‘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.’
Before considering the formation of planets, let us take stock of the interstellar raw material from which the solar nebula emerged. The bulk of the matter in the solar system consists of atoms of hydrogen and helium. Other chemical elements, which are less than one-thousandth as abundant as hydrogen, were formed by nuclear reactions in the interiors of earlier generations of stars that existed between the time of the Big Bang and the origin of the solar system 4.55 billion years ago. These heavier, later-formed nuclides were released from their parent stars into the interstellar medium by stellar winds or stellar explosions.
Surprisingly little can be said about the physical state of these elements before and during the collapse process that formed the solar system. Extrapolating from our knowledge of the present interstellar medium, the most abundant element, hydrogen, existed chiefly as the diatomic gas molecule H2 — thus justifying the often-used term “molecular cloud”. Metallic elements (most notably magnesium, silicon, and iron, the principal ingredients of rocky planets) condense into solids at the highest temperatures and are termed refractory for this reason. They combined with oxygen and other elements while still in interstellar space to form tiny grains roughly 0.1 micrometer across — only about 1,000 atoms wide. The physical state of the elements lying between H (and He) and Mg, Si, Fe in atomic mass and abundance is very poorly known. In part they occurred as a variety of molecules in the gas phase, such as CO, N2, NH3, and free oxygen. In part they condensed into solid grains, as graphite (C) and silicon carbide (SiC), for example. They also formed coatings of complex organic compounds and mantles of frozen ices on more refractory grains, though the ices would have evaporated when the grains were warmed as they fell toward the protosun.
Most
of the matter present in the original solar nebula is gone now. It was drawn
into the Sun, expelled into interstellar space, or incorporated into planets
whose internal activity has reprocessed it into some new form. However, some
of the primordial nebular material has survived and thus provides a crucial
key to learning the details of how our solar system formed. The most abundant
reservoir of unchanged nebular matter is in the form of comets. Because they
remained small and far from the Sun, effectively in deep freeze for eons, these
icy planetesimals retain most or all of the properties they had when they accreted
in the outer nebular disk.
We do not yet have direct access to comets for study, but some of their ingredients are in our laboratories. In the year 2003, however, a satellite launched by the NASA in US is expected to be landing (or crashing, depends on your viewpoint) on a comet. High abundance of information about the ingredient of the comet is expected to be obtained. Comets that approach the Sun are warmed by its heat, causing some cometary ice to evaporate and release embedded dust particles into space. Some of these are swept up by the Earth, along with other interplanetary dust particles, at which point they can be collected by research aircraft flying high in the stratosphere. Exactly which particles in these collections are cometary remains a puzzle. Viewed by electron microscope, many particles consist of clusters of tiny grains of minerals, organic compounds, and nondescript amorphous materials. Notably, these component grains tend to have roughly the same dimension, 0.1 micron, attributed to interstellar grains. Some fraction of them probably is just that, having fallen into the nebula 4.55 billion years ago. They then became embedded in the snowflakes that joined growing icy planetesimals.
Samples
of more refractory primitive material, from the inner solar nebula, are preserved
in the form of meteorites known as chondrites. These are fragments of asteroids,
bodies that were not large and geologically active enough to completely reprocess
the primitive nebular material. Although all chondrites were affected to some
degree by thermal or hydrous metamorphism in their parent asteroids, the least-altered
ones contain bona fide interstellar grains. We conclude such an origin based
on the grains’ anomalous isotopic compositions, which in each case records the
particular 
nuclear
reactions occurring in the unknown star that gave rise to it, long before our
solar system formed. Collisions between asteroids release a shower of chondritic
debris into space, some of which eventually reach the Earth’s atmosphere as
meteorites.
The solar nebula was hot near its center, tapering off to cold, then very cold, at its outermost margins. Of course, the environment near the infant Sun was warmed by its radiant energy. More important than this heat, however, was the Sun’s mass and gravitational attraction. Close to the Sun, the nebula was its thickest and densest, and all the mechanical processes affecting the nebula — infall of molecular-cloud material, relative motions of nebular gas, turbulence, shocks — were stronger there and generated more heat than they did farther from the Sun.
Think of the nebula’s falloff in temperature with heliocentric distance as defining three radial zones, like rings in a target. The innermost zone was too warm for water to condense as ice; objects forming there consisted entirely of silicate minerals and other refractory materials, ultimately becoming the terrestrial planets. The next zone was colder, water ice was stable, and a vast blizzard of snowflakes gave rise to the much larger Jovian planets. In the outermost and thus coldest zone, condensed matter was also icy. But it was too sparsely distributed to accrete into sizable planets; instead it remained dispersed in small icy planetesimals — comets — in what we now call the Kuiper belt. Remarkably, the planets assembled themselves very quickly. Although the process differed in detail from zone to zone, virtually everything was in place within 10 million years, by which time the solar nebula had largely dissipated.