Astronomy:History of Solar System formation and evolution hypotheses

Pierre-Simon Laplace, one of the originators of the nebular hypothesis

The history of scientific thought about the Formation and evolution of the Solar System begins with the Copernican Revolution. The first recorded use of the term "Solar System" dates from 1704.

Contemporary view

The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed.

There are, however, arguments against this hypothesis.

Formation hypothesis

French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his Le Monde (ou Traité de lumière) which he wrote in 1632 and 1633 and for which he delayed publication because of the Inquisition and it was published only after his death in 1664. In his view, the Universe was filled with vortices of swirling particles and the Sun and planets had condensed from a particularly large vortex that had somehow contracted, which explained the circular motion of the planets and was on the right track with condensation and contraction. However, this was before Newton's theory of gravity and we now know matter does not behave in this fashion.

Artist's conception of a protoplanetary disc

The vortex model of 1944, formulated by German physicist and philosopher Baron Carl Friedrich von Weizsäcker, which harkens back to the Cartesian model, involved a pattern of turbulence-induced eddies in a Laplacian nebular disc. In it a suitable combination of clockwise rotation of each vortex and anti-clockwise rotation of the whole system can lead to individual elements moving around the central mass in Keplerian orbits so there would be little dissipation of energy due to the overall motion of the system but material would be colliding at high relative velocity in the inter-vortex boundaries and in these regions small roller-bearing eddies would coalesce to give annular condensations. It was much criticized as turbulence is a phenomenon associated with disorder and would not spontaneously produce the highly ordered structure required by the hypothesis. As well, it does not provide a solution to the angular momentum problem and does not explain lunar formation nor other very basic characteristics of the Solar System.

The Weizsäcker model was modified in 1948 by Dutch theoretical physicist Dirk Ter Haar, in 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 (solid and liquid planets) as due to the temperature difference between the inner and outer regions, the former being hotter and the latter being cooler, so only refractories (non-volatiles) condensed in the inner region. A major difficulty is that in this supposition turbulent dissipation takes place in a time scale of only about a millennium which does not give enough time for planets to form.

The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg and later elaborated and expanded upon by Immanuel Kant in 1755. A similar theory was independently formulated by 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, Laplace refuted this idea in 1796, showing 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 may in fact 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 the planets.

Alternative theories

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. This means that the Sun should be spinning much more rapidly.

Tidal theory

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. 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. However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. 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.

The Chamberlin-Moulton model

Forest Moulton in 1900 had also shown that the nebular hypothesis was inconsistent with observations because of the angular momentum. Moulton and Chamberlin in 1904 originated the planetesimal hypothesis (see Chamberlin–Moulton planetesimal hypothesis). Along with many astronomers of the day they came to believe the pictures of "spiral nebulas" from the Lick Observatory were direct evidence of forming solar systems. These turned out to be galaxies instead but the Shapley-Curtis debate about these was still 16 years in the future. One of the most fundamental issues in the history of astronomy was distinguishing between nebulas and galaxies.

Moulton and Chamberlin suggested that a star had passed close to the Sun early in its life to cause 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 fragments, ‘planetesimals’, and a few larger protoplanets. This model received favourable support for about 3 decades but passed out of favour by the late '30s and was discarded in the '40s by the realization it was incompatible with the angular momentum of Jupiter, but a part of it, planetesimal accretion, was retained.

Lyttleton's scenario

In 1937 and 1940, Ray Lyttleton postulated that a companion star to the Sun collided with a passing star. Such a scenario was already suggested and rejected by Henry Russell in 1935. Lyttleton showed terrestrial planets were too small to condense on their own so suggested 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, involves a triple star system, a binary plus the Sun, in which the binary merges and later breaks up because of rotational instability and escapes 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, but with some solid- particle impurities ("meteor rain"), a B-cloud, with mostly hydrogen, a C-cloud, having mainly carbon, and a D-cloud, made mainly of silicon and iron. Impurities in the A-cloud form Mars and the Moon (later captured by Earth), in the B-cloud they condense into Mercury, Venus, and Earth, in the C-cloud they condense into the outer planets, and Pluto and Triton may have formed from the D-cloud.

Interstellar cloud theory

In 1943, 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. Extensions of the model, together forming the Russian school, include Gurevich and Lebedinsky (in 1950), Safronov (in 1967,1969), Safronov and Vityazeff (in 1985), Safronov and Ruskol (in 1994), and Ruskol (in 1981), 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 theory by showing that a 3rd body was not necessary and proposing that a mechanism of line accretion described by Bondi and Hoyle in 1944 would enable cloud material to be captured by the star (Williams and Cremin, 1968, loc. cit.)

Hoyle's hypothesis

In this model (from 1944) the companion went nova with ejected material captured by the Sun and planets forming from this material. In a version a year later it was a supernova. In 1955 he proposed a similar system to Laplace, and with more mathematical detail in 1960. It differs from Laplace in that a magnetic torque occurs between the disk and the Sun, which comes into effect immediately or else more and more matter would be ejected resulting in a much too massive planetary system, one comparable to the Sun. The torque causes a magnetic coupling and acts to transfer angular momentum from the Sun to the disk. The magnetic field strength would have to be 1 gauss. The existence of torque depends on magnetic lines of force being frozen into the disk (a consequence of a well-known MHD (magnetohydrodynamic) theorem on frozen-in lines of force). As the solar condensation temperature when the disk was ejected could not be much more than 1000 degrees K., a number of refractories must be solid, probably as fine smoke particles, which would grow with condensation and accretion. These particles would be swept out with the disk only if their diameter at the Earth's orbit was less than 1 meter so as the disk moved outward a subsidiary disk consisting of only refractories remains behind where the terrestrial planets would form. The model is in good agreement with the mass and composition of the planets and angular momentum distribution provided the magnetic coupling is an acceptable idea, but not explained are twinning, the low mass of Mars and Mercury, and the planetoid belts. It was Alfvén who formulated the concept of frozen-in magnetic field lines.

Kuiper's theory

Gerard Kuiper (in 1944) argued, like Ter Haar, that regular eddies would be impossible and postulated that large gravitational instabilities might occur in the solar nebula, forming condensations. In this, the solar nebula could be either co-genetic with the Sun or captured by it. Density distribution would determine what could form: either a planetary system or a stellar companion. The 2 types of planets were assumed to be due to the Roche limit. No explanation was offered for the Sun's slow rotation which Kuiper saw as a larger G-star problem.

Whipple's theory

In Fred Whipple's 1948 scenario a smoke cloud about 60,000 AU in diameter and with 1 solar mass (M_☉) contracts and produces the Sun. It has a negligible angular momentum thus accounting for the Sun's similar property. This smoke cloud captures a smaller one with a large angular momentum. The collapse time for the large smoke and gas nebula is about 100 million years and the rate is slow at first, increasing in later stages. The planets would condense from small clouds developed in, or captured by, the 2nd cloud, the orbits would be nearly circular because accretion would reduce eccentricity due to the influence of the resisting medium, orbital orientations would be similar because the small cloud was originally small and the motions would be in a common direction. The protoplanets might have heated up to such high degrees that the more volatile compounds would have been lost and the orbital velocity decreases with increasing distance so that the terrestrial planets would have been more affected. The weaknesses of this scenario are that practically all the final regularities are introduced as a priori assumptions and most of the hypothesizing was not supported by quantitative calculations. For these reasons it did not gain wide acceptance.