Uranus was discovered in 1781, but its rings were not found until 1977 Uranus was discovered in 1781, but its rings were not found until 1977. Like Uranus itself, the rings were discovered by accident. This false-color infrared image was taken by the Hubble Space Telescope. Red Sea

Gas Giants How are the gas giants similar? The four gas giants are thought to have small, dense cores and thick atmospheres that are mostly hydrogen and helium.

Gas Giants Jupiter, Saturn, Uranus, and Neptune are very different from the terrestrial planets. They are much colder because of their distance from the sun. All four planets are much larger and more massive than the terrestrial planets. They are often referred to as the gas giants because they are composed mainly of hydrogen and helium.

Gas Giants None of the gas giants have solid surfaces. If you tried to land a spaceship on one, you would sink into the planet until the intense pressure of the atmosphere crushed the ship. In fact, the pressure is so great inside each planet that hydrogen and helium exist mostly in liquid form.

Gas Giants Gas giants have many moons, most of which revolve in the same direction that the planets rotate. Each of the gas giants is surrounded by rings. A ring is a disk made of many small particles of rock and ice in orbit around a planet. Rings are so close to the planet that gravitational forces are very large, which may have prevented the ring particles from clumping together to form moons.

Gas Giants The outer planets consist of the four gas giants. Pluto, formerly considered a planet, was reclassified as a dwarf planet in 2006.

Jupiter What are the characteristics of Jupiter? Jupiter is the largest and most massive planet in our solar system.

Jupiter Jupiter’s Atmosphere Jupiter’s clouds move rapidly because Jupiter itself rotates rapidly—one day on Jupiter is less than 10 Earth hours. The brown cloud bands are colder and move faster around the planet than the white bands.

Jupiter Sometimes storms occur at the boundaries between these brown and white bands. The Great Red Spot is a huge storm that rotates around its own center like a hurricane. It is caught between two bands of winds blowing in opposite directions. The Great Red Spot was first observed in 1664 but may be much older.

Jupiter This model of Jupiter is based on photos taken by Voyager 1. The Great Red Spot, is a giant storm much larger than Earth.

Jupiter has at least 63 moons. Jupiter’s Moons Jupiter has at least 63 moons. Most of them are small, less than 200 kilometers in diameter. Ganymede and Callisto are similar in size to Mercury. Io is covered with actively erupting volcanoes. Io and Europa are somewhat smaller, about the size of our moon.

Jupiter Ganymede, Io, and Europa have metal cores and rocky mantles. Io is covered with actively erupting volcanoes. Ganymede, Europa, and Callisto are covered with ice. On Europa, the icy crust appears to rest on top of a liquid saltwater ocean. Scientists hypothesize that Europa is the most likely place in the solar system, other than Earth, to support life.

Jupiter Io is the most volcanically active body in the solar system. Io’s volcanic eruptions consist mainly of liquid sulfur. Europa may have a liquid water ocean beneath its icy surface. The dark lines are wide fractures in Europa’s crust.

Saturn What are the characteristics of Saturn? Saturn’s rings are the largest and most visible from Earth.

Saturn Saturn, the second largest planet in the solar system, is best known for its rings. The rings are made of particles of ice and ice-coated rock. These particles are generally between a few micrometers and 10 meters across, with most particles being snowball-sized. The rings are about 274,000 kilometers in diameter, yet are only tens of meters thick.

Saturn Saturn’s Atmosphere Like Jupiter, Saturn has colorful banding in its atmosphere. Saturn’s atmosphere is the largest of any planet in the solar system. Saturn also has the lowest average density, less than the density of liquid water. Saturn’s atmosphere is made mostly of hydrogen and helium, with small amounts of other elements.

Saturn Saturn has a thick atmosphere. It is surrounded by a spectacular set of rings, the largest in the solar system.

Astronomers have found at least 56 moons orbiting Saturn. Saturn’s Moons Astronomers have found at least 56 moons orbiting Saturn. Most of these moons are very small. Titan is larger than Mercury. Titan is the only planetary moon with a thick atmosphere.

Uranus What are the characteristics of Uranus? The axis of Uranus’s rotation is tilted more than 90°.

Uranus Because Uranus is so far from the sun, it is very cold. Uranus’s atmosphere is mostly hydrogen and helium. It also has a large amount of methane, which gives the planet a distinct blue-green appearance.

Uranus Structure of Uranus Like Jupiter and Saturn, Uranus has a dense core. This core is thought to be surrounded by a thick mantle layer of liquid water and dissolved ammonia. A layer of liquid hydrogen and helium surrounds the mantle.

Uranus Uranus has rings, although they are not as visible as Saturn’s. Uranus has at least 27 moons, though most are quite small.

Uranus A Tilted Planet The most unusual characteristic of Uranus is that it lies nearly on its side. Uranus rotates in a direction opposite to the direction of its revolution around the sun. Uranus’s rings and moons revolve about the same tilted axis.

Uranus Uranus’s northern and southern hemispheres alternate being exposed to the sun for many years, which causes immense storms. No one knows for sure why Uranus’s axis is so tilted. Scientists hypothesize that the tilt may have been caused by a collision with another large planetary body early in its history.

