1. Section 4: The Outer Planets

2. Objectives  After completing the lesson, students will be able to...  Describe the main characteristics of the gas giant planets;  Compare Pluto with the other planets

3. Structure of the Gas Giants  The first four outer planets—Jupiter, Saturn, Uranus, and Neptune—are very large and do not have solid surfaces.  Gas Giants—The name given to the first four outer planets.

4. Atmosphere  The gas giants exert a much stronger gravitational force than terrestrial planets due to having so much mass.  Their gravity keeps gases from escaping, which causes them to have deep atmospheres.  The composition of their atmosphere is very similar to the sun.  None of the gas giants have a solid surface.

5. Solid Core  Astronomers think that each of the gas giant planets has a partly solid core made of rock, ice, frozen carbon dioxide, and other compounds.  Each core has a greater mass than Earth, but are hard to study due to being buried so deep inside the planets.

6. Jupiter  Jupiter is the most massive planet and is more than 300 times as massive as Earth.  Atmosphere:  Thick  Made up of hydrogen and helium  Great Red Spot—swirling cloud of gas thought to be an ongoing storm, much like a hurricane

7. Jupiter’s Moons  Read this section on pages 72-73.

8. Saturn  Saturn is the second largest planet in the solar system.  Like Jupiter, it has a thick atmosphere made up mostly of hydrogen and helium.  Saturn is the only planet that is less dense than water ... That means it would float on water

9. Saturn’s Rings  Saturn’s rings are made up of chunks of ice and rock, each traveling in its own orbit.  In all, Saturn has hundreds of rings.  They are broad and thin and can be seen from different angles at times.  While Saturn isn’t the only planet with rings, they are the most spectacular of them all.

10. Saturn’s Moon  Read this section on page 74.

12. Uranus  Uranus is about four times the diameter of Earth.  It has a bluish look, because of the traces of methane in its atmosphere.

13. Uranus Discovery of UranusExploring Uranus  Uranus was discovered in 1781 by astronomer William Herschel.  It was first thought to be a comet, but was soon proven to be a planet due to the calculations of its orbit.  Uranus rotates on its axis in about 17 hours.  Its axis is tilted at an angle of about 90 degrees from the vertical.  From Earth, it appears that Uranus is rotating from top to bottom.  It is hypothesized that a large object hit Uranus and caused it to “tip” over.

14. Uranus’ Moons  The five largest moons orbiting Uranus have icy, cratered surfaces.  Other’s have traces of lava flow, which suggests that material has erupted from inside each moon.  There are a total of 20 moons orbiting Uranus.

15. Neptune  Neptune is 30 times Earth’s distance from the sun.  Neptune was discovered as a result of a mathematical prediction.  This stemmed from Uranus not following a predicted orbit.  In 1846, mathematicians calculated the orbit of the “new” planet—months later, an observer in Germany actually located it.

16. Neptune Exploring NeptuneNeptune’s Moons  In 1989, photographs recorded a Great Dark Spot (much like Jupiter’s Red Spot), and was thought to be a storm.  5 years later, the spot was gone.  Smaller spots and regions of clouds come and go from time to time.  Astronomers have discovered eight moons revolving around Neptune.  Triton is Neptune’s largest moon, and is covered with a cap of ice near its south pole.

17. Pluto and Charon  Pluto is not considered a gas giant.  It is closer to the size of the terrestrial planets.  Pluto and its single moon, Charon, have solid surfaces and masses much less than that of Earth.  Pluto and Charon are so far away from the sun that they revolve only once every 248 Earth years.

18. Pluto and Charon Discovery of Pluto and Charon Is Pluto Really a Planet?  Pluto was discovered by an American astronomer, Clyde Tombaugh in 1930.  Charon was discovered in 1978 by astronomer James Christy.  Since Pluto is so small, many astronomers do not consider it a planet—instead, they see it as being one of many small objects revolving around the sun past Neptune.  Had astronomers discovered the many other small objects before Pluto, it more than likely would not have been classified as a planet.