History of Solar System formation and evolution hypotheses

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The most widely accepted theory of planetary formation and the genesis of the Solar System 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.

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.

Alternative Hypotheses on the Solar System’s Evolution

Laplace’s widely-accepted Nebular Hypothesis wasn’t the only theory on the formation of our Solar System - there were many more, some of which are still accepted by fringe theorists. Other scientists devised their own versions of the Nebular Hypothesis.

Fred Hoyle’s version of the Nebular Hypothesis

Fred Hoyle, in 1944 and 1945, argued that the star (nicknamed Orpheus in this document) whose supernova birthed the solar nebula was the Sun’s former binary companion, with the Sun being separate from the supernova remnant (which it captured) and as old as the star itself (which, as we now know, is not). The high temperatures of the supernova allowed more nuclear fusion after the star’s death and the force of the explosion threw the star’s degenerate out of the Sun’s gravitational field altogether. The supernova may have also birthed the Local Bubble, in which the Sun resides, and Geminga may well be the remnant star.

It is now known that the Sun itself was also birthed from the star’s supernova remnant and is roughly the same age as the planets that formed.

W. H. McCrea’s version of the Nebular Hypothesis

W. H. McCrea’s version of the nebular hypothesis states that the planets also contracted from the Solar Nebula by themselves (like sibling stars), before being individually captured by the Sun. Collapsing segments of gas formed the Sun and others formed the planets; the ones that formed the planets underwent fission to form the current planets. The moons of the gas giants formed from “droplets” in the neck from which they fissioned off; this is also the origin of the asteroids. Terrestrial planets would have no major satellites. Mercury and Venus fissioned off from each other, while the Earth, the Moon, and Mars fissioned off from the same parent body. In this scheme there are six principal planets: two terrestrial, Venus and Earth, two major, Jupiter and Saturn, and two outer, Uranus and Neptune, along with the dwarf planets, which do not only include Pluto, but also the Moon, Mercury, and Mars.

There are several problems with his version of the Nebular Hypothesis; for example, all of the planets revolve around the Sun in the same direction in orbits with relatively low eccentricity, which would be highly unlikely if they were all individually captured.

Gerald Kuiper and Otto Schmidt’s version of the Nebular Hypothesis

Gerald Kuiper argued that the solar nebula, from which the protoplanetary disk and thus the planets formed, could either be co-genetic with the Sun or captured by it. Soviet astronomer Otto Schmidt was a proponent for the solar nebula originally being a part of the interstellar medium that the Sun (in its current state) passed through and captured to form the planets from. Its density distribution at the time could determine on what may form: either an entire planetary system or another G-type main-sequence star.

The major problem against the protoplanetary disk originating from the interstellar medium and being captured by the Sun is that the required time for planets to condense from the interstellar medium far exceeds the calculated age of the universe.

Harold Urey’s version of planetary accretion

American chemist Harold Urey put forward his own version for accretion and collisions between protoplanets in the 1950s where in order to retain volatile elements, the protoplanets (some of which were pure carbon, moon-sized, or gas spheres), would have to form within the protoplanetary disk and dissipate at a later stage, all while being separated from the Sun by a moderately thick and dusty halo. Pressure fell as gas was lost and the diamonds became graphite, while the gas became illuminated by the Sun. Under these conditions ionization would be present and the gas could be accelerated by magnetic fields, thus the angular momentum could be transferred from the Sun to the planets. Finally, the lunar-sized protoplanets would be destroyed by collisions with each other, with their gases dissipating, with solids being left at their cores and the smaller fragments escaping into space, while the larger fragments remained and accreted to form the planets. Urey suggested that the Moon is nothing more than an exposed planetary core.

The dust on the Moon’s surface was once estimated to be six billion years old - older than Earth - however new analysis suggests it is around the same age as Earth - 4.51 billion years. Planetary cores are made of iron and nickel due to density and buoyancy. The Moon is made of granite and silicate rocks - no way it is a planet’s exposed core.

