1. A Quick Tour of the Solar System

3. From our observations using various spacecraft and telescopes, we have learned that the eight planets have a variety of surface and atmospheric features.

4. Several planets exhibit unusual rotation. Venus has retrograde rotation. This means that the planet rotates from east to west (the reverse of Earth.) Uranus’axis is tilted at an angle of about 90°. Uranus rotates on its side.

5. Planets differ in the number of their moons. Some planets have no moons - Mercury and Venus. Others have many moons. Saturn appears to have 31 and Jupiter has 61.

6. The range of temperature extremes also varies from planet to planet.

7. The atmospheres of each planet vary as well. Only Earth and Mars have oxygen. The carbon dioxide atmosphere of Venus traps heat and produces a greenhouse effect.

8. Planets have different surface features. Mercury and Venus have mountains and cliffs Mars has polar icecaps. Mars has volcanoes. Earth, Mars, Uranus and Neptune have water.

9. Some planets have rings Jupiter, Saturn, Uranus and Neptune all have rings.

10. The farther a planet is from the sun, the less its orbital velocity. Orbital velocity means the speed at which a planet is revolving around the Sun.

11. Beyond the orbit of Neptune is an area of space we call the Kuiper belt. The Kuiper (rhymes with viper) belt is a flattish region extending from the orbit of Neptune to the orbit of Pluto or so. It contains at least 30,000 Pluto-like objects called plutinos.

12. Beyond the Kuiper belt is the outermost known area of our solar system, called the Oort Cloud. This area is spherical, and contains trillions of comet nuclei.

16. There are some other objects in the solar system. Asteroids are ”flying mountains” made of rock and metals primarily found between Mars and Jupiter in the asteroid belt. Comets are “dirty snowballs” of ice and gas and dust. Some travel from the Kuiper Belt and the Oort cloud toward the sun in an elliptical orbit. We see the tail, or cloud of dust and gas surrounding the comet’s core, as it is blown by the solar wind.

17. Chunks of metal or stone that orbit the sun are called meteoroids. They may be from the asteroid belt or from comets that have broken up. If a meteoroid falls through the Earth’s atmosphere, the streak of light it produces as it burns is called a meteor.

18. If it hits the Earth’s surface we call the meteoroid a meteorite. Some meteorites are large enough to produce craters on the Earth.

19. Certain conditions must be met for life to develop. A planet must have moderate temperatures for billions of years for life to evolve. A planet must also have liquid water.

20. Earth is in the clement zone or life zone of our star, the sun. If Earth was closer to the sun, Earth would have been too hot to support life as we know it. If Earth was more distant from the sun, it would have been covered with frozen water, and life might not have developed.