In this topic you will learn about how members of the solar system are similar to one another.

The planets Jupiter, Saturn, Uranus, Neptune, and Pluto are called the outer planets. Scientists distinguish these from the inner planets, Mercury, Venus, Earth, and Mars.

The outer planets share many properties. They tend to be much larger than the inner planets. There is a gap—the asteroid belt—that separates the inner planets from the outer planets. The outer planets also tend to rotate very rapidly. Finally, their interior structure is different from the inner planets. With the exception of Pluto, the outer planets tend to have a small solid core, surrounded by a thick gaseous atmosphere.

A comet is a ball of ice and rock that orbits the Sun. Comets come from the outer fringes of the solar system. One place where comets originate is a region just outside Pluto's orbit stretching to about 45 billion kilometers out. This region probably contains 40,000 to 70,000 objects with diameters of more than 100 kilometers. The asteroid belt has about 230 objects of more than 100 kilometers in diameter.

As a comet approaches the Sun, sunlight begins to warm the chunks of ice and rock. The comet's ice begins to thaw and forms a cloud surrounding the nucleus, or core, of the comet. As the comet gets closer to the Sun, the sunlight exerts pressure on the cloud. The pressure from the sunlight drives this cloud of material away from the nucleus, forming the comet's tail. Part of the tail always points away from the Sun.

Asteroids are rocky metallic objects in orbit around the Sun. There are also small asteroids, which are often called meteoroids. Some travel to the edge of the solar system. Others spend their time within the orbits of the inner planets. Sometimes a meteoroid collides with the atmosphere of a planet, such as Earth.

When a meteoroid hits Earth's atmosphere, the rubbing, or friction, causes the meteoroid to burn. At this stage the meteoroid is called a meteor. We usually see meteor as a bright streak of light. Most times a meteor burns up completely. Any part of a meteoroid that reaches Earth's surface is called a meteorite.