Lesson Objectives

Describe vital features that the external planets and also their moons.Compare the external planets to each other and to Earth.

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Galilean moonsgas giantsGreat Red Spotouter planetsplanetary rings


The four outer planets room farther from the Sun as well as farther indigenous Earth. Lock are lot more challenging to learn about since they are really different native our home planet.

The outer Planets

The four planets farthest from the Sun are the outer planetsFigure below shows the relative sizes of the outer planets and also the Sun. This planets space much bigger than the inside planets and also are made primarily of gases and also liquids, therefore they are likewise called gas giants.


This photo shows the 4 outer planets and the Sun, with sizes to scale. Indigenous left to right, the outer planets room Jupiter, Saturn, Uranus, and also Neptune.

The gas giants are made up primarily the hydrogen and helium, the same elements that make up most that the Sun. Astronomers think that hydrogen and helium gases comprised much that the solar device when it very first formed. Because the inside planets no have sufficient mass to hold on to these light gases, their hydrogen and also helium floated away into space. The Sun and the massive outer planets had sufficient gravity to save hydrogen and also helium native drifting away.

All of the external planets have many moons. Castle all additionally have planetary rings, composed of dust and other small particles the encircle the planet in a thin plane.


Because Jupiter is therefore large, it shows a lot of sunlight. Jupiter is extremely bright in the night sky; just the Moon and also Venus are brighter (Figure below). This brightness is all the much more impressive since Jupiter is quite far from the planet — 5.20 AUs away. That takes Jupiter around 12 earth years to orbit once approximately the Sun.


This image of Jupiter to be taken through Voyager 2 in 1979. The colors were later intensified to lug out much more details.

Jupiter is named for the king of the gods in roman inn mythology. The world is enormous, the biggest object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earth’s volume, it has only 318 times the massive of Earth. Like the various other gas giants, it is lot less thick than Earth.

Check out NASA’s world book to learn much more about Jupiter: http://www.nasa.gov/worldbook/jupiter_worldbook.html.

A sphere of Gas and also Liquid

Astronauts trying to land a spaceship on the surface ar of Jupiter would uncover that there is no solid surface at all! Jupiter is made largely of hydrogen, with some helium, and small amounts the other elements (Figure below).


Jupiter’s atmosphere is created of hydrogen and also helium. Deeper within the planet, push compresses the gases into a liquid. Part evidence argues that Jupiter may have actually a tiny rocky core of heavier elements at that center.

A Stormy Atmosphere

The upper layer that Jupiter’s atmosphere contains clouds of ammonia (NH3) in bands of various colors. This bands rotate roughly the planet, but also swirl approximately in turbulent storms. The Great Red Spot (Figure below) is an enormous, oval-shaped storm uncovered south the Jupiter’s equator. This storm is an ext than 3 times as broad as the whole Earth. Clouds in the storm revolve in a counterclockwise direction, do one complete turn every six days or so. The great Red Spot has been top top Jupiter for at the very least 300 years, since astronomers could an initial see the storm with telescopes. Perform you think the great Red clues is a permanent function on Jupiter? How could you know?


This image of Jupiter’s great Red clues (upper ideal of image) was taken by the Voyager 1 spacecraft. The white storm just listed below the great Red spot is about the very same diameter as Earth.

Jupiter’s Moons and also Rings

Jupiter has a very large number of moons — 63 have been found so far. Four are large enough and bright sufficient to be watched from Earth, using no much more than a pair that binoculars. This moons — Io, Europa, Ganymede, and also Callisto — were an initial discovered through Galileo in 1610, therefore they are sometimes referred to as the Galilean moons (Figure below). The Galilean moons are bigger than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not just the greatest moon in the solar system it is also larger 보다 the world Mercury!


This composite picture shows the four Galilean moons and also their sizes relative to the good Red Spot. From optimal to bottom, the moons space Io, Europa, Ganymede, and also Callisto. Jupiter’s an excellent Red spot is in the background. Sizes are to scale.

Scientists are particularly interested in Europa because it may be a place to uncover extraterrestrial life. What features could make a satellite so far from the sunlight a candidate for life? return the surface ar of Europa is a smooth layer of ice, over there is proof that over there is an s of liquid water under (Figure below). Europa likewise has a continual resource of energy — that is heated as it is stretched and squashed by tidal pressures from Jupiter. Plenty of missions have actually been to plan to check out Europa, including plans to drill with the ice and send a probe into the ocean. However, no together mission has actually yet to be attempted.


An intensified color image of a section of Europa’s icy surface. The surface ar ice may have actually motions similar to key tectonics ~ above Earth.

In 1979, two spacecrafts — Voyager 1 and Voyager 2 — saw Jupiter and its moons. Photos indigenous the Voyager missions showed that Jupiter has a ring system. This ring device is very faint, so that is challenging to watch from Earth.


Saturn, presented in Figure below, is renowned for its beautiful rings. Although all the gas giants have actually rings, just Saturn’s have the right to be conveniently seen native Earth. In roman mythology, Saturn was the dad of Jupiter.

