Anu Gupta & Amit Kalaria

Introduction

The study of our solar system has been a topic of interest since ancient times. From the question of life on other planets, to what makes Earth so special, have been motivating factors to learn about our place in the universe. In order to understand the solar system and the objects within it, we must first take a look at its formation through time. In this chapter, we will take you on a journey from when our solar system was first created up to the present time. Along the way, we will describe some important differences between the planets and how they are categorized. Our primary emphasis will be on how the sun plays a role in regulating our solar system, and also to compare and contrast the inner and outer planets. Hopefully by the time you finish reading this chapter, you will have gained a better understanding of our sun and its nine planets.

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The Solar Nebula

We can get an idea of how our solar system was formed by observing star formation elsewhere in our galaxy. This suggest that our solar system originated from a cloud of gas and dust, called the solar nebula, which had a mass three times that of our sun. Matter in this cloud was composed of gas and small grains of ice and dust. Initially, the solar nebula is cold with temperatures less than 50K. At these temperatures most gases freeze into ice particles. However these temperatures are not cold enough to solidify hydrogen and helium, which remain as gases.

The gravitational pull of the particles and gases causes them to collect toward the center of the solar nebula. As the concentration of matter in the center increase, density and pressure rose in magnitude. The high pressure and density caused molecules to collide and thus the temperature deep in the nebula increased. The high temperatures and enormous amount of matter characterized the early stages of our sun, called the protosun. After a few million years, temperatures in the protosun reached millions of Kelvin, igniting thermonuclear reactions; from which our sun was born.

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Our Sun

Even though our sun may seem magnificent to us, we must remember that it is just an average size star. Fortunately, due to the close proximity to Earth, we can examine it in great detail. The sun is composed of primarily two elements: hydrogen and helium (92% and 7.8%, respectively) and can be subdivided into four main parts. The innermost part, the core, is over 15,000,000 C and has a density 150 times that of water. Such severe conditions are so strong that hydrogen can actually fuse together to form helium. Every second, about 700,000,000 tons of hydrogen are converted into 695,000,000 tons of helium releasing 3.86E33 ergs of energy. This is the source of the suns heat.

Above the core resides the photosphere, which is the actual surface of the sun. Since temperatures and pressures here are not as high as found in the core, hydrogen fusion does not occur. Here we can find such things as sunspots which are very large dark spots which seem to be scattered on the suns surface. It is thought that they represent relatively cool regions of the sun (3800 C compared with surrounding temperatures of 5800 K). This phenomenon is not well understood since logic would suggest the cool regions would be quickly heated thus eliminating the sunspots. However, sunspots can be visible for days, and it has been suggested that the strong magnetic field of the sun somehow prevents warming of the dark spots. These magnetic fields are important to us because they produce flares of hot gas which are accelerated to over 900 km/hr. The particles that are emitted by these flares which interrupt radio communications and cause the northern lights on Earth, and need to effect radio-controlled devices.

Above the photosphere lies the chromosphere, which is just a gaseous layer 2500 km thick. Above this layer is the corona that has the remarkable feature of temperatures as high as 2,000,000 C. How can a relatively cool photosphere (6000 C) give rise to such a hot corona? The answer lies once again in the magnetic fields of the sun. Twisted magnetic fields carry energy from the core to the corona where the energy causes particles to move extremely fast, thus resulting in high temperatures. Such high temperatures strip electrons from their nuclei leaving behind nuclei floating in a sea of electrons. These electrons and protons are steadily discharged and travel through space at 450 km/sec. This is known as the solar wind which has important effects as we will later see.

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Formation of Inner Planets

The inner planets of our solar system, Mercury, Venus, Earth, and Mars, originated from small dust and larger particles in the solar nebula that collided with each other over millions of years forming larger and larger particles (Figure 1a). These eventually became protoplanets. The inner protoplanets were composed of materials whose condensation temperatures were high such as iron, silicon, magnesium, sulfur, aluminum, calcium, and nickel. Due to the violent impacts of large objects, such as asteroids, with the protoplanets, a great deal of heat was released. This, along with the energy from radioactive decay, produced enough heat to melt the rocky protoplanets . The heavy elements (iron, nickel, and lead) then sank to the center of the protoplanets, while the lighter elements (calcium and graphite) rose to the surface. The elements which were gases were burned off by the heat of the impact and the combination of the planet's close proximity to the sun. After time, the protoplanets cooled leaving behind dense iron cores with rocky surface formations.

Scientists are able to deduce when the core of the earth was formed by looking at the distribution of radioactive material on its surface. After long periods of time, uranium decays into 206Pb and 207Pb. Originally lead could only be found in the core of the Earth, but then radiogenic lead was found near the Earths surface. By measuring the lead to uranium ratio scientist could determine that the iron core of the Earth was formed 50-100 million years after the formation of the planet.

