Overview of the Solar System - The solar system is the collection of objects gravitationally bound to our Sun. In addition to the Sun itself, the solar system contains at least nine planets, their approximately 150 moons, a large number of asteroids (small, rocky bodies), and comets (small, icy bodies). These objects exist in the interplanetary medium, a sparse blend of dust and gas particles. The Sun is at the center of the solar system and contains most of its mass—a whopping 99.86 percent.
Moving outward from the Sun are, in order, the inner planets: Mercury, Venus, Earth, and Mars. Next is the main asteroid belt, which lies between the orbits of Mars and Jupiter. Then, there are the outer planets: Jupiter, Saturn, Uranus, Neptune, and, as we shall see, controversial Pluto. Beyond Neptune, containing Pluto, lies the disk-shaped Kuiper Belt (pronounced “koy-per”) of comets and assorted objects. Far beyond the Kuiper Belt is the Oort Cloud (rhymes with short), a giant cometary sphere completely surrounding the solar system. The planets exhibit a high degree of orderliness in their motions and in theirpositioning. For example, all the planets travel in elliptical orbits around the Sun.
And except for Pluto, all the planets and their larger moons follow orbits that lieroughly in the same plane. This plane, called the ecliptic, is deﬁned as the plane of Earth’s orbit. Further, all the planets and almost all of their moons orbit in the same direction counterclockwise (when viewed from the Sun’s north pole).
This is also the direction in which the Sun and almost all of the planets and their moons spin or rotate. Also, the planets are neatly divided such that the inner planets are small, solid, and rocky while the outer planets are large and gaseous. The solar system, like the interior of an atom, is mostly empty space. Figure 1 gives a hint of this but no illustration can be to scale because the planets are so tiny in comparison to the distances between them. To appreciate the true relative sizes of objects in the solar system, try this mental exercise. Imagine reducing the size of everything by a factor of a billion (Figure 2). Now Earth is 1.3 centimeters (cm) in diameter (the size of a grape). The Moon, the size of a pea, is a distance of 30 cm about a foot from Earth. The Sun is about 1.5 meters in diameter, the size of a Sumo wrestler. The distance between the Sun and Earth is 150 meters the length of one-and-a-half football ﬁelds. Jupiter is 15 cm in diameter, the size of a large grapefruit, and at a distance of about 12 football ﬁelds away from the Sun. Saturn, the size of an apple, is 15 football ﬁelds away from the Sun.
Tiny outermost “almost-a-planet” Pluto is about the size of an apple seed and is located at a distance of about 60 football ﬁelds from the Sun. Because distances in the solar system are so great, astronomers use the astronomical unit to measure them. One astronomical unit (AU) is about kilometers (about ) or the distance from Earth to the Sun. Table 1 gives the distances of planets from the Sun in kilometers (km) as well as in AU. The data in Table 1 also shows the division of the planets into two groups with similar properties. The inner planets Mercury, Venus, Earth, and Mars are solid and relatively small and dense. For this reason they are often called the “terrestrial planets.” The outer planets are large, have many rings and satellites, and are composed primarily of hydrogen and helium gas. The outer planets are often referred to as the “Jovian planets” because they resemble Jupiter in terms of their large sizes and gaseous compositions. 9.3 x 107 mi 1.5 x 108
|Table 1 Planetary Data|
The Scale of the Solar System
Astronomical distances are mind-boggling. Try the following problems to better appreciate the sizes of bodies in the solar system and distances between them. (Use the distance formula: 27.1.) distance = speed x time and data from Table
1. The distance between Earth and the Moon is 384,401 km. How many Earth-diameters would ﬁt between Earth and the Moon?
2. How long would it take to drive from Earth to the Moon if you drive at 55 miles per hour? State your answer in hours and in years.
3. If you could ﬂy to the Sun on a jet that moves at 1000 km/h, how long would it take? State your answer in years.
4. The diameter of the Sun is 1,390,000 km. What is its diameter stated in units of AU? How many times greater is the mean distance between Earth and the Sun compared to the diameter of the Sun?
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References and Further Reading
Conceptual Integrated Science
Paul G. Hewitt, City College of San Francisco
Suzanne A Lyons, California State University, Sacramento
John A. Suchocki, St.Michael''s College
Jennifer Yeh, University of California, San Francisco