestrial planets were too small to condense on their own and suggested that one very large proto-planet broke in two because of rotational instability, forming Jupiter and Saturn, with a connecting filament from which the other planets formed. A later model, from 1940 and 1941, involved a triple star system, a binary plus the Sun, in which the binary merged and later split because of rotational instability and escaped from the system, leaving a filament that formed between them to be captured by the Sun. Objections of Lyman Spitzer apply to this model also.Band-structure model
In 1954, 1975, and 1978, Swedish astrophysicist Hannes Alfvén included electromagnetic effects in equations of particle motions, and angular momentum distribution and compositional differences were explained. In 1954, he first proposed the band structure, in which he distinguished an A-cloud, containing mostly helium with some solid-particle impurities, a B-cloud with mostly carbon, a C-cloud having mainly hydrogen, and a D-cloud made mainly of silicon and iron. Impurities in the A-cloud formed Mars and the Moon, impurities in the B-cloud collapsed to form the outer planets, the C-cloud condensed into Mercury, Venus, Earth, the asteroid belt, moons of Jupiter, and Saturn's rings, while Pluto, Triton, the outer satellites of Saturn, the moons of Uranus, the Kuiper Belt, and the Oort cloud formed from the D-cloud.Interstellar cloud hypothesis
In 1943, Soviet astronomer Otto Schmidt proposed that the Sun, in its present form, passed through a dense interstellar cloud and emerged 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. Extensions of the model, together forming the Russian school, include Gurevich and Lebedinsky in 1950, Safronov in 1967 and 1969, Ruskol in 1981 Safronov and Vityazeff in 1985, and Safronov and Ruskol in 1994, among others 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.Ray Lyttleton modified the hypothesis by showing that a third body was not necessary and proposing that a mechanism of line accretion, as described by Bondi and Hoyle in 1944, enabled cloud material to be captured by the star.