Talk about a long-term hold – it took 10m years just to form the spinning flat disc that was later to become our solar system.
The best theory we have today about how the solar system was formed – and one that has been supported by a preponderance of data at hand – says that a portion of a primordial interstellar cloud first began to contract. As it did, its slight rotation was amplified until the contracting gas formed a spinning, flattened disc orbiting a denser core of gas and dust. The core further contracted to become a protostar containing the lion’s share of the mass of the original collapsing cloud. This process took about 10m years.
Astronomers have now seen many of these circumstellar discs of material orbiting very young stars, so this part of the theory seems in good shape. The Hubble space telescope has photographed dozens of them in the Orion nebula – check out their site to see the pictures. An amazing system for which we may now be witnessing the formation of actual planets – at least indirectly – is the beautiful HL Tauri system, where we think we can see planets sweeping out rings of gas and dust leaving gaps behind.
One feature of interstellar clouds is dust grains. Astronomers have detected them in dense clouds since the 1960s, and by carefully examining carbonaceous chrondritic meteorites, we know them to be part of the material of the early solar nebula. The solar nebula, by that point a rotating disc of dust grains and gas, continued to evolve as the sun began to form at its centre. The friction in the rotating disc caused temperatures to rise above 1,000ºK out to about 100m miles, so that the chemistry of the dust and gas favoured silicate materials and not ices. At those temperatures, methane and water, both known constituents of molecular interstellar clouds, never got a chance to join into the chemistry, except in the more distant, cooler outskirts of the disc.
As a result of the decreasing temperatures from the centre to the edge of the disc, a variety of specific chemical domains were set up, each leading to its own set of ratios for the abundance of compounds. The inner solar nebula became rich in silicate, iron and nickel compounds. The outer, cooler disc became rich in ices of every kind. This imprint still exists in the composition of the inner planets as silicate bodies and the outer moons of the major planets as water-rich, icy bodies. Once the chemical composition of the dust grains was adjusted to conform to the ambient temperature regimes within the disc, the next phase began.
This next stage, involving planet formation, has not yet been directly observed, but the HL Tauri system mentioned above may be a very promising candidate for this process. A continuation of the same physical model suggests that the dust grains normally found in interstellar clouds, which are quite sticky, begin to build up into centimetre- and kilometre-sized bodies that then settle into the mid-plane of the disc. This process may end at this scale unless the hydrodynamical and gas-dynamical conditions are just right. Too much turbulence, for example, and the small clumps will collide and break apart, never to form larger bodies. It is thought that gravitational instabilities in such a disc can also hasten the formation of very large bodies.
Like a miniature spiral galaxy, the rotating, flattened disc was unstable and tended to form a double- or multiple-armed pinwheel pattern rotating within the body of the disc. Also during this stage, considerable angular momentum was transferred out of the Sun and into the disc. The Sun contains 99% of the mass of the current solar system, but only 2% of its angular momentum. The best-known way in which the rotation of the Sun can be slowed down is through magnetic fields (this is known as magnetic breaking). These have been detected in many infant stars, so we know that for other stars like our Sun (but with ages of less than 10m-20m years) powerful magnetic fields are indeed present. Typical dust grains seen in meteorites and in interstellar space have sizes measured in microns.
To make planets, we have to get the dust and gas to come together into larger bodies. Meteoritic samples tell a rich and complex story of how this probably happened and rely on the fact that dust grains are typically very sticky. As they circulate through their local region of the disc, they collide with other grains and can grow to centimetre sizes in only a few thousand years. Studies of meteorites have revealed that these growing dust grains formed in rather hostile environments that alternated between periods of hot and cold, and in which tremendous bursts of energy (perhaps in the form of lightning?) appeared to singe them. These processes served to make their surfaces even stickier (through partial melting and then freezing). Because the rotating disc has its own gravitational field, grains precipitated out of the ‘atmosphere’ of the disc and slowly sank into the mid-plane of the disc, further narrowing the thickness of the planet formation region into a very narrow band: the present ecliptic plane.
We do not know exactly what process causes matter to make the jump to kilometre-sized and planetesimal-sized bodies. Direct collisions seem the simplest mechanism to build up larger and larger bodies, and the cratering evidence we have from dozens of bodies around the solar system show that very large bodies did indeed once exist in great numbers. Even gravity itself could have amplified this process. Such a narrow, self-gravitating disc is very unstable, and calculations suggest that it would tend to break up into even smaller clumps and inhomogeneities. The estimated size of these clumps is about a few hundred metres to a few kilometres or so – similar to the sizes of the majority of the asteroids in the asteroid belt. The number of such bodies in the primitive solar nebula is hard to imagine.