Uranus Uranus is unusual in that its axis is tilted so that the planet rotates nearly on its side.

Neptune What are the characteristics of Neptune? Neptune’s bluish color comes from the methane in its atmosphere.

Neptune Neptune is so far from the sun that it takes a very long time—165 Earth years—for it to make one revolution. Neptune has about the same composition and is about the same size as Uranus.

Neptune has clearly visible cloud patterns in its atmosphere. Thin, wispy clouds form high within Neptune’s atmosphere. Neptune’s clouds are made of methane ice crystals. Neptune also has large storms in its atmosphere. One storm, observed by the Voyager 2 space probe in 1989, was called the Great Dark Spot, but by 1994, the storm was gone.

Neptune This photo of Neptune was taken by the Voyager 2 spacecraft. The Great Dark Spot, visible near the center, was similar in size to Earth.

Neptune Neptune has thin rings very similar to the rings of Uranus. In addition, Neptune has at least 13 moons. Triton, the only large moon, has a thin atmosphere and an icy surface.

Dwarf Planets What is a dwarf planet? A dwarf planet, like a planet, is spherical and orbits the sun directly. But unlike a planet, a dwarf planet has not cleared the neighborhood around its orbit.

Dwarf Planets In 2006, astronomers established a new classification for certain objects in the solar system that share some, but not all, of the characteristics of a planet. Pluto, which for many years had been considered the ninth and most distant planet in the solar system, was reclassified as a dwarf planet.

Dwarf Planets A dwarf planet is too small for its gravity to attract all the nearby debris, so dwarf planets tend to orbit in zones that contain many other objects. Pluto is much smaller than the inner or outer planets. Its diameter is about half that of Mercury. The density of Pluto is similar to that of Triton, suggesting a mixture of ice and rock.

Dwarf Planets Pluto’s orbit around the sun is more elliptical than those of the planets. Pluto actually swings closer to the sun at times than Neptune. Like Uranus, Pluto has a rotation axis that is tilted more than 90°. Pluto also has three moons, Charon, Nix, and Hydra.

Dwarf Planets The New Horizons space probe will reach the dwarf planet Pluto and its three moons in the year 2015.

Dwarf Planets Other known dwarf planets include Eris and the asteroid Ceres. Many more solar system objects will likely be named dwarf planets once their shapes can be verified as being spherical. Not all astronomers agree on what defines a planet, and the classification of dwarf planet may be subjected to further debate.

Comets and Meteoroids What are comets and meteoroids? Comets are dusty pieces of ice and rock that partially vaporize when they pass near the sun. Meteoroids are pieces of rock, usually less than a few hundred meters in size, that travel through the solar system.

Comets and Meteoroids There are many smaller objects moving through the inner solar system in two forms. Comets are dusty pieces of ice and rock that partially vaporize when they pass near the sun. Meteoroids are pieces of rock, usually less than a few hundred meters in size that travel through the solar system.

Comets and Meteoroids Certain ancient meteoroids have been largely unaltered since the birth of the solar system. These meteoroids are the oldest remnants of the early solar system. Scientists have used radioactive dating to determine their absolute age and have found that the solar system is about 4.6 billion years old.

The Edge of the Solar System What lies in the outer solar system beyond Neptune? Most of the objects in the Kuiper belt lie in a doughnut-shaped region close to the ecliptic. Beyond the Kuiper belt lies a great reservoir of comets called the Oort cloud.

The Edge of the Solar System Astronomers estimate that thousands of solar system objects exist beyond Neptune’s orbit. These objects are found largely in two regions, the Kuiper belt and the Oort cloud. The Kuiper belt extends from Neptune’s orbit out to about 100 AU or more from the sun. Pluto is an example of a Kuiper belt object.

The Edge of the Solar System The solar system extends much farther out than the Kuiper belt. The Oort Cloud is a very sparse sphere of comets thought to encircle the solar system out to a distance of about 50,000 AU. Occasionally objects from the Oort cloud enter the inner solar system, where they appear as comets.

The Edge of the Solar System Located in the outer reaches of the solar system, the Oort cloud is a sphere of comets surrounding the sun and planets.

The planets referred to as the gas giants are composed mainly of Assessment Questions The planets referred to as the gas giants are composed mainly of nitrogen and oxygen. hydrogen and carbon dioxide. hydrogen and helium. carbon dioxide and fluorine.

The planets referred to as the gas giants are composed mainly of Assessment Questions The planets referred to as the gas giants are composed mainly of nitrogen and oxygen. hydrogen and carbon dioxide. hydrogen and helium. carbon dioxide and fluorine. ANS: C

The part of the solar system that is most distant from the sun is Assessment Questions The part of the solar system that is most distant from the sun is Neptune. Pluto. the Kuiper belt. the Oort cloud.

The part of the solar system that is most distant from the sun is Assessment Questions The part of the solar system that is most distant from the sun is Neptune. Pluto. the Kuiper belt. the Oort cloud. ANS: D

Assessment Questions Most meteroids consist primarily of ice and orbit the sun at a great distance in the Oort cloud or Kuiper belt. True False

Assessment Questions Most meteroids consist primarily of ice and orbit the sun at a great distance in the Oort cloud or Kuiper belt. True False ANS: F, comets