How does the solar nebula theory explain the early stages of the formation of the solar system?

Question

What will be an ideal response?

Answer 1

Scientists attempted for centuries to explain the origin ofthe solar system. In the eighteenth century, German philosopherImmanuel Kant (1724–1804) and French mathematician
Pierre Simon Laplace (1749–1827) independentlyargued that a solar nebula(a flattened and rotating disk ofgas and dust such as has been observed in the Orion Cloud)must have coalesced around the sun, and from this theplanets were created. This theory has survived the centuriesand is the most widely accepted explanation, althougha number of unanswered questions still remain concerningthe process of planetary formation.

The theory suggests that the solar nebula formed froma dense core in a molecular cloud that collapsed under thepressure of gravity, a process that may have also been triggeredby a shock wave from a supernova. As it collapsed,the cloud began to heat up, rotate, and spin, and matterfell into a disk that formed around the developing protosun.The rotating disk released enormous amounts of energy(produced by colliding atoms), and temperatures inthe inner disk may have reached more than 3,092 degreesFahrenheit! This would have vaporizeddust grains close to the center, but in the outer regionsof the nebula interstellar molecules, grains, and ices survived.Eventually the nebula began to cool, allowing molecules and solid particles to re-form, although the spinning disk would still have consisted of about 98.5 percent gasand only 1.5 percent dust.

The distribution of the solid particles and gases wasimportant in the subsequent story of the solar nebula. Theinner nebula contained silicates and iron compounds, whilethe outer regions included large quantities of carbon dioxide,water, and other interstellar grains inherited fromthe original molecular cloud.Eventually the matter in the inner zones began to spinmore slowly, and as it slowed was drawn in a spiral nearerto the central mass of the proto-sun. This drift toward thecenter was accentuated for fractions of an inch- to yardsized(centimeter- to meter-sized) solids, which may havedrifted toward the proto-sun at a rate of 600,000 miles(1 million kilometers) per year. Some material may havefallen into the sun, but much of it survived to form therocky terrestrial planets. Why all of the material did not fallinto the sun remains one of the unanswered questions ofthe solar nebular theory, although it may have something todo with centrifugal force in the spinning disk, which wouldhave tended to drive matter away from the center.

By 100,000 years after the start of the process, the sunprobably reached its final mass, at which time collapsingwould have finished and turbulence within the disk subsided.
This moment becomes age zero for the solar system,and a precise date for this has been determined by the radiometricdating of chondrites(primitive stony materialsfrom the asteroid belts). In December 2007, researchers atthe University of California–Davis analyzed material froma carbonaceous chondrite to propose that the solar systemis precisely 4.568 billion years old.

A large number of young stars with relatively low masshave been discovered in other regions of the Milky Way galaxyover the past decade or so, still with dust rings around
them (remnants of the original accretion disk). Astronomershave observed violent winds escaping from many ofthese stars, much more intense than the solar winds thatcurrently escape from the sun. Scientists now realize thatstars not only emit radiation, but also release particles in asteady stream, probably caused by pressure expansion inthe star; and it is these particle emissions that are called stellar winds. The first star discovered emitting this powerfulwind was the T star of the Taurus constellation, andthese winds have been known ever since as the T Tauri. Thewinds seem to emerge at the time that the accretion diskstops directly feeding the star; they are so powerful that ina few million years they disperse a huge amount of the stellarcloud. As the winds collided with the inner edge of the disk, the growth process of the sun was stopped. Only theheaviest objects in the disk were unaffected by the T Tauri,because their mass was large enough to resist the winds.The T Tauri wind also influenced the fact that there isso little hydrogen and helium in the Earth’s crust and somuch in the orbits of the gas giants. This intense solar winddrove the lighter elements (such as H and He) away outtoward the orbits of Jupiter and Saturn. This explains boththe domination of heavier elements in the Earth’s crust, andalso the great size of the gas giants.

By the time the growth process of the sun was ended bythe T Tauri, the sun had absorbed almost all of the materialin the original solar nebula. Only a very small amount wasleft over, perhaps as little as 0.1 percent. It is this tiny survivingremnant was used to make all the rest of the objects in the solarsystem, including planet Earth.