The Solar System

The Solar System is our latest unit of study.  We are working on a PowerPoint presentation about each of the planets in the solar system.  We inflated planets and hung them from the ceiling in the classroom.  We are very excited to learn more about the solar system.

Our solar system consists of the sun, nine planets (and their moons), an asteroid belt, and many comets and meteors. The sun is the center of our solar system; the planets, over 61 moons, the asteroids, comets, meteoroids and other rocks and gas all orbit the Sun.

• The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.
• The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto which is small, rocky, and has one moon).

• Asteroids (also called minor planets) are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter.
• Comets are small, icy bodies that orbit the sun. They have very long tails.
• Meteoroids are small bodies that travel through space. They are stony and/or metallic and are smaller than asteroids. Most are very tiny.

What is the order of the planets in our solar system? 

(Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) 

Here are key characteristics of each planet and how the sizes of the planets compare to each other.

• Earth- 
• only planet to support life as we know it, 
• third planet from the Sun, 
• made of rock and surrounded by a layer of gas called and atmosphere.
• Jupiter- 
• largest planet,
• has Great Red Spot,
•  has twice as much mass as all other planets put together.
• Mars-
•  the red planet, 
• most like the Earth, 
• has two moons called Deimos and Phobos.
• Mercury- 
• closest to the Sun, 
• has craters, 
• has no atmosphere
• Neptune- 
• has an icy moon called Triton, 
• bluish in color, 
• great gas ball
• Pluto- 
• smallest planet, 
• last to be discovered, 
• has moon called Charon
• Saturn- 
• has thousands of rings and most satellites, 
• second largest planet
• Uranus- 
• spins on a nearly horizontal axis, 
• made of gas and liquid gas, 
• tipped on its side when it travels.
• Venus-
• atmosphere of hot, 
• swirling clouds of carbon dioxide, 
• called the Evening Star

Types of Planets

		Terrestrial Planets - Also known as rocky planets, these bodies are composed primarily of rock and metal and have very high densities. They also tend to be relatively small in size and have slow periods of rotation. The terrestrial planets in our solar system are Mercury, Venus, Earth, and Mars. They are the planets closest to the Sun. Terrestrial planets tend to have very few natural satellites, or moons. Of the four terrestrial planets in our solar system, only two have moons. Earth has one moon while Mars has two.
		Gas Giants - Four of the outer planets in our solar system are known as gas giants. They are Jupiter, Saturn, Uranus, and Neptune. Gas giants are composed mainly of hydrogen and helium and are quite large in size. Jupiter, for example, is 1000 times larger than the earth. Gas giants also have low densities and tend have a very fast period of rotation. All four of the gas giants in our solar system have ring systems and a large number of moons. This may be due to the intense gravity of these planets. They may have more of a tendency to capture wandering asteroids and planetoids then the terrestrial planets. It is believed that the ring systems may have formed from old moons that were pulverized by the tidal forces of the planets' gravity.
		Planetoids - Planetoids is a name given to group of objects that are too large to be asteroids and too small to be planets. Thousands of these objects are known to be orbiting the outer regions of our solar system in an area known as the Kuiper Belt. In fact, many astronomers believe that the planet Pluto and its moon Charon may actually be Kuiper belt objects and not planets. Little is known about these distant objects. They are believed to be formed of the ancient material that formed the solar system. It is thought that occasional gravitational anomalies may send these objects on a path towards the inner solar system where they may be captured by the gravitational pull of one of the planets. This could explain the extremely large number of moons orbiting the gas giants.

Stars

The Sun

A Ball of Hot Gas

We start our journey through the solar system at the center. Here we find a gigantic ball of gas so massive that the immense pressure has ignited a fusion reaction. It is an average-sized yellow star known as the Sun. Here in this cosmic furnace, hydrogen atoms under unimaginable pressures are being fused into helium atoms, releasing a tremendous amount of energy. Scientists believe that this reaction has been taking place for almost 5 billion years, and will likely continue for another 5 billion years. At that time, the Sun's hydrogen supply will have been depleted, and heavier elements will begin to fuse. This will cause the star to swell to the size of a red giant, consuming most if the inner planets in the process. Eventually the Sun will end its life as a cold lightless body known as a black dwarf.

Solar Facts

The Sun is the largest body in the solar system. It has a diameter of over 863,482 miles (1,390,000 km). The nuclear reactions taking place within the Sun drive its surface temperature to over 10,832° F (6,000° C). Solar radiation is measured in wavelengths that include radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Like all other bodies in the solar system, the Sun rotates on its axis. Unlike the others, its equatorial region rotates faster that its polar regions. The equatorial region rotates once every 27 days, while the polar regions rotate every 34 days.

