Lesson Objectives

Describe an essential 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 space farther native the Sun as well as farther indigenous Earth. They are much more challenging to learn about since castle are an extremely different from our house planet.

The outer Planets

The four planets the furthest from the Sun space the outer planetsFigure below shows the family member sizes of the outer planets and the Sun. These planets are much bigger than the inside planets and are made generally of gases and also liquids, therefore they are also called gas giants.


This image shows the four outer planets and the Sun, through sizes come scale. Native left come right, the external planets room Jupiter, Saturn, Uranus, and also Neptune.

The gas giants are consisted of primarily the hydrogen and also helium, the same aspects that consist of most the the Sun. Astronomers think that hydrogen and also helium gases consisted of much the the solar system when it an initial formed. Since the inside planets no have enough mass to organize on to these light gases, your hydrogen and helium floated away into space. The Sun and the enormous outer planets had enough gravity to save hydrogen and helium from drifting away.

All of the external planets have plenty of moons. Lock all additionally have planetary rings, created of dust and other tiny particles that encircle the earth in a thin plane.


Because Jupiter is for this reason large, it shows a many sunlight. Jupiter is extremely bright in the night sky; only the Moon and also Venus are brighter (Figure below). This brightness is every the an ext impressive because Jupiter is quite far from the planet — 5.20 AUs away. The takes Jupiter about 12 planet years come orbit once approximately the Sun.


This image of Jupiter was taken by Voyager 2 in 1979. The colors were later intensified to lug out an ext details.

Jupiter is called for the king that the god in roman inn mythology. The world is enormous, the biggest object in the solar system besides the Sun. Return Jupiter is over 1,300 times Earth’s volume, it has only 318 time the massive of Earth. Prefer the other gas giants, the is lot less thick than Earth.

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

A round of Gas and Liquid

Astronauts trying to land a spaceship ~ above the surface of Jupiter would discover that over there is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts the other facets (Figure below).


Jupiter’s setting is composed of hydrogen and also helium. Deeper within the planet, push compresses the gases into a liquid. Part evidence says that Jupiter may have a small rocky core of heavier facets at its center.

A Stormy Atmosphere

The top layer the Jupiter’s atmosphere includes clouds the ammonia (NH3) in bands of different colors. This bands rotate roughly the planet, but additionally swirl approximately in rough storms. The Great Red Spot (Figure below) is one enormous, oval-shaped storm discovered south of Jupiter’s equator. This storm is more than three times as wide as the whole Earth. Clouds in the storm turn in a counterclockwise direction, make one finish turn every six days or so. The good Red Spot has been top top Jupiter for at least 300 years, since astronomers could an initial see the storm v telescopes. Perform you think the an excellent Red clues is a permanent attribute on Jupiter? How might you know?


This picture of Jupiter’s great Red point out (upper ideal of image) was taken through the Voyager 1 spacecraft. The white storm just 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 actually been found so far. 4 are large enough and bright enough to be watched from Earth, utilizing no more than a pair that binoculars. This moons — Io, Europa, Ganymede, and Callisto — were an initial discovered through Galileo in 1610, so they are periodically referred to as the Galilean moons (Figure below). The Galilean moons are bigger than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the best moon in the solar system it is also larger 보다 the world Mercury!


This composite photo shows the four Galilean moons and also their sizes family member to the great Red Spot. From optimal to bottom, the moons room Io, Europa, Ganymede, and also Callisto. Jupiter’s great Red point out is in the background. Sizes space to scale.

Scientists are an especially interested in Europa because it might be a ar to find extraterrestrial life. What features could make a satellite so far from the sun a candidate for life? return the surface of Europa is a smooth class of ice, there is evidence that over there is an ocean of fluid water under (Figure below). Europa additionally has a continual resource of energy — it is heated as it is stretched and squashed by tidal pressures from Jupiter. Plenty of missions have actually been to plan to explore Europa, consisting of plans come drill with the ice and send a probe into the ocean. However, no together mission has yet to be attempted.


An enhanced color photo of a part of Europa’s icy surface. The surface ice may have actually motions similar to key tectonics top top Earth.

In 1979, 2 spacecrafts — Voyager 1 and also Voyager 2 — went to Jupiter and also its moons. Photos from the Voyager missions showed the Jupiter has actually a ring system. This ring system is an extremely faint, so the is an overwhelming to watch from Earth.


