Lesson Objectives

Describe crucial features that the outer planets and their moons.Compare the outer planets to every other and also to Earth.

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Galilean moonsgas giantsGreat Red Spotouter planetsplanetary rings


The 4 outer planets room farther indigenous the Sun and also farther from Earth. They are much more an overwhelming to learn around since lock are very different indigenous our home planet.

The external Planets

The four planets the furthest from the Sun are the outer planetsFigure below shows the relative sizes of the outer planets and the Sun. This planets are much larger than the inner planets and are made mainly of gases and also liquids, therefore they are also called gas giants.


This photo shows the 4 outer planets and the Sun, with sizes come scale. From left come right, the outer planets are Jupiter, Saturn, Uranus, and Neptune.

The gas giants are consisted of primarily the hydrogen and helium, the same elements that make up most that the Sun. Astronomers think the hydrogen and helium gases consisted of much the the solar mechanism when it very first formed. Since the inner planets no have sufficient mass to organize on to these light gases, their hydrogen and also helium floated away right into space. The Sun and the huge outer planets had enough gravity to save hydrogen and also helium indigenous drifting away.

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


Because Jupiter is so large, it shows a many sunlight. Jupiter is very bright in the night sky; just the Moon and also Venus space brighter (Figure below). This brightness is every the an ext impressive since Jupiter is quite much from the earth — 5.20 AUs away. The takes Jupiter around 12 planet years come orbit once about the Sun.


This image of Jupiter to be taken by Voyager 2 in 1979. The colors to be later amplified to bring out much more details.

Jupiter is named for the king the the gods in roman inn mythology. The world is enormous, the largest object in the solar device besides the Sun. Return Jupiter is end 1,300 times Earth’s volume, it has only 318 time the massive of Earth. Prefer the other gas giants, the is much less thick than Earth.

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

A round of Gas and also Liquid

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


Jupiter’s environment is written of hydrogen and also helium. Deeper within the planet, press compresses the gases into a liquid. Part evidence argues that Jupiter may have actually a little rocky main point of heavier aspects at its center.

A Stormy Atmosphere

The upper layer of Jupiter’s atmosphere includes clouds the ammonia (NH3) in bands of different colors. These bands rotate about the planet, but likewise swirl roughly in unstable storms. The Great Red Spot (Figure below) is one enormous, oval-shaped storm uncovered south that Jupiter’s equator. This storm is more than three times as vast as the whole Earth. Clouds in the storm turn in a counterclockwise direction, make one complete turn every six days or so. The great Red Spot has been on Jupiter because that at the very least 300 years, since astronomers could an initial see the storm with telescopes. Execute you think the an excellent Red clues is a permanent attribute on Jupiter? How can you know?


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

Jupiter’s Moons and Rings

Jupiter has actually a very large number that moons — 63 have actually been discovered so far. Four are large enough and bright sufficient to be seen from Earth, making use of no much more than a pair of binoculars. This moons — Io, Europa, Ganymede, and Callisto — were first discovered by 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 also Eris. Ganymede is not only the greatest moon in the solar system it is even larger than the earth Mercury!


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

Scientists are an especially interested in Europa due to the fact that it may be a location to discover extraterrestrial life. What features can make a satellite so far from the sun a candidate because that life? return the surface ar of Europa is a smooth class of ice, over there is proof that over there is an s of liquid water under (Figure below). Europa additionally has a continual resource of energy — the is heated together it is stretched and also squashed through tidal pressures from Jupiter. Numerous missions have actually been planned to explore Europa, including plans to drill with the ice and also send a probe right into the ocean. However, no together mission has yet been attempted.


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

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


Saturn, displayed in Figure below, is famous for that is beautiful rings. Although every the gas giants have rings, only Saturn’s can be easily seen native Earth. In roman mythology, Saturn to be the father of Jupiter.

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


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

Like Jupiter, Saturn is made largely of hydrogen and also helium gases in the external layers and liquids at greater depths. The upper atmosphere has clouds in bands of various colors. These rotate rapidly about the planet, but there appears to be much less turbulence and fewer storms top top Saturn 보다 on Jupiter. One amazing phenomena that has actually been observed in the storms top top Saturn is the existence of thunder and also lightning (see video, below). The planet likely has a tiny rocky and also metallic core.

Cassini scientists waited years for the right problems to create the very first movie that mirrors lightning on one more planet — Saturn.http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots much 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, however he believed they could be two huge moons, one ~ above either side of the planet. In 1659, the dutch astronomer Christian Huygens realized the the functions were ring (Figure below).

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


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

The Voyager 1 and 2 spacecraft in 1980 and 1981 sent back detailed images of Saturn, the rings, and some the its moons. Saturn’s rings space made of particles of water and ice, through some dust and also rocks (Figure below). There are several gaps in the ring that researchers think have actually originated because (1) the material was cleared the end by the gravitational pull within the rings or (2) by the gravitational pressures of Saturn and of moons exterior the rings.