 

Mercury

 

 

The next stop on our tour is a small, barren world called Mercury. It is the closest planet to the Sun and the second smallest planet in the solar system. Mercury was named after the ancient Roman messenger of the gods because it moved across the sky much faster than the other planets. In fact, Mercury completes one orbit around the Sun in only 88 days! The planet has a very slow rotational period. It takes 58 days for Mercury to make a complete rotation on its axis. Because of its close proximity to the Sun, it can only be observed in the sky in the east just before sunrise or in the west just after sunset. It never appears more than 27 degrees away from the Sun.

A Hot and Cold Barren World

Mercury has almost no atmosphere. Scientists believe that a thicker atmosphere may have existed at one time, but was stripped away by the strong solar winds. The atmosphere on Earth helps to maintain a relatively uniform temperature around the planet. By contrast, the temperature on Mercury ranges from 800° F (427° C) during the day, to -279° F (-173° C) at night. It is indeed a very inhospitable world.

Features of Mercury

Mercury was visited by NASA's Mariner 10 space probe in 1974. In a series of three flybys, the probe managed to map most of the planet's surface. When the photos were received on Earth, the features might easily have been mistaken for the moon. Mercury's surface, however, is almost completely covered with craters. The moon has more open areas known as maria, which are mostly crater-free. Mercury has only one major open area known as the Caloris Basin. These open areas are thought to have been formed by more recent volcanic flows, which may have covered the older craters. The surface is also marked by cliffs which rise as much as 6300 feet (1920 m) above the valley floors.

 

Venus

 

Goddess of Love and Beauty

Our next stop on the tour brings us to a planet completely covered by whispy white clouds. This is the planet Venus, and it is the second planet from the Sun. Venus is named after the Roman goddess of love and beauty. It is also known as the jewel of the sky, and as the morning star. Venus is so bright in the sky that it has been known since prehistoric times. Venus is sometimes referred to as Earth's sister planet, as both are nearly the same size and have nearly the same mass. Venus differs from all other bodies in the solar system because it rotates from east to west. It is, in a sense, upside down. Another odd fact about this planet is that its day is longer than its year! Venus takes 225 Earth days to make one complete revolution around the Sun, while it takes 243 days for Venus to rotate once on its axis.

A Cloud Covered Greenhouse

Contrary to its outward appearance, Venus is a very inhospitable world. The planet is covered by a thick atmosphere composed primarily of carbon dioxide with almost no water vapor. It is so thick in fact, that the pressure at the surface of the planet is 92 times that of Earth. Carbon dioxide allows solar radiation in, but prevents it from escaping back into space. This works very much like a greenhouse, and because of this "greenhouse effect", temperatures on the surface rise to over 900° F (482° C). To make things worse, the thick layer of clouds in the atmosphere is composed of sulfuric acid droplets. There is no water on Venus, but pools of strong sulfuric acid may be found. Beneath its innocent and delicate whispy cloud tops, Venus could easily be mistaken for hell.

Features of Venus

Until recently, the dense cloud cover on Venus prevented every attempt to get a look at its surface. The U.S. Mariner spacecraft could only send back pictures of a world shrouded in clouds. The real breakthroughs came with Pioneer Venus in 1978, Magellan in 1990, and with two Soviet space probes in 1982. Pioneer Venus was able to do some low resolution mapping of the surface. The two Soviet Venera probes landed on the surface, but could only send back a few poorly aimed pictures before they were destroyed by the intense temperature and pressure. Nevertheless, they did show a barren rocky terrain. Magellan used high definition radar to map the planet in detail, and revealed a relatively young surface. It consists of plains covered by lava flows, and highland regions formed by geological activity. The surface is dotted by thousands of volcanoes and numerous impact craters. Venus is a very active planet undergoing constant change.

 

Earth

 

The Blue Planet

 This is the next stop on our tour, the planet Earth. It is the third planet from the Sun, and it is our home. Only slightly larger than Venus, Earth is the largest of the four terrestrial, or rocky planets. There are two very important factors that distinguish this planet from all others in the solar system. First, it is the only planet known to contain liquid water on its surface. It is the availability of this water that gives rise to Earth's second unique feature. It is the only known planet to support life!

Planet of Water, Planet of Life

The surface of Earth is literally covered with water. Nearly 3/4 of the planet's surface is liquid water. Water is also found in abundance as ice in the planet's poles and as water vapor in the atmosphere. Most of this water was thought to have been deposited by billions of years of volcanic activity. Some scientists believe that water may also have been brought to Earth by early comets. It is this water, combined with Earth's perfect distance from the sun, that has given rise to the solar system's only known life forms. The atmosphere also played an important factor, providing the right mixture of gases and helping to protect the surface from meteor impacts. Because of these perfect conditions, Earth has evolved an unimaginable diversity of millions of different life forms. Life on Earth has adapted to all types of environments, including the land, oceans, and air. At least one of these has become self-aware and has begun to explore its surroundings. We humans have become the first form of life to leave their planetary home and explore the universe.

