"The Moon is a crescent-shaped boat filling up with souls and transporting them to the sun."

-- Persian Sage, 200 AD

The recognition that stars could suddenly appear or explode and die, rather than all being permanent fixtures of the Heavens, was one of the profound philosophical wake-up calls of the Renaissance. No longer did the 'firmament' appear too firm. This recognition accelerated Human inquiry into the finite existence of our own star and the solar system about it, and systematic thought addressed the issue of how the Solar System formed.

Early hypothese that were advanced, primarily by Mathematicians and Physicists, include:

(1) Condensation: The idea that the Sun and planets cooled from a hot nebula, a cloud of dust and gas in the interstellar medium, with the spin of the nebula being important. Descartes (of the Cartesian System) advanced the original idea in 1644, while Laplace recognized the importance of spin in forming the system in 1796.

(2) Encounter: The idea that the star formed first, and then close passage of another star gravitationally 'ripped' some of the material out, which cooled in place to make the planets. This was originally advanced by de Buffon, 1785, and favored for many years, but is now thought to be untenable since the material in a star is so hot that once pulled into space it would vaporize and spread rather than consolidate into a planet.

Most current ideas involve

(3) Nebular Turbulence: A hybrid of nebular condensation ideas, modified to recognize the great heterogeneity and turbulence in a condensing spinning nebula. This is key to explaining why the planets have the angular momentum of the system.

A few of the Basic Observations that any hypothesis for Solar System Formation must explain:

1. The Planets are all in the Plane of the Ecliptic, and orbit the Sun (and for the most part rotate) in the same direction as the Sun' rotation. The orbits are almost circular.

2. The Planets have a regular spacing, with each planet having an orbital radius twice that of the Planet next nearest to the Sun. This is Bode's Law (Rn+1 = 2Rn)

3. The Planets differ systematically in composition with distance from the Sun, with the four terrestrial planets being much denser than the outer planets.

4. The Sun has 99.9% of the mass of the whole solar system. The Planets have 98% of the angular momentum of the system.

The common plane of rotation and the circular orbits strongly suggest in-situ formation of the planets simultaneous with formation of the Sun. The Angular Momentum observation is the most challenging to explain, as most nebular theories lead to the Sun spinning much faster as mass collapses into it. How could the angular momentum of the nebula be transferred out to the outer regions to offset this intrinsic effect (which is why Neutron stars spin so fast, for example)?

The Nebular Turbulence models are the current notion of how the Solar System formed: In essence the idea is that there was a nebular cloud of Gas and dust particles in a region of space, enriched by 2% in the heavier elements of the periodic chart. Probably many stars formed close together in time, perhaps with a supernova triggering localized gravitational collapse of regions of the cloud (there were many proto-stars).

Gravity drives the nebular collapse, but conservation of angular momentum cause the nebula to spin faster and faster as mass concentrates toward its center. The balance of gravity and rotation leads to a flattened disk. The central portions of the nebular disk begin to heat up do to increasing collisions of gas and dust particles, and there are turbulent eddies in the hot nebula, which transfer momentum outward from the center.

As the center of the nebula collapses, enough mass to heat up to the 10,000,000 degree temperatures required for fusion of H to He, the star begins to fire-up, and the protosun is surrounded by a spinning hot ring of nebular material with tubulent structure. The mass of the ring may have been 2%-200% of the solar mass, with much material to be driven off.

As the hot gas in the nebula cools, materials begin to freeze out of the gas, assuming solid state at temperatures that vary depending on the particular compound. This is called the Condensation Sequence, and has been replicated in the laboratory. The nebula need only to have been hotter than about 1500 C to have all of the material in gaseous form. As it cooled to 1400 C it began to freeze out Refractory Material. Some of the compounds that form solid particles at different temperatures are:

Refractory Materials Temperature, C

Al2O3 1410

CaTiO3 1200

Fe(Ni) 1150

MgSiO3 1100

Volatile Materials

FeS 430

Fe3O4 135

H2O, CH4, CO2, H2 <0

This sequence links the composition of different portions of the nebula to the temperature distribution. The main variation is with distance from the Sun, however, the edges of the nebula are cold so there is also a vertical variation across the nebula. The sequence of condensing materials has a gross similarity to the predominant composition of planets with increasing distance from the sun. Hot, close-in terrestrial planets formed from refractory materials, with the more volatile materials still being in a gas state and not being incorporated into the growing planet. Solar radiation would drive the gas outward, particularly in the T-tauri phase of the Sun, when the fusion engine turned on, and strong radiation would have cleared the inner planetary areas of the volatiles. This left the final terrestrial planets enriched in refractories.

The accretion models are either a one-step process or a two-step process. The one-step builds entire planets out of direct aggregation from the condensed mist of granules. The two-step builds them by first accumulating as 'planetisimals', small masses, which then collide to form the larger planets. The two step is favored for several reasons, including: the very low inert gas content of the Earth relative to the expected abundance from that of the nebula (i.e. the solar abundance); models of planetisimal trajectories; and the very tightly constrained ages of the meteorites.

We now think that the process of accretion of solid particles was very rapid, with collisions leading to larger and larger objects which progressively swept up the material around them by gravity. This led to planetisimals, which had a very chaotic collisional history, and these formed the planets,