1. THE INNER PLANETS

2. The four inner planets are small and dense and have rocky surfaces
The four inner planets are small and dense and have rocky surfaces. The inner planets are often called “terrestrial planets,” from the Latin word “terra” which means “Earth.”

3. Mercury
Mercury is the smallest terrestrial planet and the planet closest to the sun. It is not much larger than Earth’s moon and has no moons of its own. The inside of Mercury is most likely made up of the dense metal iron.

4. Mercury has many flat plains and craters on its surface
Mercury has many flat plains and craters on its surface. Mercury also has virtually no atmosphere due to its high daytime temperatures. The side of Mercury that faces the sun can reach temperatures of 430 degrees Celsius. However, because Mercury has no atmosphere, at night its heat escapes into space causing temperatures to drop to -170 degrees Celsius.

5. Venus
Venus is similar in mass and size to Earth that it is known as “Earth’s twin.” Although Venus and Earth have similar internal structures and densities, they are very different in other ways.

6. Venus’s atmosphere is so thick that it is always cloudy there
Venus’s atmosphere is so thick that it is always cloudy there. These clouds are mostly made of sulfuric acid. The pressure of Venus’s atmosphere is 90 times greater than Earth’s, which means you would be crushed by it if you were to stand on Venus’s surface. You could not breathe on Venus because its atmosphere is mostly carbon dioxide (which is what WE breathe out!).

7. At 460 degrees Celsius, Venus has a surface temperature hot enough to melt lead. Its surface has many volcanoes and broad plains formed by lava flows.

8. Earth
Earth has 3 main layers: the crust, mantle, and core. The crust includes the solid, rocky surface. Under the crust is the mantle, a layer of hot molten rock. Earth has a dense core made of mainly iron and nickel. The outer core is liquid, but the inner core is solid.

9. Earth is unique in our solar system in having liquid water at its surface. About 70 percent of Earth’s surface is water.

10. Earth has enough gravity to hold on to most gases
Earth has enough gravity to hold on to most gases. These gases make up Earth’s atmosphere, which extends more than 100 kilometers above its surface. Although some other planets also have atmospheres, Earth is the only one that is rich in oxygen. The rest of its atmosphere is mostly nitrogen.

11. Mars
Mars is called the “red planet” because it has a reddish color in the sky. The red color is caused by the breakdown of iron-rich rocks, which causes a rusty dust that covers Mars’s surface.

12. Mars’s atmosphere is more than 95 percent carbon dioxide (similar to Venus’s atmosphere, but much thinner), and contains few clouds. Because Mars has a transparent atmosphere, its surface is easily viewed from Earth through a telescope. Mars has temperatures that range from -140 degrees Celsius to 20 degrees Celsius.

13. Scientists think that a large amount of liquid water flowed on Mars’s surface in the distant past. For this to be true, Mars must have been much warmer and had a thicker atmosphere at that time. Today, liquid water cannot exist on Mars for very long. Since Mars’s atmosphere is so thin, liquid water would quickly turn into gas. Much of the water on Mars exists in its polar ice caps.

15. The surface of Mars is barren and covered with craters like the moon
The surface of Mars is barren and covered with craters like the moon. Some regions of Mars have giant volcanoes. Astronomers see signs that lava flowed from the volcanoes in the past, but the volcanoes are no longer active.

16. THE OUTER PLANETS

17. The first four outer planets- Jupiter, Saturn, Uranus, and Neptune- are much larger and more massive than Earth, and they do not have solid surfaces. Because these planets are so large, they are often called “gas giants.”

18. Like the sun, the gas giants are composed mainly of hydrogen and helium. Because they are so massive, the gas giants exert a much stronger gravitational force than the terrestrial planets. These planets have thick atmospheres because gravity keeps these gases from escaping into space.

19. All of the gas giants have many moons and are surrounded by a set of rings. A ring is a thin disc of small particles of ice and rock.
RINGS
MULTIPLE MOONS

20. JUPITER
Jupiter is the largest and most massive planet.

21. Jupiter has a thick atmosphere made up mainly of hydrogen and helium
Jupiter has a thick atmosphere made up mainly of hydrogen and helium. A well known feature on Jupiter’s surface is its great red spot, a storm that is larger than Earth! Similar to a hurricane on Earth, Jupiter’s storm has swirling winds that blow up to hundreds of kilometers per hour. On Earth, hurricanes usually weaken as they pass over land. Jupiter has no land to weaken the storm.
GREAT RED SPOT

22. Astronomers think that Jupiter has a dense core of rock and iron at its center. A thick mantle of liquid hydrogen and helium surrounds this core.

23. Jupiter has four large moons- Io, Europa, Ganymede, and Callisto
Jupiter has four large moons- Io, Europa, Ganymede, and Callisto. They are all larger than Earth’s moon, but are all very different from each other. Jupiter also has dozens of small moons that have been discovered in the past few years.

24. SATURN
Saturn is the second largest planet in the solar system. Like Jupiter, Saturn has a thick atmosphere made of mostly hydrogen and helium. Saturn’s atmosphere also contains storms and clouds, but they are not as big as Jupiter’s.

25. Saturn has rings around it that are made of chunks of ice and rock, each traveling in its own orbit around Saturn. Saturn has the most spectacular rings of any planet. Saturn’s rings are broad and thin, like a CD.

26. Saturn’s largest moon, Titan, is larger than the planet Mercury
Saturn’s largest moon, Titan, is larger than the planet Mercury. Four other moons of Saturn are each over 1000 kilometers in diameter!

27. URANUS
Uranus is about 4 times the diameter of Earth, but is still much smaller than Jupiter and Saturn. Uranus is a very far from the sun, making it colder than Saturn, and is surrounded by a group of thin, flat rings that are much darker than Saturn’s rings.

28. Uranus has few clouds on its surface and rotates in about 17 hours (Earth rotates every 24 hours). Uranus’s axis of rotation is tilted at an angle of about 90 degrees from the vertical (top to bottom rather than side to side).
CLOUDS ON URANUS

29. Uranus’s five largest moons have icy, cratered surfaces
Uranus’s five largest moons have icy, cratered surfaces. They also have lava flows on their surfaces, suggesting that material has erupted from inside each moon at some time. Astronomers have recently discovered more moons around Uranus, for a total of at least 27.
URANUS

30. NEPTUNE
Neptune is a cold, blue planet. Its atmosphere contains visible clouds. Scientists think that Neptune is slowly shrinking, causing the interior to heat up. As this energy rises to Neptune’s surface, it produces clouds and storms in the planet’s atmosphere.

31. Like Jupiter’s giant red spot, Neptune also had a great dark spot at one time. The great dark spot was probably a storm; however the storm did not last very long. Since then, other small spots and regions of clouds on Neptune also seem to come and go.

32. Astronomers have discovered at least 13 moons orbiting Neptune
Astronomers have discovered at least 13 moons orbiting Neptune. The largest moon, Triton, has a thin atmosphere. A region near Triton’s south pole is covered by nitrogen ice.