‘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>ROTATION AND SATELLITES

Somehow, the process of accretion imparted rotation to the planets. Planetesimals that struck a growing world on the right side, as viewed from the direction of their approach, increased the planet’s spin in a prograde sense. However, an approximately equal number of planetesimals might be expected to strike the planets’ left sides too, such that the spins imparted by the two families of accreting bodies would tend to cancel one another. If the planets grew from a very large number of small bodies, the averaging and cancellation of their many contributions should have left the planets with similar, rather slow rotation rates, all in the same direction and all about axes that were nearly perpendicular to the ecliptic plane.

However, this is not what we observe today. The planets spin at a wide variety of rates, two of them turn in a retrograde sense, and most of their rotation axes are tipped at substantial angles to the ecliptic perpendicular. This is consistent with accretion not from a large number of small bodies, but from a smaller number of large bodies, some of which were quite large. In this ease the sum of all the planetesimals’ contributions would have been unequal, leaving residual spins and tilts (obliquities) like those that characterize our planetary system. The evidence argues for such hierarchical growth, with the accretion of relatively large planetesimals occurring in the final stage.

We realize that Jupiter and Saturn did not accrete from solid planetesimals but instead mostly gathered gases directly from the nebula. Both planets spin quite rapidly, yet the manner in which they acquired their angular momenta is not well understood. But they too were subject to having their spins modified by the late addition of large planetesimals. The tilt of Saturn’s rotation axis probably requires an oblique collision, near one of its poles, from something more massive than all of the terrestrial planets put together.

The giant planets were hot when they accreted, just as the terrestrial planets were, because as nebular material fell onto them its kinetic energy was converted to heat. This heat expanded the atmospheres of the giant planets to vastly larger dimensions than they have today. Thereafter they radiated heat, cooled, and shrank. In all likelihood, as each planet shrank a disk of gas, ice, and

Deimos

dust was left in orbit around it —a small analog of the solar nebula. From these disks emerged the regular satellites and ring systems, by means more or less analogous to the formation of planets and asteroids in the solar nebula. The irregular satellites, which have high eccentricities or inclinations (or both) relative to the equatorial plane of their primary planet, are thought to have been captured from the solar system at large. These include Phoebe, Triton, and many very small satellites, among them Phobos and Deimos, the satellites of Mars.

Earth’s Moon is another story. As best we can surmise, when our planet had grown to nearly its present size, it was struck off-axis by a relatively large, fast-moving planetesimal. The energy of the impact heated the Earth and the impactor to very high temperatures, and it spalled off a plume of molten rock and vapor. Some of this debris fell back onto the Earth and some escaped to interplanetary space, but a portion of it settled into an incandescent disk orbiting the planet. We believe that such a disk would spread outward very quickly, and that once material was beyond the Roche limit, about 10,000 km above Earth’s surface, gravitational instabilities would quickly gather it into a number of moonlets. (Inside the Roche limit individual objects cannot pull together because tidal forces induced by the central planet exert dispersing forces stronger than the objects’ self-gravity.) Over a longer period of time, the hot moonlets coalesced into our Moon.