The Planets

A. What’s in Our Solar System? Our solar system consists of a central star (the Sun), planets, dwarf planets, moons, asteroids, comets, meteors, interplanetary gas, dust, and all the “space” in between. Except for Earth, the planets of the solar system are named for Greek and Roman gods and goddesses.

The Relative Size of the Planets in the Solar System

B. Planets are categorized according to size and composition Small, rocky planets are the inner planets closest to the sun gas giants are the outer planets

Inner and Outer Planets Inner Planets: Mercury Venus Earth Mars Outer Planets Jupiter Saturn Uranus Neptune

1. Characteristics of Inner Planets They are made up mostly of rock and metal. They are very heavy. They move slowly in space. They have no rings and few moons (if any). They have a diameter of less than 13,000 km.

a. Mercury Has a revolution period of 88 days. Has 1000° temperature swings from day to night because there is no atmosphere to trap heat. Is about 1/3 of Earth’s size Is about 1/3 Earth’s distance from the sun (0.39 AU)

b. Venus Has retrograde rotation – so the sun rises in the west and sets in the east! Reaches 900F at the surface due to a strong greenhouse effect. Venus has no moons and takes 225 days to complete an orbit. Is about the same size as Earth Is about 2/3 Earth’s distance from the sun (0.72 AU)

c. Earth Is the only planet known to support life! Has a surface composed of 71% water. Water is necessary for life on Earth. Is the basis for the Astronomical Unit (distance from sun to Earth = 1 AU)

d. Mars Mars appears red because of iron oxide, or rust, in its soil. Mars has two moons and takes about two years to complete an orbit. Is about ½ of Earth’s size Is about 1 ½ times as far from the sun as Earth (1.52 AU)

2. Characteristics of Gas Giants They are made up mostly of gases (primarily hydrogen & helium). They are very light for their size. They move quickly in space. They have rings and many moons. They have a diameter of 49,000 km or greater

a. Jupiter Is the largest, most massive planet. Takes about 12 years to orbit the sun. Jupiter has 17 known moons. Is about 11 times larger than Earth. Is about 5 times farther from the sun than Earth (5.19 AU)

Has many rings made of ice that extend about 260,000 miles from the surface but are less than 1 mile thick. Has 19 known moons Takes about 30 years to orbit the sun. Is almost10 times larger than Earth Is nearly ten times farther from the sun than Earth (9.5 AU) b. Saturn

c. Uranus Has 21 known moons Takes 84 years to complete one orbit. Is about 4 times larger than Earth Is about 19 times farther from the sun than Earth (19.1 AU)

d. Neptune Takes 165 years to orbit the sun Has 8 moons Is about 3.8 times larger than Earth Is about 30 times farther from the sun than Earth (30 AU)