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The Solar System

The Big Bang
Early Universe
Life and Death of Stars
Galaxy Formation
The Solar System
Exotic objects
End of the Universe
Who created the universe?
What is Time?
Life beyond Earth
NASA Missions
Particle Map
Glossary
Sources
When a supernova explodes, a giant interstellar cloud of hydrogen gas, dust and grains of heavier elements is formed as its remnants. Interstellar clouds such as these are the birthplaces of new stars. Some 5 billion years ago, just such an event occurred and the resulting gas and dust cloud became the birthplace of a new star. Fortunately for us, this new star was our Sun.

Relative sizes of planets and stars

Birth of our Solar System
Planets are born from the dust and gas leftover after the birth of stars. From one swirling cloud of dust and gas, a diverse solar system can be formed, containing a variety of unique planets each with its own distinct characteristics. The characteristics of each planet depend upon how close to its star it forms, the temperature of the zone in which it forms and the contents of the gas and dust cloud from which it forms. Atmospheric composition, ambient temperature, absence or presence of liquid water and continental land masses, planetary composition (is it a rocky planet or a gas giant?), size and mass are some of the attributes that vary with a planet's distance from its Sun and the composition of the gas cloud from which it was formed.



Early Solar System
In its early period, there may have been as many as 100 or more proto-planets. The solar system was a chaotic, dangerous place. Collisions were commonplace. Sometimes a collision would result in the complete destruction of the objects and, in other cases, one of the objects would incorporate the other as part of itself, helping it to grow larger.

Solar Nebula
The beginning of The Solar System was a supernova explosion that occurred at the end of the life of a massive star. Stars are able to exist due to a delicate balance between the expanding force of the energy produced by the fusion of hydrogen into helium and the squeezing force of gravity. When the dying star finally uses up the last of its hydrogen supply, the expanding force of fusion comes to an end and gravity wins the battle, causing the star to collapse under its own weight. If the star was massive enough (about 100 times the mass of our Sun) the star explodes in a supernova creating the nebula that will be the birthplace of a new star and, if enough heavy metals are present, planets revolving around the new star.

For a nebula to begin forming a star and planets revolving around it, there are two necessary conditions: (1) The star must exceed a certain mass, known as Jean's mass, and it must exceed a certain diameter, known as Jean's length. When both conditions are met, the cloud can begin contracting. However, there is an additional condition. If a cloud of dust and gas exists at a certain mass and size that meet the first two conditions, contraction will not begin without some kind of external force. A shockwave generated by a nearby supernova is one possibility. Another is the gravitational squeezing of the rotating cloud caused by conservation of angular momentum. Finally, passing density waves can provide the necessary force.

Birth of the Sun
The nebula is a swirling cloud of dust and gas, primarily hydrogen and some heavy metals if the dying star that produced the nebula was massive enough to create them. The nebula exists in a state of equilibrium, in which the outward force of its rotation is balanced against the squeezing force of its gravity. Some disturbance from an outside event is needed to cause the nebula to begin collapsing, thereby triggering the process of accretion which begins the process of star birth. A passing shockwave or a passing star would provide the gravitational jolt needed to trigger the process of accretion. An illustration is in order. Imagine a flat trampoline with an even distribution of marbles spread out on its surface. Now, picture what would happen if a bowling ball were rolled across the trampoline. The marbles would be drawn into a few small groups of marbles gathering right next to the bowling ball. These groups of marbles would concentrate additional weight (gravity) around themselves, attracting even more marbles to join the group. This illustrates what occurs in the nebula when the smooth composition of the nebula is acted upon by a passing gravity wave. A mass of at least 80 Jupiters is required to create enough gravitational pressure to trigger the fusion of hydrogen. When fusion begins, a star comes into existence.



The Terrestrial Planets
After the Sun was formed, there remained the planetary nebula, a swirling cloud of dust and gas, primarily hydrogen, which was the remnant of the original solar nebula from which the Sun was formed. Clumps of matter formed in the nebula and these clumps collided with other clumps and grew larger still in a process called accretion. As they grew, they became rocks and the rocks collided with other rocks, creating boulders. Eventually, their gravity increased until it was strong enough to attract other boulders, creating even larger boulders and this process of accretion became self-sustaining. The process continued until the ever-growing boulders acquired enough gravity to form a spherical shape. An object needs to be at least 500 miles in diameter for a sphere to form. At this point, the object is a proto-planet and well on its way to becoming a full-fledged planet.

