Lesson Objectives

Describe an essential features that the external planets and also their moons.Compare the external planets to every other and to Earth.

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Galilean moonsgas giantsGreat Red Spotouter planetsplanetary rings


The four outer planets room farther native the Sun and also farther from Earth. Lock are much more daunting to learn about since they are really different indigenous our residence planet.

The external Planets

The four planets the furthest from the Sun are the outer planetsFigure below shows the family member sizes the the external planets and also the Sun. These planets are much larger than the within planets and are made primarily of gases and also liquids, therefore they are likewise called gas giants.


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

The gas giants are consisted of primarily the hydrogen and helium, the same aspects that comprise most the the Sun. Astronomers think the hydrogen and helium gases comprised much of the solar system when it very first formed. Because the within planets no have enough mass to hold on to these light gases, your hydrogen and helium floated away into space. The Sun and the huge outer planets had sufficient gravity to save hydrogen and helium from drifting away.

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


Because Jupiter is therefore large, it reflects a most sunlight. Jupiter is extremely bright in the night sky; only the Moon and also Venus room brighter (Figure below). This brightness is all the an ext impressive since Jupiter is quite far from the planet — 5.20 AUs away. That takes Jupiter about 12 planet years come orbit once around the Sun.


This photo of Jupiter was taken by Voyager 2 in 1979. The colors were later enhanced to lug out much more details.

Jupiter is named for the king of the gods in roman mythology. The world is enormous, the largest object in the solar mechanism besides the Sun. Return Jupiter is over 1,300 times Earth’s volume, it has only 318 times the mass of Earth. Like the various other gas giants, it is much less thick than Earth.

Check the end NASA’s world publication to learn much 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 uncover that there is no solid surface ar at all! Jupiter is made largely of hydrogen, with some helium, and tiny amounts that other elements (Figure below).


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

A Stormy Atmosphere

The top layer the Jupiter’s atmosphere has clouds that ammonia (NH3) in bands of different colors. These bands rotate roughly the planet, but likewise swirl approximately in rough storms. The Great Red Spot (Figure below) is an enormous, oval-shaped storm discovered south that Jupiter’s equator. This storm is much more than 3 times as large as the whole Earth. Clouds in the storm revolve in a counterclockwise direction, do one finish turn every six days or so. The good Red Spot has actually been on Jupiter for at the very least 300 years, due to the fact that astronomers could very first see the storm through telescopes. Perform you think the an excellent Red point out is a permanent attribute on Jupiter? How could you know?


This image of Jupiter’s good Red point out (upper best of image) to be taken through the Voyager 1 spacecraft. The white storm just listed below the an excellent Red clues is around the same diameter together Earth.

Jupiter’s Moons and also Rings

Jupiter has a very big number of moons — 63 have actually been discovered so far. Four are huge enough and also bright enough to be viewed from Earth, utilizing no much more than a pair the binoculars. This moons — Io, Europa, Ganymede, and Callisto — were an initial discovered by Galileo in 1610, so they are sometimes referred to as the Galilean moons (Figure below). The Galilean moons are larger than the dwarf planets Pluto, Ceres, and also Eris. Ganymede is not only the best moon in the solar system it is even larger than the earth Mercury!


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

Scientists are an especially interested in Europa since it might be a place to uncover extraterrestrial life. What features could make a satellite so much from the sun a candidate for life? although the surface ar of Europa is a smooth class of ice, there is proof that there is an ocean of fluid water underneath (Figure below). Europa additionally has a continual source of power — that is heated together it is stretched and squashed by tidal pressures from Jupiter. Numerous missions have actually been to plan to check out Europa, including plans come drill with the ice and send a probe right into the ocean. However, no together mission has actually yet to be attempted.


An enhanced color picture of a section of Europa’s icy surface. The surface ar ice may have motions similar to bowl tectonics ~ above Earth.

