Our Solar System...

 Astronomers now recognize eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects (See the figure on the following page). These objects move in regular and predictable 

paths around the Sun. 

Distances in the Solar System

Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). 

Listed below is the distance from the Sun to each planet in AU (Tablebelow). The table shows how long it takes each planet to spin once on its axis. It also shows how long it takes each planet to complete an 

orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Our solar system is about 100,000 AU across from the Sun to the Oort Cloud or 1.87 light-years.

The inner planets are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. The figure below shows the relative sizes of these four inner planets.

Unlike the outer planets, which have many satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically 

active, at least early in their history. None of the inner planets hasrings. The inner planets are generally smaller than their outer planet relatives.

The four planets farthest from the Sun are the outer planets. The figure on page 31 shows the relative sizes of the outer planets and the Sun. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants.The Milky Way Galaxy is our galaxy. Home, sweet home. The Milky Way is made of millions of stars along with a lot of gas and dust. It looks different from other galaxies because we are looking at the main disk from within the galaxy. Astronomers estimate that the Milky Way 

contains 200 billion to 400 billion stars and is 100 – 120 million light-years across.