1. O UR S OLAR S YSTEM

2. T HE I NNER P LANETS Mercury, Venus, Earth, and Mars are more similar to each other than they are to the five outer planets. The four inner planets are small and have rocky surfaces. These planets are often called the terrestrial planets, from the Latin word terra, or “earth.”

3. T HE O UTER P LANETS T he first four outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger than Earth, and do not have solid surfaces. Because these four planets are all so large, they are also called the gas giants. The fifth planet, Pluto, is small and rocky like the terrestrial planets.

4. Because the gas giants have so much mass, they have a stronger gravitational force than the terrestrial planets. This prevents their gases from escaping. The composition of their atmospheres is about 75 percent, on average, hydrogen, 24 percent helium, and 1 percent other elements. None of the giant planets has a solid surface, and astronomers think that each has a partly solid core.

5. S TAR S YSTEMS AND G ALAXIES Our solar system has only one star, the sun. But more than half of all stars are members of groups of two or more stars, called star systems. Star systems with two stars are called double stars or binary stars. Those with three stars are called triple stars. The Milky Way is the galaxy in which our solar system is located. Like other galaxies, it has single stars, double stars, star systems, and lots of gas and dust between the stars.

6. The Milky Way Galaxy looks milky or hazy because stars are too close together for your eyes to see them individually. The dark blotches in the Milky Way are clouds of dust that block light from behind them. Astronomers have classified galaxies into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies.

7. A spiral galaxy is a galaxy with arms that spiral outward, like a pinwheel. Elliptical galaxies look like flattened balls. They contain billions of stars but have little gas and dust between stars. Some galaxies do not have regular shapes, so they are known as irregular galaxies.