PART 1JOBJECTS IN THE SOLAR SYSTEM 4.1 INTRODUCTION Besides the Sun, the central object of our solar system, which is a star and will be dis- cussed in more detail in Chapter 11, there are basically three types of objects in our so- lar system: planets, moons, and debris. Solar system debris is the collective term used for objects that have not become part of a planet or a moon: asteroids, comets, and me- teors. These objects will be discussed in detail in Chapter 6. Moons are objects that or- bit planets and there are two types of planet. 4.2 PLANET TYPES One of the best ways to study planets is to investigate their properties, and based on these properties compare the objects to one another. This is known as comparative planetology. The properties of planets are the quantities that we can measure such as physical properties like size, mass, and density or their orbital properties like distance from the Sun, orbital or revolution period, and rotational period. Table 4.1 lists the val- ues of these and other properties for known planets and several other objects in our so- lar system. Figure 4.1 shows bar graphs or histograms comparing the radii (the radius is the distance from the center of a planet to its edge) or size of each object listed in Table 4.1, their mass (of how much matter each is composed), and their density. Density is a com- bination of mass and size. It is a measure of how much mass per unit volume there is in something. Solids like rocks and metals are objects of high density, gases like air are of low density; and liquids like water are in between. 63 64 Part 2 The Solar System Table 4.1 Ⅲ Planetary Data Orbital Object Radius Mass Density Orbital Radius Period Rotation Period Number of Name Earth=1 Earth=1 Water=1 (AU) (Years) Earth=1 Moons 1 Mercury 0.382 0.055 5.43 0.387 0.2409 58.6 0 2 Venus 0.949 0.815 5.25 0.723 0.6152 243 0 3 Earth 1 1 5.52 1 1 0.9973 1 4 Mars 0.533 0.107 3.93 1.524 1.881 1.026 2 5 Jupiter 11.19 317.9 1.33 5.203 11.86 0.41 67 6 Saturn 9.46 95.18 0.7 9.539 29.42 0.44 62 7 Uranus 3.98 14.54 1.32 19.19 84.01 0.72 27 8 Neptune 3.81 17.13 1.64 30.06 164.8 0.67 13 9 Pluto 0.181 0.0022 2.05 39.48 248 6.39 5 10 Eris 0.183 0.0028 2.52 67.67 561 15.8 1 Planetary Radii 12 10 8 6 Radius Radius (Earth=1) 4 2 0 12345678910 Object Created with Graphical Analysis 3 by Vernier Software Vernier 3 by Analysis with Graphical Created FIGURE 4.1 Bar graphs comparing planetary radii, masses, and densities. Chapter 4 Solar System Overview 65 Planetary Masses 350 300 250 200 Mass 150 Mass (Earth=1) 100 50 0 12345678910 Object Created with Graphical Analysis 3 by Vernier Software Vernier 3 by Analysis with Graphical Created Planetary Densities 6 5 4 3 Density 2 Densities (water=1) 1 0 12345678910 Object Created with Graphical Analysis 3 by Vernier Software Vernier by 3 Analysis Graphical with Created FIGURE 4.1 (cont’d) 66 Part 2 The Solar System Examination of the radii bar graph shows that there are different-size objects (the numbers on the bar graphs match the numbers in Table 4.1). Objects 5 and 6, Jupiter and Saturn are very large compared to objects 7 and 8, Uranus and Neptune that are more medium in size while Earth and all the others are very small by comparison. The mass bar graph shows that Jupiter is by far the most massive, then Saturn with Uranus and Neptune the only others that even register on the graph. Perhaps at this point Jupiter, Saturn, Uranus, and Neptune could be considered a group of large and massive planets while Earth and all the others could be called small and less massive planets. Courtesy of NASA. of Courtesy FIGURE 4.2 Images of our solar system’s planets. The density graph shows something different. Now Mercury, Venus, Earth, and Mars have large values or high densities while the larger, massive planets and Pluto and Eris all have lower densities. The higher densities are because Mercury, Venus, Earth, and Mars are all made mostly of rocks and metals; while the lower-density objects, Jupiter, Saturn, Uranus, and Neptune, are made mostly of gases and liquids. Pluto and Eris, being so far from the Sun, are composed partly of ice. At this point it is possible to distinguish between two types of planets. Mercury, Ve- nus, Earth, and Mars are all small, of lower mass and higher density while Jupiter, Sat- urn, Uranus, and Neptune are all the opposite—large, higher mass, and lower density. Collectively the