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


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

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


This photo of Saturn and also its rings is a composite of photos taken through the Cassini orbiter in 2008

Like Jupiter, Saturn is made mostly of hydrogen and also helium gases in the outer layers and liquids at greater depths. The upper setting has clouds in bands of various colors. These turn rapidly roughly the planet, however there appears to be less turbulence and fewer storms top top Saturn 보다 on Jupiter. One amazing phenomena that has actually been it was observed in the storms ~ above Saturn is the existence of thunder and also lightning (see video, below). The earth likely has actually a small rocky and also metallic core.

Cassini scientists waited years because that the right conditions to create the very first movie that shows lightning on another planet — Saturn.http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots more videos indigenous the Cassini mission room indexed here:http://saturn.jpl.nasa.gov/video/

Saturn’s Rings

In 1610 Galileo very first observed Saturn’s rings v his telescope, yet he thought they might be two huge moons, one on either next of the planet. In 1659, the netherlands astronomer Christian Huygens realized the the attributes were rings (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 execute not touch the planet.


A color-exaggerated mosaic of Saturn and its ring taken through Cassini as Saturn eclipses the Sun.

The Voyager 1 and also 2 spacecraft in 1980 and also 1981 sent earlier detailed pictures of Saturn, that rings, and also some that its moons. Saturn’s rings space made of particles of water and also ice, with some dust and also rocks (Figure below). There are numerous gaps in the ring that scientists think have originated since (1) the material was cleared out by the gravitational pull within the ring or (2) through the gravitational pressures of Saturn and of moons external the rings.


A close-up the Saturn’s external C ring showing locations with higher particle concentration and gaps.

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

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

Saturn’s Moons

Most that Saturn’s moons room very little and only seven are large enough for heaviness to have made lock spherical. Only Titan is bigger than earth Moon at about 1.5 time its size. Titan is also larger 보다 the earth Mercury.

Scientists room interested in Titan because its atmosphere is similar to what Earth’s was like before life developed. Nitrogen is dominant and methane is the 2nd most plentiful gas. Titan may have actually a layer of liquid water and ammonia under a layer of surface ar ice. Lakes of fluid methane (CH4) and also ethane (C2H6) are discovered on Titan’s surface. Although conditions are comparable enough come those of at an early stage Earth for scientists to speculate that very primitive life might exist ~ above Titan, the extreme cold and lack of carbon dioxide make it unlikely (Figure below).


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


Uranus (YOOR-uh-nuhs) is called for the Greek god the the skies (Figure below). From Earth, Uranus is therefore faint the it to be unnoticed by old observers. Wilhelm Herschel an initial discovered the earth in 1781.


Uranus is an icy blue-green ball.

Although Uranus is really large, it is extremely far away, around 2.8 billion km (1.8 exchange rate mi) native the Sun. Light from the sunlight takes around 2 hours and 40 minute to with Uranus. Uranus orbits the sun once around every 84 earth years.

Uranus has actually a mass around 14 times the fixed of Earth, yet it is lot less dense than Earth. Heaviness at the surface ar 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 around 10% less than what you sweet on Earth.

An Icy Blue-Green Ball

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

When sunlight shows off Uranus, clouds of methane filter the end red light, providing the world a blue-green color. There are bands of clouds in the environment of Uranus, yet they are tough to check out in common light, so the planet looks like a plain blue ball.

The sideways Planet

Most of the planets in the solar system rotate on their axes in the same direction the they move around the Sun. Uranus, though, is tilted ~ above its side so that axis is practically parallel come its orbit. In other words, it rotates favor a optimal that to be turned so that it was spinning parallel come the floor. Researchers think that Uranus was more than likely knocked over by a collision with an additional planet-sized object billions of year ago.

Rings and Moons of Uranus

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


This photo from the Hubble an are Telescope reflects the pass out rings that Uranus. The planet is tilted on its side, therefore the ring are practically vertical.