  • Composition The inner Solar System, considered to be the region inside 4 astronomical units, was too warm for molecules like water and methane to condense, so the planets that formed there could only form from compounds with high melting points, such as metals like iron and nickel and rocky silicates like quartz. The inner proto-planets developed their metallic cores By the process of chemical differentiation, the heavier elements, such as, iron, nickel, etc. sank toward the center of the planetesimals while the lighter elements floated to the top. This separation and distribution of materials, dependent upon the density of the materials, was responsible for the ultimate composition and structure of the inner planets as rocky bodies with metallic cofres. This was also the reason that the inner planets developed their magnetospheres emanating from their polar regions. These rocky bodies were the embryos of the terrestrial planets, Mercury, Venus, Earth, and Mars. These metals are quite rare in the universe, comprising less than 1% of the mass of the planetary nebula, so the size of the terrestrial planets is limited by the available supply of these substances. Collisions and mergers between these planetesimals allowed them to grow to their present sizes.

  • Orbital characteristics When the terrestrial planets were forming, they remained immersed in a disk of gas and dust. The gas was less dense and therefore, orbited the Sun more slowly than the planets. The drag resulting from the differences in orbital velocity between the gases and the denser substances caused a transfer of angular momentum, resulting in a migration of the planets to new orbits. As the disk dissipated, the overall trend was for the inner planets to migrate inward, closer to the Sun, resulting in the inner planets' current orbits.
The Gas Giants
What are they? Gas giants are typically very large and massive and are composed, as the name implies, almost entirely of gases, such as hydrogen, methane and ammonia in their gaseous state - all of which are hydrogen compounds. Gas giants have no solid surface, but they do have a metallic core composed mostly of hydrogen in a liquid state. The metallic core is responsible for the huge magnetic fields encircling the gas giants. They also have a very deep atmosphere which gets very heavy the deeper you go. Jupiter could have been a star if it had enough mass.

  • Atmosphere The atmosphere of Jupiter, as an example, is about 1,000 km deep. It is almost an exact copy of the Sun's atmosphere, containing 82% hydrogen, 18% helium and trace amounts of methane, ammonia and water vapor. There is a point at which the atmosphere becomes so dense that the gases condense to their liquid states. The atmosphere is composed of alternating light and dark bands called zones and belts, respectively. The zones and belts vary in both pressure and temperature The zones are higher in the atmosphere and cooler while the belts are lower and hotter. Jupiter's rotation varys with latitude. Closer to the equator, the speed of ratation is faster than it is at the poles.

  • Composition Why are the inner planets composed mainly of metals and rocky substances, while the outer planets (Jupiter and beyond) are gas giants? There is an invisible line, the frost line, which is approximately 2.7 AU from the Sun (in the middle of the asteroid belt). Within this boundary, the temperature is high enough for hydrogen compounds, such as water and methane to exist in a gaseous state, while beyond this boundary, it is cold enough for hydrogen compounds to condense into solid ice grains. The lower temperature in the nebula beyond the frost line makes solid grains of frozen water, methane, and other hydrogen compounds available for accretion, distinguishing the compositions of planets formed beyond the frost line from the composition of the inner, rocky planets. The frost line therefore, is one distinguishing feature that caused the jovian planets (the gas giants) to form where and how they did, while causing the rocky planets to form closer to the Sun. So we can say that the varying composition of the planetary nebula, which differs in a direct relation to the varying distance from the Sun, determines whether or not a forming planet becomes a solid, rocky planet or a giant gaseous planet.
The Planets
The Asteroid Belt
The Asteroid Belt is a distinct region starting beyond Mars' orbit and ending before Jupiter's orbit, within which the asteroids revolve around the Sun. They are mostly rocky and metallic objects and some are made of frozen water ice, as well. The largest asteroids, such as Ceres, are as much as 1,000 km across, while the majority are 1 km across or less. There may be millions of these. It is believed that the asteroids are the leftover objects that did not become planets during the formation of the solar system. Jupiter and its gravity play an important role in the behavior of the asteroids. It both protects Earth and threatens it. As protector, Jupiter's gravity tends to give large asteroids a gravitational kick pushing the objects out of their orbit and toward the outer regions of the Solar System. Asteroids affected in this way will never threaten Earth as they are destined to wander through interstellar space forever, never to cross Earth's orbit. In its other role, Jupiter may give large asteroids a kick towards the Sun, creating what are known as NEO's or Near Earth Objects. These objects may cross Earth's orbit as they are propelled toward the Sun, creating the threat of a collision and, if large enough, the possibility of the extinction of all life on Earth.

The Kuiper Belt



The Oort Cloud



Having presented the background information about the Solar System, we can now proceed to discuss each planet. In the sidebar menu (the main menu), Select "Mercury" under the topic "The Solar System". Once the Mercury page is displayed, clicking on the "Previous" and "Next" buttons will cycle you through each planet in the Solar System covered by this website.