Lesson 2:How Do Objects Move in the Solar System?

Rings Around a Planet:Saturn’s rings have fascinated sky

gazers since the astronomer Galileo first saw them in 1610. We’ve learned that the rings are made of rock, gas,

and ice.

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Our Solar System A solar system is a group of objects in space that orbit a star in

the center, plus the star itself. The sun is the star in the center of our solar system.

Our solar system contains a variety of objects. These include planets and moons as well as asteroids, which are small and rocky.

A planet is a large object that orbits a star. A moon is a smaller object that orbits a planet.

In our solar system, there are EIGHT (8) planets. Pluto is now considered a dwarf planet, so there are no longer 9 planets .

Often scientists group them as the inner planets (closer to the sun) and the outer planets (farther from the sun). The groups are separated by a ring of asteroids.

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o The inner planets are those closest to the sun; they are Mercury, Venus, Earth, and Mars. These planets are alike in many ways. They all have rocky surfaces and are smaller than most of the outer planets. Also, none of the inner planets has more than two moons.

o There are also many differences—Mercury is hot enough to melt lead while Mars never gets above 68°F.

o Earth is the most unusual inner planet. Only Earth has liquid water on its surface and a large amount of oxygen in the atmosphere. This water and oxygen help support life on Earth.

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o Beyond Mars, on the far side of the asteroid belt, are the outer planets. They are Jupiter, Saturn, Uranus, and Neptune.

o These 4 planets are huge and made mostly of gases. They are often called the gas giants.

o They all have many moons, and they all are surrounded by rings that are made of dust, ice, or rock.

o Pluto is no longer considered an outer planet .

Solar System 101