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27.1 KEY IDEA The four planets closest to the sun all have a rocky crust, dense mantle layer, and very dense core. KEY VOCABULARY • inner planets • outer planets CLASSZONE.COM Examine the vast distances between planets in the solar system. Keycode: ES2701 The Inner Planets The sun is at the center of the solar system. Around it orbit nine planets, each with unique characteristics. Although astronomers have known about the existence of other planets for centuries, it’s only within the last few decades that technology has given scientists an up-close look at our nearest neighbors in space. Two Planetary Neighborhoods The planets in our solar system are divided into two groups. The four nearest the sun—Mercury, Venus, Earth, and Mars—are called the inner planets. All of the inner planets have rocky crusts, dense mantle layers, and very dense cores. Because of their earthlike characteristics, these planets are sometimes called the terrestrial (earthlike) planets. Just beyond the orbit of Mars is a belt of small bodies called asteroids. This asteroid belt separates the inner planets from the outer planets, Jupiter, Saturn, Uranus, Neptune, and Pluto. The first four of these planets, called the Jovian, or Jupiter-like, planets, are considerably larger than Earth. They are gaseous, with an outer layer that is mostly hydrogen gas. Closer to the planet’s center, the hydrogen is compressed to a hot liquid. The Jovian planets are much less dense than Earth, and all have ring systems. Pluto is the oddity of the solar system. It is not dense enough to be considered a terrestrial planet, and it is too small to be a Jovian planet. You will read more about the outer planets in the next section. THE SOLAR SYSTEM This illustration shows the relative sizes of the planets but does not represent the distance between them. The Inner Planets The Asteroid Belt Mercury 588 Venus Earth Mars