Lesson Objectives

Describe crucial features of the outer planets and also their moons.Compare the external planets to each other and also to Earth.

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Galilean moonsgas giantsGreat Red Spotouter planetsplanetary rings


The four outer planets are farther native the Sun and also farther native Earth. They are lot more challenging to learn around since lock are really different indigenous our home planet.

The external Planets

The 4 planets the furthest from the Sun space the outer planetsFigure below shows the relative sizes of the external planets and also the Sun. These planets are much larger than the within planets and are made mostly of gases and also liquids, so they are likewise called gas giants.


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

The gas giants are consisted of primarily that hydrogen and also helium, the same facets that consist of most that the Sun. Astronomers think the hydrogen and helium gases consisted of much of the solar system when it an initial formed. Due to the fact that the within planets didn’t have enough mass to host on to these light gases, your hydrogen and also helium floated away right into space. The Sun and the huge outer planets had sufficient gravity to keep hydrogen and also helium native drifting away.

All of the external planets have numerous moons. They all additionally have planetary rings, composed of dust and also other tiny particles that encircle the planet in a slim plane.


Because Jupiter is therefore large, it shows a the majority of sunlight. Jupiter is incredibly 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 much from the planet — 5.20 AUs away. It takes Jupiter around 12 planet years to orbit once around the Sun.


This photo of Jupiter to be taken by Voyager 2 in 1979. The colors to be later enhanced to carry out more details.

Jupiter is named for the king that the gods in roman mythology. The world is enormous, the largest object in the solar mechanism besides the Sun. Although Jupiter is end 1,300 time Earth’s volume, it has only 318 time the mass of Earth. Prefer the various other gas giants, it is much less dense 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 on the surface of Jupiter would find that there is no solid surface at all! Jupiter is made largely of hydrogen, with some helium, and small amounts of other facets (Figure below).


Jupiter’s environment is composed of hydrogen and helium. Deeper within the planet, pressure compresses the gases right into a liquid. Some evidence suggests that Jupiter may have a tiny rocky main point of heavier elements at its center.

A Stormy Atmosphere

The top layer that Jupiter’s atmosphere has clouds that ammonia (NH3) in bands of various colors. This bands rotate about the planet, but additionally swirl roughly in turbulent storms. The Great Red Spot (Figure below) is one enormous, oval-shaped storm discovered south the Jupiter’s equator. This storm is an ext than three times as wide as the entire Earth. Clouds in the storm revolve in a counterclockwise direction, make one finish turn every six days or so. The great Red Spot has been top top Jupiter because that at least 300 years, because astronomers could very first see the storm v telescopes. Execute you think the great Red spot is a permanent function on Jupiter? How could you know?


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

Jupiter’s Moons and also Rings

Jupiter has actually a very big number the moons — 63 have been discovered so far. 4 are large enough and also bright sufficient to be checked out from Earth, using no much more than a pair of binoculars. This moons — Io, Europa, Ganymede, and Callisto — were an initial discovered by Galileo in 1610, for this reason they are occasionally 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 biggest moon in the solar mechanism it is even larger than the world Mercury!


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

Scientists are an especially interested in Europa because it may be a place to find extraterrestrial life. What features might make a satellite so much from the sunlight a candidate for life? although the surface ar of Europa is a smooth layer of ice, over there is proof that over there is an s of fluid water under (Figure below). Europa likewise has a continual resource of energy — that is heated together it is stretched and squashed by tidal forces from Jupiter. Countless missions have been plan to check out Europa, consisting of plans to drill with the ice and send a probe into the ocean. However, no together mission has yet been attempted.


An amplified color picture of a part of Europa’s icy surface. The surface ice may have actually motions comparable to key tectonics on Earth.

In 1979, 2 spacecrafts — Voyager 1 and Voyager 2 — checked out Jupiter and its moons. Photos native the Voyager objectives showed the Jupiter has a ring system. This ring device is really faint, so the is daunting to observe from Earth.


Saturn, shown in Figure below, is renowned for that beautiful rings. Although every the gas giants have actually rings, only Saturn’s can be conveniently seen indigenous Earth. In roman mythology, Saturn to be the dad of Jupiter.

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


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

Like Jupiter, Saturn is made mainly of hydrogen and also helium gases in the outer layers and liquids at higher depths. The upper environment has clouds in bands of different colors. These rotate rapidly approximately the planet, yet there appears to be less turbulence and fewer storms top top Saturn than on Jupiter. One exciting phenomena that has been it was observed in the storms ~ above Saturn is the existence of thunder and also lightning (see video, below). The world likely has a little rocky and metallic core.

Cassini researchers waited years for the right conditions to produce the an initial movie that mirrors lightning on another planet — Saturn.http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots an ext videos indigenous 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, yet he believed they can be two large moons, one top top either next of the planet. In 1659, the dutch astronomer Christian Huygens realized that the functions were rings (Figure below).

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


A color-exaggerated mosaic that Saturn and its rings 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 rings, and some that its moons. Saturn’s rings are made of particles of water and also ice, through some dust and rocks (Figure below). Over there are several gaps in the ring that scientists think have actually originated since (1) the product was cleared the end by the gravitational traction within the ring or (2) through the gravitational pressures of Saturn and of moons exterior the rings.


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

The rings to be likely formed by the breakup of one of Saturn’s moons or from product that never 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 their origin is as yet unknown.

