This article was originally published in February 2020.
Talk about a long-term hold – it took 10 million 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.
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
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 100 million 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.
The dust grains normally found in interstellar clouds begin to build up into centimetre-
and kilometre-sized bodies that settle
into the mid-plane of the disc
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.
All we have to do is look at the surfaces of the inner planets, moons and even the asteroids themselves, to see that an intense period of bombardment occurred by objects of about this size. The density of these bodies in any cubic kilometre of the nebula must have been very high, because the amount of dust material available in the solar nebula was several percent of the mass of the Sun, and no solar process could eject these centimetre-sized pellets once formed. As for the gas in the nebula, that’s another story. We know that Sun-like stars go through what is called a T-Tauri phase as their nuclear fires are turning on. This unleashes a tremendous solar wind that washes through the inner solar nebula and scours out all of the gas. This phase ended about 20 million years after the solar nebula and proto-Sun started to form.
The small bodies collided and merged, and it is
estimated that it took less than 100 million years
to form a body as large as the Earth
The small bodies collided and merged, and it is estimated that it took less than 100 million years to form a body as large as the Earth. Initially, the planetesimals grew by direct collision, like a tennis ball hitting a basketball dead-on. But as the body grew in size to a few hundred kilometres or more, its own gravity field began to steer surrounding dust and matter into a capture cone so that the planetesimal could sweep out more material that was present along its orbit.
Towards the end of this runaway accretion process, larger and larger bodies were available as the solar nebula continued to age and the distribution of body sizes relentlessly got bigger. Although initially the body sizes may have been only a kilometre across, by the end of the planet formation process, bodies several hundred or even thousand kilometres in size may have been colliding. One of these apparently ploughed into the Earth and tore off enough material to form our moon. A second one may have collided with Venus and tipped its rotation axis; a third one smacked into Mercury and tore off its crustal material leaving only its mantle behind. A fourth one may have tipped Uranus on its side before this planet had formed its own satellite system.
The inner planets formed rather slowly over time, but the outer gas giants followed a different pattern. Once a planet reaches ten to 20 times the mass of the Earth, its gravitational field becomes so strong that, in the cooler outer regions of the solar system where gas moves sluggishly, even this gas can be trapped by the growing planet. The planet then grows exponentially. The details are still in dispute, but it does not seem to be too hard to create a Jupiter-sized body in a few tens of millions of years. The discovery of Jupiter-sized planets around other stars tells us that these bodies do not stay put but probably drift inwards in the solar nebula during formation. Jupiter, for instance, may have formed near the orbit of present-day Saturn and drifted inwards due to viscous and gravitational forces within the disc. In some discs, these huge bodies could even drift all the way into their star and evaporate. As they move inwards, they would eject any forming planets already present – including proto-Earths.
The composition of these planets, and their atmospheres, depended on where they were in the circumstellar disc, because the inner disc had a temperature of over 1,000ºK and the outer limits were at 20ºK. In the region we call the inner solar system, compounds rich in silicates, iron and nickel would be in thermodynamic equilibrium. In the outer solar system beyond Jupiter, methane, ammonia and water ices would be abundant. This accounts for why the inner planets and asteroid belt are rocky bodies and why the moons of the outer planets are all giant balls of ices. This ‘chemical equilibrium’ model is very powerful and could be used to predict the composition of undetectable planets in other solar systems, knowing only their mass and distance from their star.
After the T-Tauri phase had removed the free gas within the disc and blasted away the original atmospheres of the inner planets, new atmospheres outgassed from the planets’ interiors, setting the stage for interesting surface chemistry. Even cometary bodies colliding with these planets may have unloaded significant quantities of enriched molecular material – and great quantities of water – but not as much as what would have been outgassed from inside the planets themselves.
For the next billion years, the planets would continue to be bombarded by large asteroids until these free bodies had been completely swept out or ejected from the inner solar system. Today, there still exist pockets of these ancient bodies throughout the solar system – and we must be constantly on the alert for potentially devastating impacts.