A close-up of Saturn’s external C ring showing locations with greater particle concentration and 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 initially 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 also their beginning is together yet unknown.

Saturn’s Moons

Most of Saturn’s moons are very small and just seven are big enough for gravity to have made lock spherical. Only Titan is larger than earth Moon at around 1.5 time its size. Titan is even larger 보다 the planet Mercury.

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


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


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


Uranus is an icy blue-green ball.

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

Uranus has a mass about 14 time the fixed of Earth, yet it is lot less dense than Earth. Gravity at the surface ar of Uranus is weaker than on Earth’s surface so if you to be at the optimal of the clouds on Uranus, you would weigh about 10% less than what you weigh on Earth.

An Icy Blue-Green Ball

Like Jupiter and Saturn, Uranus is composed greatly of hydrogen and also helium, v an external gas layer the gives means to liquid on the inside. Uranus has 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 that methane filter out red light, giving the earth a blue-green color. There space bands the clouds in the atmosphere of Uranus, yet they are tough to watch in common light, therefore the earth looks prefer a level blue ball.

The party Planet

Most the the planets in the solar mechanism rotate on their axes in the very same direction that they move around the Sun. Uranus, though, is tilted on its side so that axis is virtually parallel to its orbit. In other words, it rotates choose a peak that to be turned so the it to be spinning parallel come the floor. Researchers think that Uranus was probably knocked over by a collision with another planet-sized thing billions of year ago.

Rings and also Moons the Uranus

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


This picture from the Hubble space Telescope mirrors the pass out rings that Uranus. The earth is tilted ~ above its side, so the rings are almost vertical.

Uranus has 27 well-known moons and all but a couple of of castle are named for characters from the plays of william Shakespeare. The five biggest moons that Uranus — Miranda, Ariel, Umbriel, Titania, and also Oberon — are presented in Figure below.


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


Neptune, shown in Figure below, is the only significant planet that can’t be viewed from earth without a telescope. Researchers predicted the existence of Neptune prior to it to be discovered since Uranus did not constantly appear specifically where it must appear. Castle knew that the gravitational traction of one more planet beyond Uranus must be influence Uranus’ orbit.

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


This image of Neptune to be taken through Voyager 2 in 1989. The good Dark Spot watched on the left center in the photo has since disappeared, but a similar dark clues has appeared on another component of the planet.

In plenty of respects, Neptune is comparable to Uranus (Figure below). Neptune has slightly much more mass 보다 Uranus, but it is slightly smaller sized in size. Neptune is lot farther indigenous the sun at virtually 4.5 billion kilometres (2.8 billion mi). The planet’s slow orbit way that it takes 165 earth years to go once roughly the Sun.


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

Extremes that Cold and Wind

Neptune’s blue color is mostly since of frozen methane (CH4). As soon as Voyager 2 went to Neptune in 1986, there to be a big dark-blue spot the scientists called the an excellent Dark Spot, southern of the equator. When the Hubble space Telescope took photos of Neptune in 1994, the good Dark Spot had disappeared but an additional dark point out had appeared north that the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune.

The transforming appearance of Neptune is resulted in by its unstable atmosphere. The winds top top Neptune space stronger 보다 on any kind of other world in the solar system, reaching speeds the 1,100 km/h (700 mi/h), close to the speed of sound. This extreme weather surprised astronomers, since the world receives tiny energy native the sunlight to power weather systems. Neptune is also one the the coldest areas in the solar system. Temperatures at the top of the clouds are about -218oC (-360oF).

Neptune’s Rings and Moons

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

Neptune has actually 13 well-known moons. Triton, shown in Figure below, is the only one of them the has enough mass to be spherical in shape. Triton orbits in the direction opposite to the orbit that Neptune. Scientists think Triton walk not form around Neptune, however instead was recorded by Neptune’s gravity together it pass by.


This photo Triton, Neptune’s largest moon, was taken by 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 generally of hydrogen and helium. They have actually thick gaseous outer layers and also liquid interiors.The external planets have many moons, and planetary rings.Jupiter, by far the biggest planet in the solar system, has bands of various colored clouds, and also a long-lasting storm called the good Red Spot.Jupiter has an ext than 60 moons consisting of the four largest, the Galilean moons.Europa has an ocean of fluid water under a great of ice wherein life may have actually formed.Saturn is smaller sized than Jupiter however has a large system of beautiful rings.Titan’s atmosphere is comparable to early Earth’s and the moon can harbor primitive life.Uranus and also Neptune to be discovered relatively recently since 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 has solid winds. That is dark spots room storms in Neptune’s atmosphere.

Review Questions

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

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2. Why are the outer planets called gas giants?

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

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

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

6. What causes gaps in Saturn’s rings?

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

8. What liquid is discovered on the surface 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 inside planets are small and rocky, when the external planets are huge and gaseous. Why might the planets have developed into these two groups?We have disputed the Sun, the planets, and the moons the the planets. What various other objects can you think that that can be discovered in our solar system?