Section 4: The Outer Planets

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

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.

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.

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.

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

Jupiter’s Moons  Read this section on pages

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

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.

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

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

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.

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.

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.

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.

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.

Pluto and Charon Discovery of Pluto and Charon Is Pluto Really a Planet?  Pluto was discovered by an American astronomer, Clyde Tombaugh in  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.