The effects of solar radiation have boiled off many of the lighter elements such as hydrogen and helium, from the inner protoplanets. These planets consist therefore primarily of solid material and thus have been termed terrestrial planets.

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Formation of Outer Planets

5~The outer planets of our solar system, Jupiter, Saturn, Uranus, Neptune, and Pluto, began their formation in a similar fashion to the inner planets. Their rocky cores also incorporated gases which were present in the environment, but unlike the inner planets, due to the effects of solar radiation and their far distance from the sun (Figure 1b), the outer planets did not lose their acquired gases. These gases, mainly the lighter elements of hydrogen and helium, were not boiled off and thus increased the surface area of the planets leading to an increase in volume. The term Jovian (Jupiter like) has been given to Jupiter, Saturn, Uranus, and Neptune since these four planets exhibit similarities. Even though Pluto is considered an outer planet, it is an exception in that it does not contain a dense gaseous atmosphere.

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Orbits and Paths.

The combined effects of gravity and angular momentum caused the once shapeless solar nebula cloud to turn into a rotating disk whose center was hot, yet its outer edges cold. This disk formation explains why all the planets lie in nearly the same plane. If you could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. The reason for this motion of the planets is due to the gravitational pull exerted on them by the sun. The sun has a force of gravity so strong, that it can pull in an object as far away as Pluto. Why then, do planets not fall straight into the sun? The answer to this lies in the definition of centripetal acceleration. The planets are attracted towards the center of the sun, in a manner which constantly changes their direction, but not magnitude of tangential velocity. The planets would fall in towards the sun, but are going too fast to actually do this. If there was no centripetal force being exerted on planets, they would just go off into space in a direct line. If on the other hand the planets were halted in their motion and then let go, they would fall into the sun. Therefore, the combination of these two characteristics explains why planets revolve in a circular/elliptical path. The planets are trying to travel in a straight line, but are not able to maintain their paths due to the effects of the suns gravity.

All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat. Some of the planets are distinct in their revolutions. For example, Mercury exhibits a 3 to 2 spin-orbit coupling. This means that for every two complete orbits around the sun, the planet makes three complete rotations about its axis. Planets also vary in the shape of their orbits. For example, Pluto's orbit is so elliptical that it is sometimes closer than Neptune to the Sun. Thus, it is the farthest planet from the sun only part of the time.

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Comparison Between Inner and Outer Planets

Due to the varied distances from the sun and the different modes of formation, the outer and inner planets have many unique characteristics Table 1. For example, the sizes of the inner planets are much smaller than those of the outer planets. This is because the outer planets are in a gaseous state while the inner planets are solid and more compact. Consequently, the density of the inner planets is greater than that of the outer planets. Despite the density differences, the mass is smaller for the inner planets than the outer planets. One suggested reason for this is that the rocky planets have a iron core whereas the outer planets may not. Another explanation is that although the outer planets are larger, they are primarily composed of gases, which are characteristically less dense than solids.

As we travel away from the sun, the temperature rapidly drops. Pluto, the farthest planet, is so cold that it exhibits the temperature conditions of the original solar nebula. These vast temperature differences have contributed greatly to the unique characteristics of planets. For example, Mercury has no atmosphere because it would be burned up by the intense heat of the sun. Pluto however has no atmosphere because the gases are frozen and consequently fall to the ground. However, when Pluto comes closest to the sun, its atmosphere resumes a gaseous state similar to that of the outer planets. The atmospheres of the outer planets are very prominent compared with those of the inner planets. For example, Jupiter exhibits differential rotation due to the immense volume of its atmosphere. That is, the core rotates at one speed, but the surrounding gases move at a different speed. Scientists were able to deduce that Jupiter is composed of a large quantity of gases by observing that Jupiters equator and poles move at two different speeds. Such observances are key to inferencing facts about other planets.

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Similarities Within the Terrestrial Planets

The inner planets have many similarities. It is thought that at one point in time, water may have flowed on all the planets. This gave scientists the hope that life may exist on other planets but through scientific research this theory has been disproved for planets in our solar system. It is true, however, that water was present in some form on all the terrestrial planets. On Venus and Mercury, it boiled off long ago due to the close proximity of the planets to the sun. On Mars, it is thought that water still resides at the poles, however it is beneath the surface and never melts. Earth exhibits the proper conditions which allow water to exist in its liquid form. This is a significant fact about the inner planets of the solar system because it can explain the atmospheric differences found amongst them. Mercury for instance has no atmosphere due to its close orbit with the sun. The lack of atmosphere on Mercury contributes to the intense heat on the surface. Venus however, has a very thick atmosphere, yet surface temperatures are hotter on Venus than Mercury. The reason offers important lessons for our Earth (discussed later).

