‘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 2: THE GIANT PLANETS

Farther out in the solar nebula, it was cold enough for its water to exist as ice. In this second zone, snowflakes were 10 times more abundant, by volume, than the silicate dust particles. This follows because oxygen — the main ingredient in water — is more abundant in the solar system than magnesium, silicon, and iron combined. Clearly the planetesimals that collected in the outer nebula, and ultimately the planets that formed from them, would have very different compositions than those of the terrestrial planets. However, the largest worlds, Jupiter and Saturn, do not have water as their main ingredient; instead they consist mostly of hydrogen and helium — a composition closer to the Sun’s than to an icy planetesimal’s. (Actually, in December 1995 the Galileo probe found the outermost Jupiter atmosphere contains much less oxygen than average solar-system matter, not more. But this superficial layer may not be representative of the planet as a whole.)

Jupiter and Saturn’s compositions were not established by accreting snowflakes of pure hydrogen and helium, because temperatures in the solar nebula were not nearly cold enough to permit either of these gases to condense. Instead, these planets most likely gathered the bulk of their mass directly from the nebula, wholesale, without discriminating between solids and gases. Thus their compositions are essentially that of the nebula and, in turn, that of the Sun.

There are two ways in which they might have gathered in nebular material. Large cores or nuclei of ice and dust may have accreted first, much as terrestrial planetesimals did. When they became massive enough, their gravity began to attract and hold the nebular gas. The more gas these icy nuclei collected the heavier they became, and the greater their attraction for even more gas. The other possibility is that the very early solar nebula was massive enough to go through periods of gravitational instability, as described earlier. One form this instability might have taken, especially in the outer disk, was the separation and pulling together of gaseous, self-gravitating protoplanets massive enough to resist being dispersed by later tidal forces. In this case, nuclei of solid material would not have been required to get the process started.

Any good model for formation of the giant planets must explain why they differ in composition with radial distance from the Sun. Although hydrogen and helium dominate the compositions of Jupiter and Saturn, Uranus and Neptune consist mostly of the elements that form ices: oxygen, carbon, and nitrogen. In the icy-nuclei model, this trend could have occurred if the planetary nuclei grew so slowly out beyond Saturn that, by the time they were massive enough to attract gases, the nebula had largely dissipated. On the other hand, if all the giant planets started out as gaseous protoplanets, they would have needed to attract and absorb icy planetesimals — which enriched their proportions of O, C, and N to varying degrees.

The various planet-forming processes just described are interdependent and must have taken place in a particular sequence. However, the exact timing of these events is not well understood. There are two possible sequences as they relate to the chronology of star and disk formation laid out earlier. Other scenarios are also possible.

According to the upper schedule, things happened — or began to happen — very early, while interstellar material was still actively falling into the nebular disk. Meteorite researchers, however, discount this option because the isotopic record in meteorites seems to argue against such an early beginning. Specifically, some inclusions present in chondrites once contained the short-lived radionuclide aluminum-26, whereas other inclusions in the same chondrites never did. If the latter inclusions did not form until the early solar system’s Al²⁶ had decayed to indetectability, and if chondrite accretion had to wait until these inclusions became available, the sequence would have been delayed for several million years.

The lower schedule adheres to the A1²⁶ constraint, in that chondrites and terrestrial planets do not begin to form until well after interstellar collapse has ended and the Sun and protostellar disk are in place. Another constraint assumed here (one that cannot be proven with absolute certainty) is that the terrestrial planets were made of chondritic material, so their formation had to follow after that of the chondrites.