Working through this chapter of the study guide will enable you to:

1. Describe how our planet Earth moves through space and responds to the gravitational pull of the Sun and other nearby celestial objects.
2. List the other planets that orbit the Sun, show that they follow Kepler's laws, and tell how they fit into the general structure of our solar system.
3. Distinguish between the terrestrial and Jovian planets and give general descriptions of each planet and its associated satellites.
4. Show how asteroids and comets fit into the overall description of our solar system.
5. Discuss the origin of our solar system and comment on the possibility of the existence of additional solar systems associated with other stars in the universe.

Discussion

The positions of the stars and the motions of the planets in our night sky has captured the interest of people for centuries. In order to explain these astronomical phenomena, it is first necessary to understand how Earth rotates and revolves as it moves through space. These motions not only account for our celestial travel, they give us an explanation of everyday occurrences such as the sequence of day and night and the periodic nature of the seasons of the year.

The last few years have seen a tremendous increase in our knowledge of not only our solar system, but of the entire universe. We now have access to data about the planets and their moons that could only have been dreamed about by earlier astronomers. With this wealth of information, we are now able to understand much more about the composition and structure of our solar system, but there are still a lot of questions that have not been completely answered. Future satellite observations and unmanned missions to explore our solar system promise to bring us even more interesting data about the Sun and the nine planets that revolve around it.


Section  15.1The Solar System: An Overview

The ancient idea that Earth was the center of our solar system, often referred to as the geocentric theory, has been replaced by the Sun-centered or heliocentric theory. Several early scholars such as Heracleides and Aristarchus saw the advantages of a Sun-centered theory, but the transition from one belief to the other was not easy or rapid. Then, as always, people thought of themselves as very important. Earth appeared solid and unmoving under their feet, and the stars did not seem to change their relative positions, as people thought they should if Earth were moving rapidly through space. For these reasons, a great deal of opposition was raised to the idea that Earth was not the center of the solar system, which at that time was considered the entire universe.

In an attempt to solve problems related to the geocentric theory, a rather complex mathematical system involving circles within circles was developed by Ptolemy in the second century A.D. to account for the observed positions of the Sun and planets in the sky. This epicycle theory was not very easy to use or particularly accurate in its predictions. Finally, in the mid-1500s, the Polish astronomer Copernicus developed mathematical concepts that could be used to predict future positions of the planets more easily because it placed the Sun at the center of our solar system. Copernicus's theory still required epicycles because he continued to use uniform concentric circles in his calculations, but the Sun-centered theory at last had a strong mathematical basis to build on. Unfortunately, Copernicus died the same year that his ideas were first published, which may have contributed to the fact that his heliocentric theory was not immediately accepted by most people.

After the death of Copernicus, his ideas, together with detailed astronomical data gathered by the Danish astronomer, Tycho Brahe, were used by Johannes Kepler in formulating three laws of planetary motion. Keplers three laws are:

1. All planets move in elliptical paths around the Sun, with the Sun at one focus of the ellipse.
2. An imaginary line joining a planet to the Sun sweeps out equal areas in equal periods of time.
3. The square of the sidereal period of a planet is proportional to the cube of its mean distance from the Sun.

Kepler's laws finally put the heliocentric theory on firm enough scientific footing that it was at last accepted, at least by most well-educated people. About 100 years later, Sir Isaac Newton integrated the work of Copernicus and Kepler into a publication called Principia Mathematica, in which his new concept of gravitational attraction explained why Kepler's laws really worked.

Gravity was also responsible for the initial formation of our solar system. The condensation theory describes this process as starting with a large swirling cloud of cool gas and dust and culminating in an orderly and structured system of planets that orbit the Sun today. This general scenario for the formation of planets around a new star may even have led to the development of similar solar systems around other stars like our Sun in other parts of the universe.

Our current concept of the solar system places the Sun, which contains nearly 99.9% of the mass of the entire system, at the center of nine revolving planets. There are four inner or terrestrial planets (Mercury, Venus, Earth, and Mars) and four outer or Jovian planets (Jupiter, Saturn, Uranus, and Neptune), plus Pluto. Pluto is far from the Sun and does not fit into the general classification of a Jovian planet. Some astronomers feel that it more closely resembles a large asteroid, but it does fit the general definition of a planet, so it retains that classification.