Venus has been referred to as Earths sister planet. The reason for this is its size, density, and distance from the sun resembles that of Earth Table 1. Therefore, it was once thought that the environment on Venus would be similar to that of Earth (especially since they formed from about the same position in the solar nebula). Since Venus is closer to the Sun than the Earth, three things led to the formation of a dense atmosphere on Venus:

Thus, the atmosphere of Venus became hot, acidic and full of carbon dioxide. Both water and carbon dioxide absorb radiation and prevented it from escaping. This is known as the greenhouse effect. Due to the large amount of water and carbon dioxide in Venus atmosphere, heat is trapped on the surface thus causing temperatures to rise beyond that of even Mercury. The effect is so strong that Venus is regarded as having a runaway greenhouse effect.

Since Earth and Venus are "sister" planets, people on Earth should realize the dangers associated with greenhouse gases. If we continue to add greenhouse gases to our atmosphere on Earth, it will cause the temperature to rise (like it did on Venus). This, in turn, will cause more reactions to occur and more gases to be realesed. Eventually, just like Venus, Earth will have a runaway greenhouse effect. By preventing the builup of these problematic gases in our atmosphere, we can save Earth from a fate similar to that of its sister planet.

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Similarities Amongst the Outer Planets

Just as there are similarities between the terrestrial planets, the Jovian planets also share similar characteristics. For example, violent storms are very prominent on Jupiter, Saturn, and Neptune. These storms are so intense that observers on Earth can see them. On Jupiter, there is an area located near its equator known as the Great Red Spot which is a giant hurricane like storm with winds gusting up to over 500 km per hour. The size of the Great Red Spot is as big as the entire surface area of the Earth. On Neptune, even stronger wind forces (up to 1100km per hour) are observed near the equator. Ironically, this area has been termed the Great Dark Spot (in contrast to Juptiers Great Red Spot).

The formations of the storms on Jupiter's Great Red Spot and Neptune's Great Dark Spot differ. The initial understanding of storm formations on The Great Red Spot was first understood after many flybys of spacecrafts which were sent past Jupiter. The first spacecraft to fly past Jupiter was Pioneer 10 in December of 1973. It sent back many colorful photographs of Jupiter which were seen for the very first time. Within a four year time period three more flybys of spacecrafts were sent past Jupiter. The photographs reveal many dynamic changes occured near the area of the Great Red Spot. Photographs from earlier space satellites show a broad white zone dominated the area of the Great Red Spot while later photographs show a dark band begining to form. Through careful examination of cloud and wind motions it was discovered that north of the spot the winds blew to the west while south of the spot winds blew to the east. This opposing wind pattern created a counterclockwise circulation of wind around the Great Red Spot which in turn led to the creation of the violent storms. Recent flybys past Neptune showed pictures of the Great Dark Spot for the first time in August of 1989 when Voyager 2 flew past Neptune. The photographs also revealed a counterclockwise motion around the region of the Great Dark Spot however, the violent storms of Neptune were formed in a different manner. It is believed that methane decomposes near the core into carbon and hydrogen. The carbon crystallizes into diamond releasing great amounts of heat energy that is carried to the surface, which is what fuels the power of the wind and forms these extremely violent storms on the Great Dark Spot.

Another interesting feature of the Jovian planets is the presence of rings which encircle the planets. The rings are composed of tiny ice particles, dust and rock. The most prominent rings are found around Saturn. It is believed that these rings may be fragments of moons that never formed or perhaps moons that were pulled in and destroyed by the intense gravitational field of the planets. The latter of these theories was deduced by observations of how moons interact with planets. For example, the moon causes tides on the Earth, and the Earth causes seismic rumbling on the moon. Similarly, a planet with the magnitude as large as Jupiter can have profound interactions with its moons. Its interactive forces are strong enough to melt the rock of its closest moon Io.

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Asteroids

Early astronomers noted that there was an abnormally large gap between the orbits of Mars and Jupiter. They suggested that another planet may be found here. But actually, small rocky metallic objects were found orbiting the sun similar to how planets orbit the sun. These rocky objects are known as asteroids, and the location in space that they are found is known as the asteroid belt (Figure 2). One theory suggests that they are remnants of a massive collision between a former planet and some object. But more likely, they represent material from the solar nebula that never coalesced into a planet. Since these objects represent what the early solar system was made from, they are of great interest to astronomers. By analyzing asteroids, we can get a better idea of the composition of our solar system early in time.