Herndon’s version of planetary accretion

American scientist James Herndon’s model proposes that the inner planets were originally the cores of former gas giants that formed from condensation of material within them. Earth was originally the core of a Neptune-like ice giant originally 300 times more massive than the Earth today whose mass compressed its core (Earth) to a diameter only two-thirds of today. These gas giants would not last long because the Sun, when it was in its T Tauri phase, stripped the gases and ices away from the planets, leaving behind only their rocky cores. Mercury did not form fully from its parent gas giant and the Sun’s eruptions stripped part of its gases away, to the region between Mars and Jupiter. There Mercury’s gas fused with oxidized condensate from the outer Solar System, solidifying to form meteorites, the Asteroid Belt, and the iron oxide coating of Mars. Differences between the inner planets depend on how much they were compressed when they were formerly gas giant cores.

Encounter hypothesis

The encounter hypothesis states that the Solar System’s planets were formed when a star passed by the Sun around 5 billion years ago. Hot gas was stripped off both stars, which cooled and contracted to form the planets. The other star’s material became Jupiter, Saturn, Uranus, and Neptune while the material from the Sun became Mercury, Venus, Earth, and Mars. The earliest hypothesis on how the Solar System was formed, it has the advantage of explaining why the planets orbit and rotate (except for Venus) in the same direction, why the Sun has the least angular momentum in the Solar System, and explains why the terrestrial planets are denser than the gas and ice giants.

The major problem with this theory the probability of it happening: such an encounter is very, very, very unlikely (much more than you think) and it is expected that such an event has never occurred in the universe yet.

Chamberlin-Moulton planetesimal hypothesis

This hypothesis proposed in 1905 by Thomas Chamberlin and Forest Moulton is nothing more than a cross between the accepted Nebular Hypothesis and the Encounter Hypothesis. Put simply, after the intruding star’s gravity pulls matter out of the Sun (also with the help of the mechanism behind solar prominences), the matter begins to flatten into a rotating disk around the Sun - the protoplanetary disk - from which planetesimals form, which in turn accrete the remaining material, forming the planets.

Again, the major problem against this hypothesis is the unlikelihood of the stellar encounter, although the protoplanetary disk accreting into planets is still widely accepted.

Lyttleton’s scenario

Ray Lyttleton proposed a scenario in which a companion star to the Sun collided with a passing star. Henry Russell already rejected such a scenario (though it may have been more likely considering that the Sun may have been born in an open star cluster). However, Lyttleton continued saying that the merged star later split into two because of rotational instability, forming Jupiter and Saturn, with the filament forming from the “neck” where the two broke off forming the other planets. A later model from 1940 involves the Sun in a triple star system, in which the Sun’s companions merge but later break up due to rotational instability, with the filament forming from the “neck” between them being captured by the Sun to form all the planets. The stars (blue stragglers) then leave the system.

Lyttleton’s idea of a companion star to the Sun colliding with another star has recently been proven correct with finds of a neutron star collision near the Sun around the time when the Solar System was born (see above). The other half on planetary formation has been refuted.

Band structure model

In the mid-1900’s, Swedish astrophysicist Hannes Alfven proposed the band structure model in which he proposed the existance of four molecular clouds by the Sun:

  • The A cloud, which contained mostly helium and iron
  • The B cloud, mostly hydrogen
  • The C cloud, mostly carbon
  • The D cloud, mostly silicon and iron.

The A cloud condenses to form Mars and the Moon, the B cloud condenses to form Jupiter, Saturn, Uranus, and Neptune, the C cloud condenses to form Mercury, Venus, and the Earth (or the B cloud forms Mercury, Venus, and Earth while the C cloud forms the outer planets), while the D cloud collapses to form Pluto and Triton. Just like how a prevailing theory states that the Asteroid and Kuiper Belts never formed a large planet in the first place, in his theory it is probable that the two are the remnants of the C and D clouds respectively.

Fission theories

Greek philosopher Anaxagoras and French mathematician Rene Descartes believed that various celestial bodies are fissioned off their parent bodies. This “fissioning off” is due to centrifugal forces results from the body’s rapid spin. Bulges appear on the equator, which spin off and become independent.

George Darwin, son of Charles Darwin, advocated that the Moon was once part of the Earth before the rapidly spinning proto-Earth’s centrifugal forces spun it off. The Pacific Ocean’s basin is supposedly the hole left by the Moon’s separation from Earth.

