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Lesson 4-01 Solar System Overview

Page history last edited by Chai Nakpiban 10 years, 8 months ago


LESSON 4.01 – Solar System Overview

Standard:  ES1.a Students know how the differences and similarities among the sun, the terrestrial planets, and the gas planets may have been established during the formation of the solar system.




In this lesson you will learn that a solar system is a group of heavenly bodies consisting of a star and the planets and other objects orbiting around it. We are most familiar with our own solar system, which includes Earth, seven other major planets, and the sun. Our solar system also includes many smaller objects that revolve around the sun, such as dwarf planets, meteoroids, and comets; and a thin cloud of gas and dust known as the interplanetary medium. More than 100 moons, also called satellites, orbit the planets.



An orbiting solar telescope known as the Solar and Heliospheric Observatory (SOHO) studies the sun's interior, its atmosphere, and the solar wind, a stream of electrically charged particles that flow from the sun's surface. The European Space Agency launched the telescope in 1995. Image credit: NASA/ESA/Solar & Heliospheric Observatory

Besides the sun, Earth, and Earth's moon, many objects in our solar system are visible to the unaided eye. These objects include the planets Mercury, Venus, Mars, Jupiter, and Saturn; the brightest asteroids; and occasional comets and meteors. Many more objects in the solar system can be seen with telescopes.




The sun is the hub of a huge rotating system of eight (possibly nine) planets, their satellites, and numerous smaller bodies. An estimated 99.85 percent of the mass of our solar system is contained within the sun. The planets collectively make up most of the remaining 0.15 percent. As Figure 1 shows, the planets, traveling outward from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (now considered a dwarf planet; not a true planet).




Figure 1 Orbits of the Planets The positions of the planets are shown to scale along the bottom of the diagram.



Guided by the sun’s gravitational force, each planet moves in an elliptical orbit, and all travel in the same direction. The nearest planet to the sun—Mercury—has the fastest orbital motion at 48 kilometers per second, and it has the shortest period of revolution. By contrast, the most distant planet, Pluto, has an orbital speed of 5 kilometers per second, and it requires 248 Earth-years to complete one revolution.

Imagine a planet’s orbit drawn on a flat sheet of paper. The paper represents the planet’s orbital plane. The orbital planes of seven planets lie within 3 degrees of the plane of the sun’s equator. The other two, Mercury and Pluto, are inclined 7 and 17 degrees, respectively.



The Planets: An Overview

Careful examination of Table 1 shows that the planets fall quite nicely into two groups.



The terrestrial planets—Mercury, Venus, Earth, and Mars—are relatively small and rocky. (Terrestrial = Earth-like.) The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are huge gas giants. (Jovian = Jupiter-like.) Small, cold Pluto does not fit neatly into either category.





Size is the most obvious difference between the terrestrial and the Jovian planets. The diameter of the largest terrestrial planet, Earth, is only one-quarter the diameter of the smallest Jovian planet, Neptune. Also, Earth’s mass is only 1/17 as great as Neptune’s. Hence, the Jovian planets are often called giants. Because of their distant locations from the sun, the four Jovian planets and Pluto are also called the outer planets. The terrestrial planets are closer to the sun and are called the inner planets. As we shall see, there appears to be a correlation between the positions of these planets and their sizes.



Density, chemical makeup, and rate of rotation are other ways in which the two groups of planets differ. The densities of the terrestrial planets average about five times the density of water. The Jovian planets, however, have densities that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density only 0.7 times that of water, which means that Saturn would float if placed in a large enough water tank. The different chemical compositions of the planets are largely responsible for these density differences.




The Interiors of the Planets

The planets are shown to scale in Figure 2. The substances that make up the planets are divided into three groups: gases, rocks, and ices. The classification of these substances is based on their melting points.



Figure 2 The planets are drawn to scale. Interpreting Diagrams How do the sizes of the terrestrial planets compare with the sizes of the Jovian planets?



1.    The gases—hydrogen and helium—are those with melting points near absolute zero (−273°C or 0 kelvin).



3.    The rocks are mainly silicate minerals and metallic iron, which have melting points above 700°C.



5.    The ices include ammonia (NH3), methane (CH4), carbon dioxide (CO2), and water (H2O). They have intermediate melting points. For example, H2O has a melting point of 0°C.



The terrestrial planets are dense, consisting mostly of rocky and metallic substances, and only minor amounts of gases and ices. The Jovian planets, on the other hand, contain large amounts of gases (hydrogen and helium) and ices (mostly water, ammonia, and methane). This accounts for their low densities. The outer planets also contain substantial amounts of rocky and metallic materials, which are concentrated in their cores.



The Atmospheres of the Planets

The Jovian planets have very thick atmospheres of hydrogen, helium, methane, and ammonia. By contrast, the terrestrial planets, including Earth, have meager atmospheres at best. A planet’s ability to retain an atmosphere depends on its mass and temperature, which accounts for the difference between Jovian and terrestrial planets.



Simply stated, a gas molecule can escape from a planet if it reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers per second. Any material, including a rocket, must reach this speed before it can escape Earth’s gravity and go into space.



A comparatively warm body with a small surface gravity, such as our moon, cannot hold even heavy gases, like carbon dioxide and radon. Thus, the moon lacks an atmosphere. The more massive terrestrial planets of Earth, Venus, and Mars retain some heavy gases. Still, their atmospheres make up only a very small portion of their total mass.



In contrast, the Jovian planets have much greater surface gravities. This gives them escape velocities of 21 to 60 kilometers per second—much higher than the terrestrial planets. Consequently, it is more difficult for gases to escape from their gravitational pulls. Also, because the molecular motion of a gas depends upon temperature, at the low temperatures of the Jovian planets even the lightest gases are unlikely to acquire the speed needed to escape.




  1. Take notes on the above information.
  2. Turn in your notes.
  3. Play this (http://www.nasa.gov/audience/forkids/kidsclub/flash/games/levelfive/KC_Solar_System.html) game
  4. View a interactive activity:**
    1. Click on Geode
    2. Click on Astronomy
    3. Click on “A. The Planets: An Overview”


Take the 4.01 Quiz


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