Saturn’s massive is about 95 times the mass of Earth, and also its volume is 755 times Earth’s volume, making it the second largest planet in the solar system. Saturn is additionally the least dense planet in the solar system. It is less thick than water. What would happen if you had a big enough bathtub to placed Saturn in? Saturn would float! Saturn orbits the sunlight once around every 30 earth years.


This image of Saturn and also its rings is a composite of images taken by the Cassini orbiter in 2008

Like Jupiter, Saturn is made mainly of hydrogen and helium gases in the outer layers and liquids at greater depths. The upper atmosphere has clouds in bands of various colors. These rotate rapidly around the planet, yet there seems to be less turbulence and also fewer storms ~ above Saturn 보다 on Jupiter. One interesting phenomena that has been it was observed in the storms ~ above Saturn is the visibility of thunder and also lightning (see video, below). The planet likely has a little rocky and metallic core.

Cassini researchers waited years because that the right conditions to create the first movie that reflects lightning on an additional planet — Saturn.http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots an ext videos from the Cassini mission space indexed here:http://saturn.jpl.nasa.gov/video/

Saturn’s Rings

In 1610 Galileo very first observed Saturn’s rings through his telescope, however he assumed they could be two huge moons, one ~ above either side of the planet. In 1659, the dutch astronomer Christian Huygens realized that the attributes were rings (Figure below).

Saturn’s ring circle the planet’s equator and appear tilted because Saturn chin is tilted about 27 degrees. The rings carry out not touch the planet.


A color-exaggerated mosaic that Saturn and also its rings taken by Cassini as Saturn eclipses the Sun.

The Voyager 1 and 2 spacecraft in 1980 and also 1981 sent back detailed images of Saturn, the rings, and also some the its moons. Saturn’s rings room made of corpuscle of water and also ice, v some dust and also rocks (Figure below). Over there are numerous gaps in the ring that researchers think have actually originated since (1) the product was cleared the end by the gravitational traction within the ring or (2) by the gravitational pressures of Saturn and of moons outside the rings.


A close-up the Saturn’s external C ring showing areas with greater particle concentration and also gaps.

The rings were likely created by the breakup of among Saturn’s moons or from product that never accreted right into the planet as soon as Saturn originally formed.

An computer animation of dark spokes in Saturn’s ring is seen here:http://en.wikipedia.org/wiki/File:Saturn_ring_spokes_PIA11144_300px_secs15.5to23_20080926.ogv. The spokes show up seasonally and also their origin is together yet unknown.

Saturn’s Moons

Most the Saturn’s moons are very small and only seven are large enough for heaviness to have made castle spherical. Only Titan is larger than earth Moon at around 1.5 time its size. Titan is also larger 보다 the earth Mercury.

Scientists space interested in Titan due to the fact that its setting is similar to what Earth’s to be like before life developed. Nitrogen is dominant and also methane is the second most plentiful gas. Titan may have a class of liquid water and also ammonia under a great of surface ice. Lakes of fluid methane (CH4) and ethane (C2H6) are found on Titan’s surface. Although problems are comparable enough come those of early Earth for scientists to speculate that extremely primitive life may exist top top Titan, the too much cold and lack the carbon dioxide make it i can not qualify (Figure below).


This composite photo compares Saturn’s largest moon, Titan (right) to planet (left).


Uranus (YOOR-uh-nuhs) is called for the Greek god of the sky (Figure below). Native Earth, Uranus is for this reason faint the it to be unnoticed by ancient observers. Wilhelm Herschel first discovered the planet in 1781.


Uranus is an icy blue-green ball.

Although Uranus is very large, the is incredibly far away, around 2.8 billion kilometres (1.8 billion mi) from the Sun. Irradiate from the sunlight takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the sun once about every 84 planet years.

Uranus has a mass about 14 time the massive of Earth, yet it is lot less thick than Earth. Heaviness at the surface of Uranus is weaker 보다 on Earth’s surface ar so if you to be at the top of the clouds on Uranus, you would certainly weigh around 10% less than what you sweet on Earth.

An Icy Blue-Green Ball

Like Jupiter and also Saturn, Uranus is composed mainly of hydrogen and helium, through an outer gas layer that gives method to fluid on the inside. Uranus has actually a higher percentage that icy materials, such as water, ammonia (NH3), and also methane (CH4), than Jupiter and also Saturn.

When sunlight mirrors off Uranus, clouds the methane filter out red light, giving the world a blue-green color. There space bands the clouds in the setting of Uranus, yet they are tough to see in regular light, therefore the earth looks like a plain blue ball.

The sideways Planet

Most of the planets in the solar system rotate on your axes in the same direction that they move roughly the Sun. Uranus, though, is tilted ~ above its side so that axis is nearly parallel come its orbit. In other words, it rotates prefer a height that to be turned so that it to be spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized thing billions of years ago.

Rings and Moons of Uranus

Uranus has a faint mechanism of ring (Figure below). The rings circle the planet’s equator, but since Uranus is tilted ~ above its side, the ring are virtually perpendicular come the planet’s orbit.