The orbit of asteroids is variable. If one comes too close to a planet, the force of that planets gravity could cause the asteroid to fall and hit the planet's surface. As an asteroid falls toward the surface of a planet, it heats up by friction with the atmosphere and begins to glow; this is termed a meteor. Most meteors burn before hitting the surface but a few actually reach it. The impact of meteors can cause craters on the surface to form which, as we learned, is what gave the terrestrial planets different terrains and shapes.

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Moons

Many of the planets in our solar system have small rocky satellites orbitting them. These objects, called moons, have various theories behind their origin. There are three theories about the creation of our moon:

Of the three theories that were created about moon formation the most accepted is the "collision" theory. The reason for its popular acceptance is that the other two theories have proof against them. For example, the fact that, other than iron, most of the elements in the moon and Earth are present in similar amounts would disprove the capture theory. If the moon was captured by the Earth, the chances of it having similar composition is too low. As far as the spinning theory, it is improbable that the Earth could have rotated so fast. For this theory to be valid, calculations have shown that the Earht must have spun so fast that the length of a day would be only two hours! For these reasons it is believed that gigantic asteroids collided with the surface of the Earth and a portion of the Earth broke off and thus formed the moon. It has been suggested that this collision is the same one which resulted in the formation of the Earth's core as the collisional energy was sufficient to melt the planet. It has even been speculated that at one point Pluto was a moon. Pluto's close resemblance with many of the moons of Neptune have led researchers to believe that Pluto may been an escaped moon that was once a part of Neptune. This would explain the many exceptions for Pluto in regards to the outer and inner planets charecteristics.

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Conclusion

Throughout this chapter we have led you from the formation of the solar system to its current status. By outlining the different characteristics of the sun, planets, moons, asteroids and other objects out there, hopefully you have gained a better understanding of our solar system. It should be kept in mind that the two general classes given to the planets (the inner and outer planets) share differences primarily due to their distances from the sun. Although the inner planets share similarities within themselves, as do the outer planets, we must note that every planet is unique and has its own special features.

There are so many theories behind the formation of the solar system that perhaps one day we will physically be able to venture out to other planets to gain a better understanding about them instead of having to read about them. With the successful journey to our moon, scientists are trying to endeavor further. Plans are already underway to send a manned shuttle to Mars. The mystery behind our solar system will not be solved for a long time, but with the advent of new technology and an increased understanding of scientific knowledge, we continue to get closer to understanding our Sun and its nine planets.

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Final Thoughts

When studying something as abstract as space, we must remember that there is no definitive answer to any questions, only theories. There are many theories to everything we have discussed, and throughout this chapter, we have listed many of those assumptions (such as the three theories of moon formation). In other cases, we have given the most widely accepted ideas. We must keep in mind that these are only ideas. Throughout our research, we too have come up an ideas. As mentioned before, it is unknown whether or not the two inner most planets, Mercury and Venus, have iron cores. Well we believe that they may not. If all moons are formed at the same time as the iron core (as believed for the Earth), and these two planets don't have moons, they must not contain an iron core either. We know that this is just a deduction based on ideas we have encountered. But it is these types of hypotheses that must be made and re-made over and over in order to gain a more concrete understanding of the truth behind the sun and its nine planets. Perhaps someday we will find an answer to all the questions which have baffled us since the beginning of time.

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Reference/Source List

Book Sources:

Web Sources:

Class Notes:

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Table 1

Data table comparing characteristics of the Nine Planets and the Sun.

Comparison Between Inner and Outer Planets

Similarities Within the Terrestrial Planets

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Figure 1a. Inner Planets mean distance from the sun.

Formation of the Inner Planets

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Figure 1b. Jovian Planets mean distance from the sun.

Formation of the Outer Planets

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Figure 2. Asteroid belt located between Mars and Jupiter.

Asteroids

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Glossary

chromosphere: name given to the region of the sun which is located

core: The innermost region of the core in which fusion occurs.

corona: Characterized by its low density and high temperature, this is

crater: Indentations on the surface of planets which may be caused by the

density: Term which gives the ratio of mass to volume.

flare: A sudden eruption of energy where large amounts of radiation and

fusion: The process of two nuclei joining together to make a different element.

greenhouse effect: Term given to the effect of heating caused by blocking

ice: Name given to solid forms of water, methane, and ammonia.

Kelvin (K): Temperature scale used to give information. Absolute zero is

photosphere: Surface of the sun. This is where sunspots are found.

protoplanet: The name given to a planet while it is being formed.

protosun: Name given to the early sun while it was still being developed.

solar nebula: The gas cloud from which the solar system originated.

solar wind: A continuous flow of energetic particles and gas which are

sunspot: Dark areas of the photosphere which are due to relatively low

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