Summary of Chapter

The origin of the Solar System cannot be deduced from its present state, though this state is an essential guide for the construction of theories, as are observations of other planetary systems, of star formation, and of circumstellar discs.

Most astronomers believe that solar nebular theories offer the correct explanation of the origin of the Solar System. In these theories the Solar System, including the Sun, forms from a contracting fragment of a dense interstellar cloud. As the fragment contracts it becomes disc shaped, and at its centre the proto-Sun begins to form. Dust in the inner disc evaporates. As the temperatures in the disc decline, dust condenses and, along with pre-existing dust, settles towards the mid plane of the disc, where it coagulates into planetesimals, and these undergo accretion to form planetary embryos. In the inner Solar System embryos come together and accrete smaller bodies, ultimately to form the terrestrial planets, consisting of rocky materials. In current theories the Moon is the result of a collision between a massive embryo and the Earth late in the Earth's formation.

In the outer Solar System most embryos reach several Earth masses, the result of fewer embryos forming and the condensation of water beyond the ice line. These embryos - called kernels - thus have an icy-rocky composition. They are generally too far apart to come together but are massive enough to capture nebular gas, mainly hydrogen and helium, a process that stops when the proto-Sun goes through its T Tauri phase and blows the gas out of the Solar System. This is the core-accretion model. Icy-rocky planetesimals are also captured, and this capture continues at a low rate today. The rate of growth of the giants decreases with increasing solar distance, so the T Tauri phase, if correctly timed, can explain the decrease in mass from Jupiter, to Saturn, to Uranus and Neptune, and the associated decrease in mass of the hydrogen-helium envelopes. For Uranus and Neptune it is better for them to have formed closer to the Sun than we find them today, otherwise their kernels, particularly that of Neptune, could well have formed long after the nebular gas had disappeared. Such migration is easily attained in the models by outward migration of the fully formed Uranus and Neptune (after the nebular gas is cleared) through the scattering of planetesimals. This causes Saturn to migrate outwards slightly and

Jupiter inwards slightly. We can thus account not only for the existence of giants beyond the terrestrial planets, but also for the broad differences between them.

An alternative (less favoured) means of forming the giant planets (though still within the context of solar nebular theories) is by a one-stage process in which each giant forms from a fragment of the nebula that contracts to become a protoplanet, and then contracts further to become the giant. This is the gravitational instability model.

Estimates of the time it took for the Solar System to evolve from the formation of a nebular disc to the virtual completion of its formation are of the order of 100 Ma for the terrestrial planets, and about 10 Ma for the giant planets including migration.

The distribution of angular momentum in the Solar System is thought to be the result of the transfer of angular momentum by the proto-Sun to the disc through turbulence in the disc. The Sun also lost much of its angular momentum via its T Tauri wind, and through the trapping by the Sun's magnetic field of ions in the solar wind.

The disc of gas and dust that gave birth to the planets would have been rotating in the same direction as the solar rotation, giving rise to prograde planetary orbits roughly in the same plane. The axial inclinations are less well ordered partly because of off centre acquisition of material as the planets grew, and, at least in the case of Saturn, probably because of a resonance.

The asteroids are the result of failed accretion due to the gravitational influence of Jupiter. Jupiter also stunted the growth of Mars, though it speeded the final stages of growth of the other terrestrial planets. The comets are thought to be icy-rocky planetesimals that become active when they enter the inner Solar System. There are two distinct source populations. First, the far-flung Oort cloud, which is thought to be the result of icy planetesimals flung out by the giant planets during their migration. Second, the Edgeworth-Kuiper (E-K) belt extending from just beyond Neptune, which is thought to be a mixture of icy planetesimals and embryos, some having formed in situ, the others having been pushed out by any outward migration of Neptune. Pluto is probably a large member of the E-K belt, and so too might be Triton. There are a few EKOs known to be larger than Pluto.

The rings and most of the satellites of the giants are derived from discs of material in orbit around the giants. The Moon is thought to have originated from the impact on Earth of an embryo 10-15% of Earth's mass.

Solar nebular theories are successful in that they account for most of the broad features of the Solar System in Table 2.1.

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

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