Lesson 4, Chapter 3

 The four outer planets – Jupiter, Saturn, Uranus, and Neptune are much larger and more massive than Earth and they do not have solid surfaces.  They are often called the gas giants.  All the gas giants have many moons and each are surrounded by a set of rings.  A ring is a thin disk of small particles of ice and rock.

 Jupiter is the largest and most massive planet.  Jupiter has a thick atmosphere, made mostly of hydrogen and helium.  Jupiter’s atmosphere also has the Great Red Spot, which is a storm that is larger than Earth.  Astronomers think that Jupiter has a dense core of rock and iron at its center.  Jupiter has 4 moons – Io, Europa, Ganymede and Callisto.

 Saturn has the most spectacular rings of any other planet and is the second largest planet in the solar system.  Saturn’s atmosphere contains storms and clouds.  Saturn’s largest moon Titan, is larger than the planet Mercury.

 Uranus looks blue-green because of the traces of methane in its atmosphere.  Uranus is surrounded by a group of thin, flat rings.  Uranus’s axis of rotation is tilted at an angle of about 90 degrees from the vertical. Astronomers believe that billions of years ago Uranus was hit by an object that knocked it on its side.  Uranus’s five largest moons have icy, cratered surfaces. They also have lava flow on their surfaces. Uranus has at least 27 moons.

 Neptune is a cold, blue planet. Its atmosphere contains visible clouds.  Neptune was discovered as a result of mathematical predictions.  Like the Great Red Spot on Jupiter, the Great Dark spot about the size of Earth is also thought to be a giant storm on Neptune.  Astronomers have discovered at least 13 moons orbiting Neptune. The largest being Triton. Triton’s south pole is covered by nitrogen ice.

 Pluto has a solid surface and is much smaller and denser than the outer planets.  Pluto is so far from the sun that it revolves around the sun only once every 248 Earth years.  Until recently, Pluto was considered to be the ninth planet in our solar system. It is now considered to be a dwarf planet. It orbits the sun and it round in shape, but unlike a planet, a dwarf planet has not cleared out the neighborhood around its orbit.