Saturn’s Moons

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

Scientists space interested in Titan because its environment is similar to what Earth’s was like before life developed. Nitrogen is dominant and also methane is the second most abundant gas. Titan may have a great of fluid water and ammonia under a class of surface ar ice. Lakes of fluid methane (CH4) and ethane (C2H6) are found on Titan’s surface. Although problems are similar enough to those of early on Earth for researchers to speculate that incredibly primitive life may exist on Titan, the extreme cold and lack of carbon dioxide make it i can not qualify (Figure below).


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


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


Uranus is an icy blue-green ball.

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

Uranus has actually a mass around 14 time the mass of Earth, yet it is much less dense than Earth. Heaviness at the surface ar of Uranus is weaker 보다 on Earth’s surface ar so if you to be at the peak of the clouds ~ above Uranus, you would certainly weigh around 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, v an external gas layer that gives way to fluid on the inside. Uranus has a higher percentage the icy materials, such together water, ammonia (NH3), and also methane (CH4), 보다 Jupiter and also Saturn.

When sunlight shows off Uranus, clouds the methane filter out red light, offering the earth a blue-green color. There space bands the clouds in the environment of Uranus, however they are tough to watch in typical light, for this reason the world looks like a level blue ball.

The party Planet

Most that 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 almost parallel come its orbit. In other words, that rotates favor a top that was turned so the it was spinning parallel come the floor. Researchers think the Uranus was more than likely knocked over by a collision with one more planet-sized thing billions of years ago.

Rings and Moons that Uranus

Uranus has actually a faint device of rings (Figure below). The rings circle the planet’s equator, but because Uranus is tilted top top its side, the ring are almost perpendicular come the planet’s orbit.


This picture from the Hubble room Telescope shows the faint rings the Uranus. The planet is tilted top top its side, so the rings are nearly vertical.

Uranus has 27 recognized moons and all yet a couple of of castle are called for characters from the plays of william Shakespeare. The five biggest moons the Uranus — Miranda, Ariel, Umbriel, Titania, and also Oberon — are shown in Figure below.


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


Neptune, displayed in Figure below, is the only significant planet the can’t be checked out from earth without a telescope. Scientists predicted the visibility of Neptune prior to it to be discovered because Uranus walk not always appear specifically where it must appear. Lock knew the the gravitational pull of an additional planet beyond Uranus must be influence Uranus’ orbit.

Neptune was discovered in 1846, in the position that had been predicted, and also it was called Neptune because that the roman god that the sea because of that bluish color.


This image of Neptune to be taken by Voyager 2 in 1989. The great Dark Spot watched on the left center in the picture has because disappeared, yet a similar dark spot has showed up on another component of the planet.

In plenty of respects, Neptune is similar to Uranus (Figure below). Neptune has slightly an ext mass 보다 Uranus, however it is slightly smaller sized in size. Neptune is lot farther native the sun at nearly 4.5 billion km (2.8 exchange rate mi). The planet’s slow orbit method that the takes 165 earth years to walk once roughly the Sun.


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

Extremes of Cold and Wind

Neptune’s blue color is mostly because of frozen methane (CH4). When Voyager 2 saw Neptune in 1986, there to be a large dark-blue spot the scientists named the great Dark Spot, south of the equator. As soon as the Hubble room Telescope took images of Neptune in 1994, the good Dark Spot had actually disappeared but one more dark clues had appeared north of the equator. Astronomers think the both of this spots stand for gaps in the methane clouds ~ above Neptune.

The changing appearance of Neptune is resulted in by its stormy atmosphere. The winds top top Neptune room stronger than on any type of other planet in the solar system, getting to speeds that 1,100 km/h (700 mi/h), close come the rate of sound. This extreme weather surprised astronomers, due to the fact that the planet receives tiny energy indigenous the sun to strength weather systems. Neptune is likewise one the the coldest locations in the solar system. Temperatures at the peak of the clouds are around -218oC (-360oF).

Neptune’s Rings and also Moons

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

Neptune has 13 well-known moons. Triton, shown in Figure below, is the only among them that has sufficient mass to it is in spherical in shape. Triton orbits in the direction opposite to the orbit the Neptune. Researchers think Triton walk not form around Neptune, but instead was recorded by Neptune’s gravity as it happen by.


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

Fly through 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 room all gas giants made mostly of hydrogen and also helium. They have actually thick gaseous outer layers and also liquid interiors.The external planets have many moons, and also planetary rings.Jupiter, by far the biggest planet in the solar system, has bands of various colored clouds, and a long-lasting storm referred to as the good Red Spot.Jupiter has much more than 60 moons consisting of the 4 largest, the Galilean moons.Europa has an ocean of liquid water under a class of ice where life may have actually formed.Saturn is smaller than Jupiter yet has a large system that beautiful rings.Titan’s environment is comparable to beforehand Earth’s and also the moon could harbor primitive life.Uranus and also Neptune to be discovered reasonably recently because they space so much away.Uranus is tilted ~ above its side, probably due to the fact that of a previous collision v a big object.Neptune is really cold and has strong winds. The dark spots are storms in Neptune’s atmosphere.

Review Questions

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

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2. Why room the outer planets dubbed gas giants?

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

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

5. Why could Europa be a most likely place to find extraterrestrial life?

6. What reasons gaps in Saturn’s rings?

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

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

9. Why is Uranus blue-green in color?

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

Further reading / Supplemental Links

Points come Consider

The within planets are little and rocky, while the outer planets are big and gaseous. Why could the planets have created into these two groups?We have debated the Sun, the planets, and the moons that the planets. What various other objects deserve to you think that that deserve to be found in our solar system?