Historically, two basic scenarios of the origin of the Solar System have been discussed. In one, the Solar System formed as a byproduct of the Sun’s formation. The material left over from the Sun’s formation is the material out of which the planets formed. The idea was first discussed by Rene Descartes in 1644, and was elaborated upon by Immanuel Kant, and farther by Pierre Simon de Laplace.
In the other scenario, originally proposed by Georges Leclerc de Buffon, the material to form the planets was ripped from the Sun by the effects of a passing object, possibly a comet.
It is believed that the Solar System is the remnant of the material that collapsed to form the Sun. The original cloud might have been spherical and was rotating, since we know that the Solar System has angular momentum.
The result of the rotation is that collapse perpendicular to the axis of rotation is retarded, while that parallel to the axis of rotation continued. This means that the spherical cloud flattened to form a disk. It is the disk out of which the planets probably formed.
Once the planets had formed, the debris not included in the planets was mostly cleared away by a very strong wind from the Sun. This would have been when the Sun was going through a T Tauri phase, and its wind would have been much stronger than it is today. The peak mass loss rate may have been mass of the Sun per million years.
The wind carried sufficient energy and momentum to sweep out the debris and stop the infall into the solar nebula.
The Sun has only 2% of the angular momentum in the Solar System, but it would be expected that most of the angular momentum is in the central condensation. To explain this, it has been proposed that the material to form the planets fell slowly into the cloud around the already forming Sun.
If you want to know about the search for life on Mars and Mars exploration check Life on Mars: The fascinating history of Mars
Gases, Ices and Rocks
In following the evolution of the solar nebula, we must keep track of three types of materials: gases, ices and rocks. Most of the mass was in the gas (as most of the mass of the interstellar medium is in gas). However, gas cannot be held to a growing planet by gravity, so it escapes from all but the largest objects. The ices are water (H2O), carbon dioxide (CO2) and nitrogen (N2), along with some ammonia (NH3) and methane (CH4). These make up 1.4% of the mass of the Solar System.
The rocks are iron oxides and silicates of magnesium, aluminum and calcium. Some of the iron was metallic and some of it was in iron sulfide (FeS). They can only be destroyed at high temperatures, in excess of 2000 K. They make up 0.44% of the mass (not including the Sun) in the Solar System. They are particularly prominent in the inner planets, while the ices are prominent in the outer planets.
The accretion of the nebula probably took place over 10,000 to 1,00,000 years. The first step in the process was for small grains to clump together. The grains collided, sometimes making larger ones, and sometimes breaking into smaller ones. The process produced many grains about 1 cm in size. These grains were large enough to settle through the gas in the plane of the nebula.
This brought the clumps closer together, and allowed for even more collisions. Calculations indicate that the thin sheet of grains could then clump into objects with sizes of a few kilometers (essentially asteroid sized objects). About 1000 of these could then form a group held together by their own gravity. At that point, the groups were spinning too fast to collapse completely.
Eventually these groups served as the cores for farther condensation of bodies orbiting at the same distance from the Sun.
If you are interested in space exploration, you might want to see V-2: Story of the First Object to reach Space.
The Temperature Factor
Different parts of the Solar System then evolved differently because of the fall-off in solar radiation with distance from the Sun. The collapsing nebula had a higher temperature in the center (near the forming Sun) than at the edge. The temperature falls off as the square root of the distance from the Sun.
When the temperature was about 3000 K near the center, it was a few hundred kelvins in the regions of planetary formation. It also falls by a factor of about five between the orbits of Venus and Neptune. Therefore, different materials condensed at different distances from the center.
Another factor affecting the nature of forming planets was a fall-off in the density of material as one goes farther from the Sun. When a uniform interstellar cloud collapses, it develops a higher density in the center than at the outside. In fact, ultimately the highest density center becomes the star.
In the higher density regions near the center, the material is also moving faster, as a result of infall, converting gravitational potential energy into kinetic energy. The higher density and higher speeds near the center meant that collisions also played an important role in shaping the gas.