This photo from the Hubble an are Telescope mirrors the faint rings of Uranus. The world is tilted ~ above its side, so the ring are practically vertical.

Uranus has actually 27 recognized moons and all but a couple of of them are named for personalities from the theatre of wilhelm Shakespeare. The 5 biggest moons that Uranus — Miranda, Ariel, Umbriel, Titania, and also Oberon — are displayed in Figure below.


These Voyager 2 photos have been resized to present the relative sizes of the 5 main moons the Uranus.


Neptune, presented in Figure below, is the only major planet that can’t be viewed from earth without a telescope. Scientists predicted the existence of Neptune before it to be discovered since Uranus walk not always appear specifically where it have to appear. They knew the the gravitational traction of an additional planet beyond Uranus need to be affect Uranus’ orbit.

Neptune was discovered in 1846, in the position that had actually been predicted, and also it was named Neptune for the roman god the the sea due to the fact that of its bluish color.


This photo of Neptune was taken by Voyager 2 in 1989. The an excellent Dark Spot seen on the left center in the snapshot has since disappeared, yet a comparable dark clues has appeared on another component of the planet.

In plenty of respects, Neptune is similar to Uranus (Figure below). Neptune has actually slightly more mass 보다 Uranus, but it is slightly smaller sized in size. Neptune is lot farther native the sunlight at virtually 4.5 billion km (2.8 billion mi). The planet’s slow-moving orbit way that the takes 165 earth years to walk once approximately the Sun.


Neptune’s composition is the of a gas giant: (1) top atmosphere, (2) environment composed of hydrogen, helium and also methane gas, (3) mantle that water, ammonia and methane ice, (4) main point of rock and ice.

Extremes the Cold and also Wind

Neptune’s blue color is mostly since of frozen methane (CH4). Once Voyager 2 checked out Neptune in 1986, there to be a huge dark-blue spot the scientists called the an excellent Dark Spot, south of the equator. As soon as the Hubble an are Telescope took pictures of Neptune in 1994, the good Dark Spot had disappeared but one more dark clues had appeared north that the equator. Astronomers think the both of these spots represent gaps in the methane clouds on Neptune.

The changing appearance that Neptune is resulted in by its stormy atmosphere. The winds ~ above Neptune room stronger 보다 on any type of other world in the solar system, getting to speeds the 1,100 km/h (700 mi/h), close come the rate of sound. This extreme weather surprised astronomers, due to the fact that the earth receives little energy from the sunlight to power weather systems. Neptune is additionally one of the coldest places in the solar system. Temperatures at the optimal of the clouds are around -218oC (-360oF).

Neptune’s Rings and Moons

Neptune has actually faint rings of ice and dust the may change or disappear in reasonably short time frames.

Neptune has actually 13 recognized moons. Triton, shown in Figure below, is the only one of them the has sufficient mass to it is in spherical in shape. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton go not kind around Neptune, but instead was caught by Neptune’s gravity as it passed by.


This picture Triton, Neptune’s largest moon, to be taken by Voyager 2 in 1989.

Fly through Neptune’s moon Triton by watching this video: http://www.space.com/common/media/video/player.php?videoRef=mm32_SunDeath#playerTop

Lesson Summary

The 4 outer planets are all gas giants made primarily of hydrogen and helium. They have actually thick gaseous outer layers and also liquid interiors.The outer planets have plenty of moons, as well as planetary rings.Jupiter, by far the largest planet in the solar system, has actually bands of different colored clouds, and also a long-lasting storm called the good Red Spot.Jupiter has an ext than 60 moons consisting of the 4 largest, the Galilean moons.Europa has actually an s of fluid water under a layer of ice wherein life may have actually formed.Saturn is smaller than Jupiter but has a large system the beautiful rings.Titan’s environment is similar to at an early stage Earth’s and also the moon could harbor primitive life.Uranus and also Neptune were discovered fairly recently due to the fact that they space so far away.Uranus is tilted ~ above its side, probably since of a past collision v a huge object.Neptune is very cold and has solid winds. That is dark spots room storms in Neptune’s atmosphere.

Review Questions

1. Name the outer planets a) in order indigenous the sun outward, b) from biggest to smallest by mass, and c) from largest to smallest by size.

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2. Why are the external planets called gas giants?

3. Just how do the an excellent Red clues and great Dark spot differ?

4. Name the Galilean moons, and also explain why they have that name.

5. Why can Europa be a likely place to uncover extraterrestrial life?

6. What reasons gaps in Saturn’s rings?

7. Why are scientists interested in the atmosphere of Saturn’s moon Titan?

8. What liquid is uncovered on the surface ar of Titan?

9. Why is Uranus blue-green in color?

10. What is the surname of Neptune’s biggest moon?

Further reading / Supplemental Links

Points to Consider

The inside planets are small and rocky, while the external planets are big and gaseous. Why can the planets have created into these two groups?We have discussed the Sun, the planets, and also the moons of the planets. What various other objects have the right to you think the that can be found in ours solar system?