planets that are grouped with Earth are called Earth-like or terrestrial planets; those grouped with Jupiter are called Jupiter-like or Jovian planets. Notice that Pluto and Eris do not fit with either category. Courtesy of NASA. of Courtesy FIGURE 4.3 The Planets in order of their distance from the Sun. Chapter 4 Solar System Overview 67 Looking at other data from Table 4.1, all the terrestrial planets are closer to the Sun and therefore have faster orbital periods while the Jovian planets are the opposite, far- ther from the Sun with longer orbital periods. Rotational periods do not seem to fit the categories as well. All of the Jovian planets have rotation periods similar to each other and all the terrestrial planets have longer periods, but as can be seen from Table 4.1, Ve- nus and Mercury have especially long periods. Again, note Pluto and Eris not fitting in either category. They are far from the Sun like the Jovian planets but have longer rota- tion periods like the terrestrial planets. Also, due to their large mass and therefore greater gravitational pull, the Jovian planets all have many moons and rings. Rings are tremendous numbers of smaller particles all in similar orbits around a Jovian planet causing the appearance of a ring around a planet when viewed from a distance. Ring systems will be discussed in more detail in Chapter 8. Table 4.2 is a comparison of the properties of the terrestrial and Jovian planets. Table 4.2 Ⅲ Properties of Terrestrial and Jovian Planets Terrestrial Planets Jovian Planets Members Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Size Smaller Larger Mass Low mass Great mass Density High Low Composition Rock and Metal Gas and Liquid Distance Close to Sun Far from Sun Rotation Slower Faster Moons Few or none Many Rings No Yes 4.3 THE KUIPER BELT As observed several times, Pluto and Eris do not fit into either of the major planet cate- gories and could in fact be classified together as very small, low mass, icy-rocky objects (thus their medium density) that are very far from the Sun. These are precisely the char- acteristics of the objects in what is known as the Kuiper belt. First proposed by Gerard Kuiper in 1951, many small icy objects, which have also been called “trans-Neptunian” objects and “ice dwarfs,” have now been observed be- yond the orbit of Neptune. There are thousands of Kuiper belt objects known to exist including several discovered more recently that rival the size of Pluto such as Eris that may be as large or larger than Pluto. In the summer of 2006, Pluto lost its status as one of the solar system’s planets. The International Astronomical Union (IAU) is a group of astronomers from throughout the world that meets every third year and makes such decisions. They reclassified Pluto, along with Ceres, the largest member of the inner solar system asteroid belt located be- tween Mars and Jupiter and several other objects large enough to be spherical, as dwarf planets. The asteroid belt, Pluto, and other Kuiper belt objects will be discussed in more detail in Chapter 6. 68 Part 2 The Solar System CHAPTER 4 PART 1JTERMINOLOGY Comparative planetology Kuiper belt Planet Debris Mass Jovian Density Moon Terrestrial Dwarf planet Radius PART 2JTHE FORMATION OF THE SOLAR SYSTEM 4.4 THE SOLAR NEBULA Our Sun was formed by a gravitational collapse within a gigantic cloud of mostly hy- drogen gas and dust in the otherwise nearly empty interstellar space between the stars in our galaxy. The leftover material surrounding the not yet shining protosun was called the solar nebula. Eventually the protosun accumulated enough material from the neb- ula to become massive enough to put sufficient pressure on its core to raise the core temperatures high enough for nuclear fusion to occur. This process provided the energy necessary for the Sun to give off light and heat or to shine and thus become a star. The processes of star formation and nuclear fusion will be discussed in more detail in Chap- ter 11. Courtesy of NASA. of Courtesy FIGURE 4.4 Solar nebulae around protostars. 4.5 THE ROCKK METAL CONDENSATION LINE The leftovers of the solar nebula were the material from which the planets of our solar system would form. Initially, temperatures were so hot that most of the solar nebula re- mained gaseous but as the young Sun cooled, temperatures reached a point where solid rocks and metals could begin to condense out of the nebula.