Current Theory on the Formation of the Solar System - Part I
This is the first of four articles on how our solar system formed. They describe the planet-making process from its initial beginnings from an interstellar dust and gas cloud to the achievement of the stable planetary arrangement that we have today. This four part series recounts the currently accepted theory of how our solar system and presumably other solar systems most likely formed. Certainly not all solar systems will look like ours. Factors including the size and composition of the initial dust cloud and the mass of the resulting central sun would vary widely and therefore the collection of planets would also be expected to vary greatly from that of our own system. The recent discovery of extra-solar planets may support this view. Most of those newly discovered planets are much more massive than Earth and orbit close to their primaries. These results may be due to the use of the current methods employed to discover them. Further improvement in current technology will enable us to detect Earth-sized extra-solar planets. This is needed to fill out our data with other solar systems to make the formation theory depicted here compatible with the expected wide variety of extra-solar planetary systems.
This first article will describe how the initial dust cloud formed what is called the solar nebula and which quite rapidly coalesced into a central star and planets. Part II will describe how the planet cores and outer layers (mantles, crusts, and gas envelopes) formed. Part III discusses how the size and density of the planets is related to where they formed within the solar nebula. A discussion on planetary magnetic fields is included there, too. The final part addresses the presence of satellites and planetary atmospheres. Parts II through IV will appear in NightTimes over the next few months.
The Solar Nebula.
How the solar system formed, creating a single, central star and a group of planets and other objects orbiting around the Sun is not completely understood. However from observation of the solar system as well as the study of interstellar gas clouds and apparent star formation within our Galaxy a generally accepted consensus of how the Sun and planets formed has been obtained. There is evidence remaining today that provides indications of this formation process. Among this evidence is the fact that the planets orbit the Sun in the same direction and at about the same plane. This gave rise to the concept, first proposed in the eighteenth century by Kant and de Laplace, that the entire solar system, Sun, planets, moons, asteroids, and comets evolved from a rotating cloud of gas and dust called the solar nebula.
This cloud would have had an initial rotation. Something disturbed this cloud and it began a slow collapse. This disturbance could have been caused by shock waves from a nearby supernova but regardless of what initiated the formation process, the contraction once begun continued to progress apparently without further input. Each particle within the cloud and all of the parts of the cloud exerted gravitational attraction on each other. This led to a continued gravitational collapse of the cloud toward its center of gravity. As these particles spiraled inward they would have increased the speed of rotation of the solar nebula according to the law of conservation of angular momentum. As they gained speed during their inward fall their inward motion would begin to be countered by their translational motion. This translational motion, or centrifugal force, slowed the inward spiral of the particles. Other particles that were close to the axis of rotation of the spinning nebula would not acquire enough centrifugal force to avoid falling into the central nebula. These two effects would tend to flatten out the nebula into a rotating disk with most of the mass collecting at the center.
Most of the particles would end up in the center of the solar nebula where they would eventually form the central star. Energetic collisions of the in-falling particles released thermal energy heating the central portion of the nebula in what is called Kelvin-Helmholtz contraction. Over time the rest of the disk would cool as its energy radiated away. The increasingly hotter center formed the protostar that continued its evolution into our Sun. The material left rotating around the protostar formed an accretion disk. The planets formed from this disk. This is why all of the planets orbit in the same direction and almost in the same plane.
Accretion into Planets
When the material that remained in the disk had cooled off sufficiently it began to condense. Near the protostar the temperature was hottest while further away from the center the temperatures steadily declined. Metals, such as iron, nickel compounds, and metal oxides have high condensation temperatures so they condensed out first. According to isotope dating of meteorites the metals condensed about 4.55 to 4.56 billion years ago and the rocks made of silicates condensed next, between 4.4 to 4.55 billion years ago. Further from the center, where temperatures are lower, volatile compounds such as methane, ammonia and water condensed and formed ices. So metals and rocky material tended to dominate in that part of the nebula closest to the hot protostar where high temperatures prevented the formation of ice and in the colder regions farther from the protostar more of the substances with lower condensation temperatures proliferated.
As the cloud cooled these compounds and molecules formed dust particles. These dust particles would collide, stick together, and form larger particles. This process continued until they were the size of boulders. These boulders, now called planetesimals, continued to accrete more material due to their increasing gravitational attraction as they swept around the protostar. During this early accretion process the solar nebula would have contained many of these planetesimals along with large amounts of gas, dust, and ices. As they orbited around the central star collisions between the planetesimals would have been common and quite energetic, creating larger planetesimals as well as breaking others apart. This process of planetesimal growth required between several hundred thousand to around 20 million years to complete. Those furthest away would take the longest. Eventually these planetesimals will grow to planet-size.
In the inner solar system the newly formed planets would grow larger by collecting the metals and rocks that were prevalent. However, they were unable to capture and hold the more energetic molecules of gases such as hydrogen and helium. The outer solar nebula was cooler so these molecules were less energetic. In these cooler regions, large amounts of water ice, compounds of methane, ammonia, as well as other substances could condense. These substances, together with rocky material, were available to form the planetesimals. These planetesimals would collide and eventually grow to a mass several times that of Earth. Once they had acquired a significant gravity they could capture molecules of hydrogen and helium as well as other volatiles and rapidly increased in size and gravitational influence. They would quickly accumulate all the material near their orbit in a runaway growth spurt. The less massive bodies in the outer solar nebula that would eventually form the planets of Uranus and Neptune would not be able to retain huge amounts of the lightest gasses such as hydrogen and helium. Heavier elements and compounds such as nitrogen, carbon, water, silicon, and iron would come to dominate their structure. Therefore, the Jovian planets are modeled to have cores containing rock and ice that are covered by deep, hydrogen-rich layers in the case of Jupiter and Saturn while heavier elements and compounds such as nitrogen and water cover Uranus and Neptune.
At the center of the nebula the protostar became the Sun. During its formation it expelled its outer layers several times into space at high speed. This would sweep the remaining gas and dust out of the solar system and the accretion process would come to an end. Whenever the last collisions may have occurred between the remaining planetesimals and planets, they may have been very energetic and significantly affected the planets, their orbits, and alignments. After about a hundred million years the planet-sized bodies would achieve stable orbits and the solar system was very much as it is today.