All of the nine planets revolve around the Sun in a counterclockwise direction while they spin on their respective axes. The farther a planet is from the Sun, the longer it takes to complete one revolution around the Sun, as indicated by Kepler's third law. Since the nine planets orbit in nearly the same plane, they often tend to line up with the Sun as observed from Earth. A planet in direct line with and on the same side of Earth as the Sun is said to be in conjunction, whereas any planet on the opposite side of the Earth from the Sun is in opposition.

In addition to the Sun and the nine planets, our solar system contains about 60 satellites or moons revolving around the various planets. There are also thousands of asteroids, most of which revolve around the Sun between the orbits of Mars and Jupiter, plus billions of smaller bodies known as meteoroids. The mass of all of the planets, moons, asteroids, comets, and meteoroids makes up only about 0.13% of the total mass of the solar system, so the Sun is quite obviously the major component, possessing well over 99% of its total mass.


Section  15.2The Planet Earth

Early scholars generally believed that planet Earth was stationary in space and served as the center of the entire universe. Today we know that Earth is one of nine planets that revolve around a typical star, the Sun. Earth also rotates on its own axis as it travels through space. These two facts make it obvious that Earth is neither stationary nor the center of the universe.

The rotation of Earth on its axis is responsible for the regular cycle of light and darkness that we refer to as day and night. This motion can be demonstrated experimentally by letting a heavy weight swing back and forth on a long cable. Such a device is called a Foucault pendulum. Careful study shows that Earth rotates under the swinging weight by an angle of 15^° each hour or by a total of 360^° (one complete rotation) every 24 hours, when the motion is observed at the North Pole or South Pole.

Close observation of the stars nearest to Earth reveals a periodic change in their position with respect to more distant background stars. This apparent motion, known as parallax, can be attributed to the yearly revolution of Earth around the Sun. This yearly revolution is the basis for the time period known as the solar year and is a primary factor in the regularly changing seasons.

Our planet is a nearly perfect sphere with a diameter of about 6.4 x 10⁶ m, or nearly 8000 mi. Actually, Earth is slightly flattened at the poles due to its rotational motion and thus takes the shape of an oblate spheroid. This distortion, however, is so slight that an observer from space would easily mistake Earth for a perfect sphere.

As seen from space, Earth is a very bright object since it tends to reflect a large portion of the sunlight that strikes its surface. This is due to the fact that the water and clouds, which cover large portions of Earth's surface, are light-colored and reflect light easily. The fraction of incident sunlight reflected from any celestial object is called its albedo. The albedo of Earth is about 0.33, whereas that of the Moon is only 0.07. Earth appeared much brighter to the astronauts who viewed it from the Moon's surface than the Moon appears to us from here on Earth. Earth revolves around the Sun at an average distance of 1.5 x 10⁸ km, a distance defined as one astronomical unit (AU). The astronomical unit is frequently used when discussing distances related to the other planets within our solar system.


Section  15.3The Terrestrial Planets

The inner, or terrestrial, planets are all quite similar to Earth. They have iron-rich cores and rocky surfaces, and all have an atmosphere of gases surrounding them. Mercury, however, is so close to the Sun that very little gas remains near its surface, resulting in an atmospheric pressure nearly 10,000 times less than that of Earth. All of these planets are relatively small and generally rotate counterclockwise as they revolve around the Sun. Venus is the exception, rotating in a clockwise direction. Only three satellites are found orbiting the inner planets—our own Moon, which orbits Earth, and two very small moons called Phobos and Deimos, which orbit Mars.

Mercury is the closest planet to the Sun, followed in order by Venus, Earth, and Mars. Many interesting features related to each of these planets are given in the textbook and presented in a special double page Spotlight presentation. In addition to the four terrestrial planets, the asteroids are often considered part of the inner solar system. Their characteristics are presented in Section 15.5 of this chapter.


