‘Our cradle planet, the Earth, and the Mars, where we are staying, are two terrestrial planets within the Solar System. Since ancient time, people all over the Earth had make conjectures about the formation of astronomical objects, especially their home.’

‘I know that human’s technology has improved a lot since the New Stone Age; and now, you can even produce artificial human.’

‘Yes, and the theory about the formation of the Solar System is more advanced than the legends in the prehistoric periodic. Not only to explain phenomenon, but our theory can also predict before discovery.’

EVOLUTION OF THE UNIVERSE>FORMATION OF THE SOLAR SYSTEM>ZONE 1: THE TERRESTRIAL PLANETS

Terrestrial planets were not made by simply sweeping up the refractory interstellar dust grains that fell into the nebula. We know this from the makeup of chondritic meteorites, which are samples of terrestrial planetary material as it first accreted. Chondrites consist in large part of small igneous spheroids called chondrules. These were once molten, which required temperatures in excess of 1,5000 to 1,9000 K. When the chondrules cooled and solidified they must have been dispersed in space, not aggregated as we see them now, in order to have maintained their droplike shapes. Over a century ago the English microscopist Henry Clifton Sorby argued that chondrules must have formed as “detached glassy globules, like drops of a fiery rain”. Sorby further surmised that chondrules were “residual cosmic matter, not collected into planets, formed when conditions now met with only near the surface of the Sun extended much further out from the centre of the solar system” — a remarkably prescient assessment.

Efforts to simulate chondrules in laboratory furnaces have shown that a sample must be cooled relatively rapidly, in roughly an hour, to reproduce the properties of meteoritic chondrules. This means the chondrules could not have been melted by the ambient temperature of the innermost nebula, because there is no way to cool matter so quickly in that setting. Instead the workings of the inner nebula must have involved pervasive, local, impulsive, high-energy events that drastically — but very briefly — affected its dispersed silicate material. We have little clue to the nature of these energetic pulses. Nor is it clear whether they occurred during the violent first million years of disk history, while interstellar material was still falling into the nebula, or in the 10 million years thereafter, when the nebula was thinner and more quiescent. Most researchers favor the latter period.

Severe thermal processing is capable of changing the chemical composition of planetary material, by selectively boiling off the more volatile chemical elements; these volatile elements are then free to recondense, perhaps somewhere else. Chemical fractionations of this sort must have occurred in the solar nebula; we see the evidence in individual ehondrules, in their bulk aggregations (such as chondrites), and even in the planets themselves. For example, potassium, which is moderately volatile, exists in the Earth at only about one-fifth the abundance (relative to nonvolatile elements) that is present in average solar-system material.

Eventually, chondrules and dust began to stick together, creating larger and larger clods of chondritic material. This is a crucial moment in the history of planet formation, but like much else in the story it is very poorly understood. The gravitational attraction between such small objects is much too feeble to get the process of accretion started. Small particles that collided at less than about 1 meter per second might have stuck together because of Van der Waals attraction (weak, short-range forces caused by the uneven distribution of electrostatic charge). However, clumps bonded by Van der Waals forces alone would not be strong enough to survive mutual collisions in the nebula’s turbulent zones. The onset of accretion would be easier to understand if some stronger “glue” were available.

There is no question that sticking did occur. Many chondrules in chondrites are coated with rims of dust particles gathered before they became grouped with other chondrules. Once the clusters exceeded a few centimeters across they became too heavy to be pushed around by turbulence in the gas, whereupon they settled and concentrated near the mid plane of the nebula.

They also began spiraling in toward the Sun. An orbiting solid object, like a bit of chondrite, maintains a simple balance between centrifugal force (directed outward) and the Sun’s gravity (inward). However, a parcel of gas in the nebula would have been pushed outward not only by centrifugal force but also by the gradient of gas pressure, which decreased outward in the disk. Consequently, gas required less centrifugal force and orbital velocity than solids did to remain in a given orbit, so any solid object in the same orbit traveled a little bit faster than the gas surrounding it. The resulting drag continually slowed the particle down, thus forcing it to spiral slowly inward through the nebula. As chondritic clusters crept inward they swept up and accumulated dust, loose chondrules, and smaller aggregations. Once such an object grew to a dimension of a kilometer or so, the gas-drag effect became relatively small, and the object’s motion toward the Sun ceased. At about this size we dignify masses that were growing in the nebula with the name planetesimal.

Imagine being a passenger on the surface of one of these 1-km planetesimals. At the stage of evolution just described, the nebula teems with countless other planetesimals of similar size, but they are so widely dispersed that most of the time we can’t even see any of them. Because of slight differences in their orbits, the planetesimals move relative to one another: a combination of up-and-down motion relative to the ecliptic plane, due to varying orbital inclinations, and in-plane motion with respect to one another due to orbital eccentricity. Because of these, every so often one of the other planetesimals zooms past us, and then quickly disappears in the distance. During the closest brushes, each planetesimal gives a tiny gravitational tug to the other, which changes the orbits of both slightly. And if we stay aboard our planetesimal long enough (maybe 1,000 years), eventually it will hit something of comparable size. If the collision velocity is slow, the two objects will merge into a single, larger mass; if fast, they knock each other apart. Either way, our ride is over!

High-speed computers allow us to study how planetesimals grow in such a system, and how fast it happens. Early on, planetesimal growth probably does not occur uniformly; instead, something like “the rich get richer and the poor get poorer” occurs. The largest objects experience runaway growth at the expense of their smaller cousins, and in some 20,000 years hundreds of bodies roughly the size of the Moon have been produced. Computer simulations have shown that such a population of bodies, through mutual orbital perturbations, coalescences, and catastrophic collisions, will eventually evolve into a family of bodies similar to the terrestrial planets. These simulations are limited in many ways, which makes them incapable of exactly reproducing Mercury, Venus, Earth, and Mars. However, they do show that the terrestrial planets reached nearly their full size in about 10 million years, and then continued to sweep up large planetesimals for another 100 million years.

The impacts of accreting planetesimals deposited huge amounts of kinetic energy in the planets being formed, enough to partly melt them. The earliest history of planetary surfaces was a chaos of solidifying crustal slabs, erupting lava, and giant explosions caused by the arrival of more planetesimals.