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As a result of the temperature and density variations, we can think of planetary formation as occurring in three zones: (1) the terrestrial planets, (2) the giant planets, and (3) comets.
Near the Sun, the temperature was too high for most of the gas (especially the H2) to have survived the star formation process. So solid materials had to be involved. The original building blocks for the terrestrial planets where chondrules. These were heated to temperatures of 1500 to 1900 K, and then cooled.
Chondrules that we can study in meteors suggest that, to give their particular structure, the heating and cooling took place very quickly, possibly over a few hours. This would mean that the early solar nebula was rocked with energetic events. These chondrules would have had a range of sizes, but typical sizes might have been a few millimeters.
There were so many of these chondrules, and their relative speeds were so low (about 1 m/s), that they eventually began to stick together. It has been speculated that the van der Waals force could have been involved. This created small aggregates of chondrules, which could also grow by sweeping up dust. Eventually they grew to sizes of about 1 km.
At that point, we call them planitesimals. Even though there were many planitesimals, they were distributed over a large volume of space, so encounters between planitesimals were rare, maybe once per thousand years. Computer simulations show that these collisions eventually made larger objects, and after about 20,000 years several Moon-sized objects should have appeared.
After about ten million years, these objects collected to form most of the four terrestrial planets, though these planets probably continued to sweep up planitesimals for 100 million years. These collisions were constantly reforming the surfaces of the planets through violent events.
You might want to read Exoplanets: Super-Earth and Sub-Neptune types Found Transiting a Star.
The Asteroid Belt
The outer edge of the inner zone is the asteroid belt. There is a large gap between Mars and Jupiter suggesting that there was room for another planet to form. We don’t expect a gap, since we expect that the material in the solar nebula would have been falling off gradually in abundance, and we know there was enough material farther out to form the giant planets.
The most likely explanation is that the early formation of the very massive Jupiter prevented the formation of a planet. This could have been either by Jupiter somehow preventing the formation of the more massive planitesimals, or by Jupiter somehow removing them after they had formed.
In the second zone, material was far enough out for water ice to exist. Since O is more abundant than the elements that are important in dust grains (e.g. Si, Mg, Fe), particles of water ice (essentially snowflakes) would have been more abundant than dust particles in the second zone. It is thought that Jupiter and Saturn formed initially from planitesimals made up primarily of water ice.
These planitesimals would have formed in a manner similar to those for the rocky planitesimals that formed the terrestrial planets. However, once Jupiter and Saturn had enough material to exert strong gravitational forces, then they would have collected all of the interstellar material (mostly gas and a little dust) that was near them. This resulted in two very massive planets.
You might want to see Voyager Probes: Top 10 Amazing Facts To Make You Awestruck.
The Ice Giants
Uranus and Neptune formed in the outer parts of the second zone. The icy planitesimals would have filled a larger volume of space, meaning fewer collisions, and less chance for growth than the ones that started Jupiter and Saturn. There would therefore have been less gravity to hold interstellar gas in.
Furthermore, the density of interstellar gas was lower the farther one got from the center of the solar nebula. So, Uranus and Neptune are just the result of the buildup of icy planitesimals, and are dominated by ices. Their compositions are therefore different from those of Jupiter and Saturn.
Most of the satellite systems probably grew from a disk forming around the planet. The satellites whose orbits are close to the ecliptic and are not too eccentric were probably made in this way. Satellites with very inclined or eccentric orbits may have been captured.
You might want to see ASTROCHEMISTRY: RELEVANCE OF CHEMISTRY IN THE STUDY OF COSMOS.
The Kuiper Belt
In the third zone, beyond Neptune, ice/rock planitesimals were formed. However, they fill such a large volume of space that gravitational encounters are very rare. This means that they cannot collect into a planet. The ones from Neptune’s orbit out to 50 AU formed the Kuiper Belt. Those farther out formed the Oort cloud. Occasionally one of these objects has its orbit perturbed, and enters the inner Solar System as a visible comet.
Also see Betelgeuse is Surprisingly Smaller, Closer to Us.
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