Swiss astronomer Louis Jacot, in the 1900s, jumped on the idea’s bandwagon, believing that moons are fissioned off their parent planets, which in turn are fissioned off their parent stars, which in turn are fissioned off from the centers of their galaxies due to the expanding universe. He claimed that due to this, the orbits of planets around the Sun are growing spirals, not circular orbits or ellipses, and that moons are moving away from their planets (which is true in the Earth’s Moon’s case). Jacot stated that planets were expelled, one at a time, from the Sun - the furthest to the Sun currently being the first expelled, with the closest being the last - and that one of them - Phaeton - shattered in its expulsion leaving the asteroid belt. Moons start off as centrifugal expulsions from their parent planets - some shatter of course, leaving the rings - and Earth is eventually going to expel another moon (which would be disastrous for life on Earth today).

The differences between the inner and outer planets can be explained by vortex behavior and expulsion order. Mercury’s relatively eccentric orbit is explained by its recent expulsion and Venus’s slow and reversed rotation due to having been expelled second to last.

Fringe astronomer Tom Van Flandern believed that the Sun’s fast rotation caused it to expel six pairs of twin planets, or twelve planets in all:

  • Venus
  • Earth
  • Maldek
  • Planet K
  • Planet LHB-A
  • Jupiter
  • Planet LHB-B
  • Saturn
  • Uranus
  • Neptune
  • Planet T
  • Planet X

Venus was twinned with Earth, Maldek with Planet K, Planet LHB-A with Jupiter, Planet LHB-B with Saturn, Uranus with Neptune, and Planet T with Planet X. Six planets have exploded so far, and these were helium-dominated or gas giants. Planet T exploded to form the Kuiper Belt and Planet X exploded to form the Oort cloud and its comets. For the two pairs of Planet LHB-A and Jupiter and Planet LHB-B and Saturn, the smaller and inner partner in each pair was subject to enormous tidal stresses from the Sun and other planets, causing them to blow up before they could fission off moons. Planets LHB-A and LHB-B exploded, causing the Late Heavy Bombardment. As they had no moons and were gas giants they left no trace of themselves. Twins Planet K and Maldek exploded or collided to form the asteroid belt and short-period comets. Solid planets fission off only one moon and Mercury was the moon of Venus but drifted away because of the Sun’s gravity. Mars was the moon of Maldek.

There are many problems with these fission theories. For one, the Moon (which consists mostly of granite and silicates) is not made of oceanic crust (which consists mostly of basalt), but rather material from the proto-Earth’s mantle before Theia’s material “contaminated it”. The Pacific Ocean’s floor is only 200 million years old, whereas the Moon is much older. One major argument against exploding planets and moons is that there would not be an energy source powerful enough to cause such explosions.

Capture theory

Proposed by Michael Mark Woolfson in 1964, this theory proposes that the Sun’s gravity and tidal forces completely ripped apart a nearby forming low-density protostar with a diameter 4,000 times greater than the Sun today (the size of Pluto’s orbit). Since the Sun is denser than the protostar and much smaller (more compact), it had a much greater gravitational pull than the protostar. The Sun’s gravity ripped off the protostar’s diffuse material, which condensed to form the planets.

Woolfson added the concept of a collision in a later version of the hypothesis. As the protostar is ripped apart by the Sun, its material originally forms seven planets, in order:

  • “A” (nicknamed “Enyo” after the Greek goddess of war, the sister of Ares)
  • “B” (nicknamed “Bellona” after the Roman equivalent of Enyo; Mars’s sister)
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

The orbits of Enyo and Bellona are eccentric, and as a result, they collide with a force powerful enough to briefly cause deuterium-deuterium nuclear reactions. Enyo, with a mass twice that of Neptune, is ejected out of the Solar System, while Bellona, a third of the mass of Uranus, is shattered by the nuclear reactions, forming the Earth, Venus, Mercury, asteroid belt, and comets. Mars, the Moon, Pluto, Haumea, Makemake, Eris, and V774104 (2015 TH367) are the former satellites of Enyo.

There are a few problems with this theory; for example, it again suggests that the Sun and the planets are not the same age. However, it has been accepted that planets like “Enyo” have been ejected from the Solar System and that Triton was captured by Neptune.