Section  15.4The Jovian Planets and Pluto

With the exception of Pluto, the Jovian, or outer, planets are all similar to the largest planet in our solar system, Jupiter. These planets are often referred to as gas giants because they are all made up primarily of hydrogen and helium gas, with only relatively small amounts of the heavier elements, most of which are concentrated in their rocky cores. These planets have much lower densities than the inner planets, although overall they contain much more mass. This means that they must, on the average, be much larger in size. The outer planets have many more satellites than the smaller inner planets. The outer planets are also characterized by ring systems composed of dust and rock fragments, the most prominent of which can be quite easily observed, even with a small telescope, around Saturn. In order outward from the Sun, the outer planets are Jupiter, Saturn, Uranus, and Neptune. Pluto not only does not resemble the other outer planets but also has the most eccentric orbit, which occasionally brings it somewhat closer to the Sun than Neptune. Thus Pluto and Neptune share the distinction of sometimes being the farthest known planets from the Sun. Fly-by missions by space probes launched from Earth have sent back new and fascinating information about the outer planets, much of which is presented in the textbook.


Section  15.5Other Solar System Objects

There are many objects in our solar system much smaller then even the smallest planet or moon. These objects are also influenced by the Sun's gravitational pull and are heated by the Sun's radiant energy. Most of these objects are much too small for us to normally see directly as individual entities even with our best telescopes, but occasionally one of them will approach the Sun, and therefore our planet Earth, and their interactions with the solar wind or the atmosphere of Earth make them spectacular sights to see. Comets and meteors are the best examples of these.

The various other objects in our solar system fall into several distinct groups including the asteroids, meteoroids and meteors, comets, and interplanetary dust. Each of these contributes to our understanding of the structure of our solar system and the processes that formed it. This Section covers these objects in some detail and points out their normal locations and orbits within the solar system, and it also describes what is currently known about their size, shape, and composition.


Section  15.6The Origin of the Solar System

Explanations of the formation of our solar system are grouped as either catastrophe theories or evolutionary theories. The theory currently accepted by most scientists comes from the evolutionary group and is called the condensation theory. In this theory a cold, swirling cloud of gas and dust was perturbed by some mechanism, such as a nearby supernova explosion, and began to collapse due to its own mutual gravitational attraction. As the cloud collapsed, its overall rate of rotation increased because of conservation of angular momentum, and a flattened disk spread out in the plane of rotation. Most of the gas and dust continued to collapse into the central region, where a gigantic protosun developed. Rotational shearing forces and density fluctuations of the remaining material in the disk led to the remaining gas and dust being collected into localized regions that developed into protoplanets and other smaller objects within our solar system.

As the protoplanets continued to orbit the newly formed Sun, their concentrated masses pulled additional nearby gas and dust together in a process called accretion. Eventually, nearly all of the free material surrounding the Sun became part of these protoplanets, which continued to evolve into the planets that make up our solar system. The continued collapse of gas and dust clouds in the central portion of our solar system produced increased temperature and density until the Sun evolved into a full-fledged star. It is generally believed that asteroids are simply remnants of material that was never pulled in to become part of a planet or moon. The comets formed far out in the original cloud and are very small aggregates of this primordial material that remains scattered about in the outer regions of the solar system in an area known as the Oort Cloud.


Section  15.7Other Planetary Systems

Because the condensation theory involves a general accretion process that could accompany the formation of any star, it is generally believed that many other planetary systems may exist throughout the universe. However, because planets are very small and do not give off light, it has so far not been possible to see any in other solar systems with even our largest telescopes. Data on pulsars indicate a variation in pulse rate that could be explained by the presence of planet-size objects near some neutron stars. The existence of other planetary systems in the universe is just beginning to be verified by data that show a small wobble superimposed on the motion of a star, caused by the gravitational interaction of that star with a large planet. This information is best found by studying Doppler shift changes in the spectra of these stars. As of this printing, around 80 stars have been found that seem to have one or more planet-size objects in orbit around them.

In addition to the direct observation of other solar systems, another approach is being tried to locate such systems. Scientists are currently scanning the skies for radio signals that may have been sent by intelligent life on planets in other solar systems. Although, no confirmed signals from other civilizations have yet been detected, our receiving technology is becoming increasingly sophisticated and programs like META and BETA will be scanning on millions of different frequencies in future years to continue the attempt to communicate with other sources of intelligent life.

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