Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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Chapter 10

Read Chapter 10 from Discovering Astronomy.


WebAssign -- Chapter 7. Answer homework questions covering Chapter 7.

The gas giants, or Jovian planets, are Jupiter, Saturn, Uranus, and Neptune. Compared to the terrestrial planets, these worlds are larger and further apart. I guess you can say that compared to the terrestrial planets, the gas giants are "worlds apart."

(Picture of Jupiter taken by Cassini in 2000)


Jupiter is the largest planet in the solar system. It's mass is one thousandth of the mass of Sun. (Remember that in the Moons of Jupiter lab, you measured its mass and found it to be about 0.001 solar masses.)

It has beautiful bands and a giant storm (called the Great Red Spot) in its atmosphere. The storm is larger in diameter than Earth. Jupiter's atmosphere is 90% hydrogen and 10% helium (and trace amounts of methane).

Jupiter has 63 moons, 23 of which have been discovered since 2003. Its largest four moons are called the Galilean moons and are Io, Europa, Ganymede, and Callisto.

(Picture taken with the Hubble Space Telescope in 2004)


All of the gas giants have rings, but Saturn's rings are the most spectacular. Rings with this much mass might be a unique phenomenon in the universe. Because the rings are so bright (meaning that they contain little dust), it's plausible that Saturn's rings are relatively young, perhaps a few hundred million years old.

The reason is that small moons, called shepherd satellites, that are in the rings are gaining angular momentum. Due to a fundamental law of physics called conservation of momentum, the material in the rings are losing angular momentum. Perhaps within a few hundred million years, much of the ring material will lose enough angular momentum that it will fall into Saturn.

The material in the rings ranges in size from dust-sized particles to house-sized boulders.

(Picture taken by Voyager 2 in 1986.)


Uranus' axis of rotation is tilted only 8° from the ecliptic plane. That means that it's rotating on its side! Its blue-green color is caused by an abundance of methane (CH4) in its atmosphere.

Uranus has rings which were first discovered when Uranus occulted (i.e. passed in front of) a star. Astronomers observed that the brightness of the star diminished just before and just after the planet passed in front of the star. This indicated the presence of rings. Later, the rings were photographed by Voyager 2.

(Picture taken by Voyager 2 in 1989.)


Neptune's discovery is an amazing story. Scientists observed Uranus' orbit and found that it deviated from what was theoretically calculated based on the gravitational pulls of Sun and the other planets. As a result, it was proposed that another planet existed which caused the deviations in Uranus' orbit, and its position was actually predicted.

After it was predicted in 1845, it was then quickly discovered in 1846. Neptune's period of revolution about Sun is 146 years. Thus, it has only orbited one time since it was first discovered.