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

Saturn’s mass is around 95 time the fixed of Earth, and its volume is 755 time Earth’s volume, making the the 2nd largest world in the solar system. Saturn is also the least dense planet in the solar system. That is less thick than water. What would take place if you had a big enough bathtub to put Saturn in? Saturn would certainly float! Saturn orbits the sun once about every 30 planet years.


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

Like Jupiter, Saturn is made mostly of hydrogen and helium gases in the outer layers and liquids at higher depths. The upper setting has clouds in bands of various colors. These revolve rapidly approximately the planet, yet there appears to be much less turbulence and also fewer storms on Saturn than on Jupiter. One amazing phenomena that has been observed in the storms top top Saturn is the presence of thunder and lightning (see video, below). The earth likely has a small rocky and also metallic core.

Cassini researchers waited years for the right conditions to create the very first movie that mirrors lightning on another planet — Saturn.http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots an ext videos native the Cassini mission room indexed here:http://saturn.jpl.nasa.gov/video/

Saturn’s Rings

In 1610 Galileo first observed Saturn’s rings v his telescope, yet he assumed they could be two big moons, one top top either side of the planet. In 1659, the dutch astronomer Christian Huygens realized that the attributes were ring (Figure below).

Saturn’s rings circle the planet’s equator and also appear tilted due to the fact that Saturn chin is tilted about 27 degrees. The rings perform not touch the planet.


A color-exaggerated mosaic that Saturn and its ring taken by Cassini together Saturn eclipses the Sun.

The Voyager 1 and 2 spacecraft in 1980 and 1981 sent earlier detailed images of Saturn, that is rings, and also some that its moons. Saturn’s rings space made of corpuscle of water and ice, with some dust and rocks (Figure below). Over there are number of gaps in the ring that researchers think have actually originated because (1) the product was cleared the end by the gravitational traction within the rings or (2) through the gravitational pressures of Saturn and also of moons exterior the rings.


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

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

An animation of dark spokes in Saturn’s rings is watched here:http://en.wikipedia.org/wiki/File:Saturn_ring_spokes_PIA11144_300px_secs15.5to23_20080926.ogv. The spokes appear seasonally and also their beginning is together yet unknown.

Saturn’s Moons

Most of Saturn’s moons room very small and just seven are big enough for gravity to have made them spherical. Only Titan is larger than earth Moon at about 1.5 time its size. Titan is also larger than the world Mercury.

Scientists are interested in Titan due to the fact that its setting is comparable to what Earth’s to be like prior to life developed. Nitrogen is dominant and also methane is the 2nd most abundant gas. Titan may have a class of liquid water and ammonia under a layer of surface ice. Lakes of fluid methane (CH4) and ethane (C2H6) are found on Titan’s surface. Although problems are similar enough come those of early on Earth for scientists to speculate that incredibly primitive life may exist ~ above Titan, the extreme cold and also lack of carbon dioxide make it unlikely (Figure below).


This composite picture compares Saturn’s biggest moon, Titan (right) to earth (left).


Uranus (YOOR-uh-nuhs) is named for the Greek god of the skies (Figure below). From Earth, Uranus is for this reason faint that it was unnoticed by old observers. Wilhelm Herschel very first discovered the earth in 1781.


Uranus is an icy blue-green ball.

Although Uranus is an extremely large, the is very far away, around 2.8 billion km (1.8 exchange rate mi) indigenous the Sun. Irradiate from the sunlight takes about 2 hours and 40 minute to with Uranus. Uranus orbits the sunlight once around every 84 earth years.

Uranus has a mass about 14 time the mass of Earth, however it is much less dense than Earth. Heaviness at the surface of Uranus is weaker 보다 on Earth’s surface so if you to be at the optimal of the clouds top top Uranus, you would weigh about 10% less than what you weigh on Earth.

An Icy Blue-Green Ball

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

When sunlight reflects off Uranus, clouds the methane filter out red light, giving the world a blue-green color. There are bands of clouds in the setting of Uranus, but they are tough to check out in typical light, therefore the earth looks favor a level blue ball.

The party Planet

Most the the planets in the solar system rotate on their axes in the very same direction that they move about the Sun. Uranus, though, is tilted top top its next so that axis is practically parallel to its orbit. In various other words, that rotates favor a height that to be turned so that it was spinning parallel come the floor. Researchers think the Uranus was probably knocked end by a collision with an additional planet-sized thing billions of years ago.

