The Outer Planets

The four planets farthest from the Sun are the outer planets. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants or Jovian planets.

The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. Since the inner planets did not have enough mass to hold on to these light gases, their hydrogen and helium floated away into space. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away. (Outer Planets | Earth Science, n.d.)

All of the outer planets have numerous moons. They all also have planetary rings, composed of dust and other small particles that encircle the planet in a thin plane.

Jupiter

Because Jupiter is so massive, it reflects much sunlight and appears bright in the night sky; only the Moon and Venus are brighter. This brightness is all the more impressive because Jupiter is quite far from the Earth, 5.20 AUs away. It takes Jupiter about 12 Earth years to orbit once around the Sun.

Jupiter is named for the king of the gods in Roman mythology. The planet is enormous, the most massive object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earth’s volume, it has only 318 times the mass of Earth. Like the other gas giants, it is less dense than Earth. Hypothetically, astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface. Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements.

The upper layer of Jupiter’s atmosphere contains clouds of ammonia (NH3) in bands of different colors. These bands rotate around the planet, but also swirl around in violent storms. The Great Red Spot is an enormous, oval-shaped storm found south of Jupiter’s equator. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days. The Great Red Spot has been on Jupiter for at least 300 years since astronomers could first see the storm through telescopes.

Jupiter’s Moons

Jupiter has 63 moons orbiting it that we are aware of so far. Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These four moons, Io, Europa, Ganymede, and Callisto, were first discovered by Galileo in 1610 and are referred to as the Galilean moons. The Galilean moons are more significant than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the biggest moon in the solar system, but it is also even larger than the planet Mercury.

Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life. Although the surface of Europa is a smooth layer of ice, there is evidence that there is an ocean of liquid water underneath. Europa also has a continual source of energy; it is heated as it is stretched and squashed by tidal forces from Jupiter. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted.

In 1979, two spacecraft, Voyager 1 and Voyager 2 visited Jupiter and its moons. Photos from the Voyager missions showed that Jupiter has a faint ring system. This ring system is very faint, so it is difficult to observe from Earth. Recently, NASA launched the Juno satellite to study Jupiter.

Saturn

Saturn is famous for its beautiful rings. Although all the gas giants have rings, only Saturn’s can be easily seen from Earth. In Roman mythology, Saturn was the father of Jupiter. Saturn’s mass is about 95 times the mass of Earth, and its volume is 755 times Earth’s volume, making it the second-largest planet in the solar system. Saturn is also the least dense planet in the solar system. It is less dense than water, meaning that Saturn would float on water. Saturn orbits the Sun once about every 30 Earth years.

Like Jupiter, Saturn is made mostly of hydrogen and helium gases in the outer layers and liquids at greater depths. The upper atmosphere has clouds in bands of different colors. These rotate rapidly around the planet, but there seems to be less turbulence and fewer storms on Saturn than on Jupiter. One interesting phenomenon that has been observed in the storms on Saturn is the presence of thunder and lightning. The planet likely has a small rocky and metallic core.

In 1610 Galileo first observed Saturn’s rings with his telescope, but he thought they might be two large moons, one on either side of the planet. In 1659, the Dutch astronomer Christian Huygens realized that the features were rings. Saturn’s rings circle the planet’s equator and appear tilted because Saturn itself is tilted about 27 degrees. The rings do not touch the planet.

The Voyager 1 and 2 spacecraft in 1980 and 1981 sent back detailed pictures of Saturn, its rings, and some of its moons. Saturn’s rings are made of particles of water and ice, with some dust and rocks. There are several gaps in the rings that scientists think have originated because 1) the material was cleared out by the gravitational pull within the rings or, 2) by the gravitational forces of Saturn and of moons outside the rings. The rings were likely formed by the breakup of Saturn’s moons or a material that never accreted into the planet when Saturn formed initially.

Saturn’s Moons

Most of Saturn’s moons are very small, and only seven are large enough for gravity to have made them spherical. Only Titan is more massive than Earth’s Moon at about 1.5 times its size. Titan is even more massive than the planet Mercury. Scientists are interested in Titan because its atmosphere is similar to what Earth’s was like before life developed. Nitrogen is dominant, and methane is the second most abundant gas. Titan may have a layer of liquid water and ammonia under a layer of surface ice. Lakes of liquid methane (CH4) and ethane (C2H6) are found on Titan’s surface. Although conditions are similar enough to those of early Earth for scientists to speculate that extremely primitive life may exist on Titan, the extreme cold and lack of carbon dioxide make it unlikely.

Uranus

Uranus (YOOR-uh-nuhs) is named after the Greek god of the sky. From Earth, Uranus is so faint that it was unnoticed by ancient observers. William Herschel first discovered the planet in 1781. Although Uranus is enormous, it is incredibly far away, about 2.8 billion km (1.8 billion mi) from the Sun. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus, and the planet orbits the Sun once about every 84 Earth years. Uranus has a mass of about 14 times the mass of Earth, but it is much less dense than Earth. Gravity at the surface of Uranus is weaker than on Earth’s surface, so if a human were at the top of the clouds on Uranus, they would weigh about 10 percent less than what they would weigh on Earth.

Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, including water, ammonia (NH3), and methane (CH4) than Jupiter and Saturn. When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in Uranus’ atmosphere, but they are hard to see in normal light, so the planet looks like a plain blue ball.

Most of the solar system planets rotate on their axes in the same direction as they move around the Sun. Uranus is tilted on its side, so its axis is almost identical to its orbit. It rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago.

Uranus’s Moons

Uranus has a faint system of rings. The rings circle the planet’s equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planet’s orbit. Uranus has 27 known moons, and all but a few of them are named after characters from the plays of William Shakespeare. The five biggest moons are Miranda, Ariel, Umbriel, Titania, and Ober-on.

Neptune

Neptune is the only major planet that cannot be seen from Earth without a telescope. Scientists predicted Neptune’s existence before it was discovered because Uranus did not always appear exactly where it should appear. They knew that the gravitational pull of another planet beyond Uranus must be affecting Uranus’ orbit.

Neptune was discovered in 1846, in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color. In many respects, Neptune is similar to Uranus. Neptune has slightly more mass than Uranus, but it is somewhat smaller in size. Neptune is much farther from the Sun at nearly 4.5 billion km (2.8 billion mi) than Uranus. The planet’s slow orbit means that it takes 165 Earth years to go once around the Sun.

Neptune’s blue color is mostly because of frozen methane. When Voyager 2 visited Neptune in 1986, there was a sizeable dark-blue spot that scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared, but another dark spot had appeared north of the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune.

The changing appearance of Neptune is caused by its turbulent atmosphere. The winds on Neptune are more substantial than on any other planet in the solar system, reaching speeds of 1,100 km/h (700 mi/h) close to the sound speed. This extreme weather surprised astronomers since the planet receives little energy from the Sun to power weather systems. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about -218 degrees Celsius (-360 degrees Fahrenheit). Neptune has faint rings of ice and dust that may change or disappear in reasonably short time frames.

Neptune’s Moons

Neptune has 13 known moons. Triton is the only one of them that has enough mass to be spherical. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton did not form around Neptune, but instead was captured by Neptune’s gravity as it passed by.