Uranus has actually 27 known moons and also all but a few of lock are named for personalities from the plays of wilhelm Shakespeare. The 5 biggest moons of Uranus — Miranda, Ariel, Umbriel, Titania, and also Oberon — are presented in Figure below.


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


Neptune, presented in Figure below, is the only significant planet the can’t be checked out from earth without a telescope. Researchers predicted the existence of Neptune before it was discovered due to the fact that Uranus did not constantly appear precisely where it must appear. They knew that the gravitational traction of another planet beyond Uranus must be influence Uranus’ orbit.

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


This image of Neptune to be taken by Voyager 2 in 1989. The an excellent Dark Spot viewed on the left facility in the picture has due to the fact that disappeared, but a similar dark point out has appeared on another part of the planet.

In countless respects, Neptune is similar to Uranus (Figure below). Neptune has actually slightly an ext mass 보다 Uranus, but it is slightly smaller in size. Neptune is lot farther indigenous the sun at practically 4.5 billion kilometres (2.8 billion mi). The planet’s slow-moving orbit way that that takes 165 earth years to go once about the Sun.


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

Extremes the Cold and also Wind

Neptune’s blue color is mostly because of frozen methane (CH4). As soon as Voyager 2 visited Neptune in 1986, there to be a huge dark-blue spot the scientists named the great Dark Spot, southern of the equator. When the Hubble an are 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 that both of this spots stand for gaps in the methane clouds ~ above Neptune.

The transforming appearance the Neptune is caused by its stormy atmosphere. The winds top top Neptune space stronger than on any kind of other world in the solar system, reaching speeds of 1,100 km/h (700 mi/h), close to the rate of sound. This too much weather surprised astronomers, due to the fact that the planet receives small energy native the sunlight to strength weather systems. Neptune is additionally one of the coldest locations in the solar system. Temperatures at the top of the clouds are about -218oC (-360oF).

Neptune’s Rings and Moons

Neptune has actually faint ring of ice and also dust that may adjust or disappear in relatively short time frames.

Neptune has 13 known moons. Triton, displayed in Figure below, is the only among them that has enough mass to be spherical in shape. Triton orbits in the direction opposite come the orbit of Neptune. Researchers think Triton go not form around Neptune, but instead was caught by Neptune’s gravity as it happen by.


This image Triton, Neptune’s biggest moon, was taken through Voyager 2 in 1989.

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

Lesson Summary

The four outer planets space all gas giants made primarily of hydrogen and also helium. They have actually thick gaseous external layers and also liquid interiors.The outer planets have numerous moons, and also planetary rings.Jupiter, by far the biggest planet in the solar system, has actually bands of different colored clouds, and also a long-lasting storm referred to as the good Red Spot.Jupiter has much more than 60 moons including the 4 largest, the Galilean moons.Europa has actually an s of fluid water under a layer of ice wherein life may have formed.Saturn is smaller sized than Jupiter yet has a big system the beautiful rings.Titan’s setting is comparable to at an early stage Earth’s and also the moon can harbor primitive life.Uranus and Neptune were discovered relatively recently due to the fact that they space so far away.Uranus is tilted ~ above its side, probably because of a previous collision through a huge object.Neptune is an extremely cold and also has solid winds. That dark spots space storms in Neptune’s atmosphere.

Review Questions

1. Name the outer planets a) in order native the sunlight outward, b) from largest to the smallest by mass, and also c) from largest to the smallest by size.

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2. Why room the outer planets referred to as gas giants?

3. How do the great Red clues and great Dark clues differ?

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

5. Why might Europa be a likely place to discover extraterrestrial life?

6. What reasons gaps in Saturn’s rings?

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

8. What fluid is discovered 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 analysis / Supplemental Links

Points to Consider

The within planets are small and rocky, when the outer planets are large and gaseous. Why can the planets have created into these two groups?We have debated the Sun, the planets, and the moons the the planets. What other objects can you think of that deserve to be uncovered in our solar system?