Rings and also Moons of Uranus

Uranus has a faint mechanism of ring (Figure below). The ring circle the planet’s equator, but due to the fact that Uranus is tilted on its side, the rings are virtually perpendicular to the planet’s orbit.


This photo from the Hubble space Telescope reflects the faint rings the Uranus. The world is tilted on its side, therefore the ring are practically vertical.

Uranus has actually 27 well-known moons and all but a couple of of them are called for personalities from the plays of william Shakespeare. The 5 biggest moons of Uranus — Miranda, Ariel, Umbriel, Titania, and Oberon — are shown in Figure below.


These Voyager 2 photos have been resized to display the loved one sizes the the five main moons the Uranus.


Neptune, shown in Figure below, is the only significant planet that can’t be seen from earth without a telescope. Scientists predicted the visibility of Neptune prior to it to be discovered since Uranus go not always appear exactly where it have to appear. Lock knew that the gravitational traction of one more planet beyond Uranus must be affecting Uranus’ orbit.

Neptune was found in 1846, in the position that had been predicted, and it was named Neptune because that the roman god that the sea since of its bluish color.


This picture of Neptune was taken by Voyager 2 in 1989. The good Dark Spot viewed on the left facility in the snapshot has since disappeared, yet a similar dark clues has appeared on another part of the planet.

In countless respects, Neptune is comparable to Uranus (Figure below). Neptune has actually slightly an ext mass than Uranus, yet it is slightly smaller in size. Neptune is much farther from the sun at almost 4.5 billion km (2.8 exchange rate mi). The planet’s slow-moving orbit means that that takes 165 earth years to walk once about the Sun.


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

Extremes of Cold and also Wind

Neptune’s blue shade is mostly since of frozen methane (CH4). As soon as Voyager 2 checked out Neptune in 1986, there was a large 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 good Dark Spot had actually disappeared but another dark clues had showed up north of the equator. Astronomers think the both of these spots represent gaps in the methane clouds on Neptune.

The changing appearance of Neptune is led to by its rough atmosphere. The winds top top Neptune space stronger 보다 on any kind of other world in the solar system, getting to speeds that 1,100 km/h (700 mi/h), close to the rate of sound. This too much weather surprised astronomers, because the planet receives little energy from the sun to strength weather systems. Neptune is likewise one the the coldest areas in the solar system. Temperature at the peak of the clouds are about -218oC (-360oF).

Neptune’s Rings and Moons

Neptune has faint ring of ice and dust that may readjust or disappear in reasonably short time frames.

Neptune has actually 13 recognized moons. Triton, displayed 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 come the orbit that Neptune. Scientists think Triton go not form around Neptune, however instead was recorded by Neptune’s gravity together it happen by.


This photo Triton, Neptune’s largest moon, was taken by Voyager 2 in 1989.

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

Lesson Summary

The four outer planets are all gas giants made generally of hydrogen and helium. They have actually thick gaseous outer layers and also liquid interiors.The outer planets have countless moons, and planetary rings.Jupiter, by much the biggest planet in the solar system, has actually bands of different colored clouds, and a long-lasting storm dubbed the good Red Spot.Jupiter has much more than 60 moons including the four largest, the Galilean moons.Europa has actually an ocean of fluid water under a great of ice where life may have actually formed.Saturn is smaller than Jupiter however has a big system that beautiful rings.Titan’s environment is comparable to at an early stage Earth’s and also the moon can harbor primitive life.Uranus and also Neptune were discovered reasonably recently because they are so far away.Uranus is tilted on its side, probably since of a past collision v a large object.Neptune is really cold and also has strong winds. The dark spots space storms in Neptune’s atmosphere.

Review Questions

1. Surname the external planets a) in order from the sun outward, b) from biggest to smallest by mass, and c) from biggest to smallest by size.

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2. Why space the external planets called gas giants?

3. How do the an excellent Red spot and an excellent Dark spot differ?

4. Surname the Galilean moons, and explain why they have actually that name.

5. Why can Europa it is in a most likely place to uncover extraterrestrial life?

6. What causes gaps in Saturn’s rings?

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

8. What liquid is found on the surface of Titan?

9. Why is Uranus blue-green in color?

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

Further analysis / Supplemental Links

Points come Consider

The inside planets are little and rocky, when the outer planets are huge and gaseous. Why might the planets have created into these two groups?We have disputed the Sun, the planets, and the moons of the planets. What other objects deserve to you think the that deserve to be uncovered in ours solar system?