Features of Earth

Earth is a rocky world that is still very much alive and active. Volcanic activity continually creates new land and Earthquakes frequently reshape the landscape. Earth's surface is constantly changing. The crust is divided up into several "plates" (somewhat like a cracked eggshell) that float on a sea of molten rock. Where these plates collide, the land is thrust upwards to create enormous mountain ranges. The tallest of these mountains, Mt. Everest, rises 29,028 feet (8850 m) above sea level. In some places, one plate sinks underneath another forming deep valleys and trenches. The deepest of these, the Marianas trench, plunges seven miles (11 km) beneath the sea. Uneven heating of Earth's atmosphere, combined with its rotation causes complex weather patterns. Wind and rain gradually erode the mountains and create vast sandy plains. Even though the planet was bombarded by asteroids and meteors throughout its early history, erosion has wiped out most of the craters. Some can still be found, however. Meteor crater in Arizona is over a mile (1.6 km) wide and almost 600 feet (183 m) deep. It is believed that large meteors may have caused mass extinctions throughout Earth's history. One of these may have been responsible for wiping out the largest animals that ever lived - the dinosaurs. Although the planet may seem calm and tranquil now, its surface is highly active and ever-changing on the wider geologic scale of time.

 

Mars

 

God of War

 This is the planet Mars, and it is the fourth planet from the sun. It was named after the Roman god of war because its color reminded observers of a bloody Roman battlefield. Early astronomers believed they could see faint, dark lines crossing the surface of the planet. Some believed these to be artificially built canals. This gave rise to some of the early speculation of intelligent life on Mars. These lines were later found to be an optical illusion. Later unmanned missions to the planet found no evidence of the so-called canals. Still, there is perhaps no other body in the solar system that has intrigued us more than Mars. To this very day, it is still one of the most likely candidates for extraterrestrial life. In fact, meteorites recently discovered in Antarctica are believed to have originated from the red planet. These ancient Martian rocks may contain fossils of tiny bacteria. If this turns out to be true, it will be the first evidence that life evolved on other planets besides Earth.

The Red Planet

The bright red color of Mars has earned it the name of red planet. This unique color is caused by the chemistry of its surface. Mars does have a thin atmosphere, which helps cause iron compounds to oxidize. It is this iron oxide in the rocks and sand that gives the planet it reddish-brown color. It could very appropriately be called the "rusty" planet. We have learned a great deal about Mars in the last few decades. Several unmanned space missions have been sent to explore the red planet. Several Mariner spacecraft orbited the planet in the mid and late 1960's. In 1975, two Viking spacecraft landed on the surface, sending back the first color pictures of this alien world. Recently, the Mars Pathfinder mission visited the planet. A small roving vehicle known as the Sojourner Rover explored the surface sending back the sharpest pictures to date. Scientists continue to be fascinated with Mars because of its similarities to Earth. Some have even suggested that we may be able to colonize the planet in the not too distant future. If the planet does turn out to be completely lifeless, it may be a candidate for a process known as terraforming. This would involve using various chemical and biological means to make the planet warmer and the atmosphere more hospitable for human life.

Features of Mars

Mars is a small, rocky world about half the size of Earth. As small as it is, however, it is a planet of extremes. Mars is the home of the largest known volcano in the solar system. Olympus Mons is an extinct volcanic crater that rises 15 miles above the Martian surface. That is over twice the height of the tallest mountain on Earth, Mt. Everest. It is a scar so large it can be seen from Earth. Another gigantic feature of the red planet is Valles Marinares. This is the Martian equivalent of the Grand Canyon. This one is much bigger, however. Valles Marinares stretches 3000 miles (4829 km) along the surface of Mars and is 435 miles (700 km) wide at its widest point. If this canyon existed on Earth, it would stretch across the continental United States from coast to coast! During the Martian winters, the poles of the planet are covered with large sheets of water ice. Several features that resemble river beds suggest that there may have been running water on Mars at one time. The thin atmosphere of Mars is composed mainly of carbon dioxide. Winds of up to 300 miles per hour whip up enormous dust storms that constantly erode and reshape the surface of the planet. With an average surface temperature of 60 degrees below zero, it may not seem very hospitable. But with its thin atmosphere and abundance of water, Mars remains the most likely candidate for life anywhere in the solar system. A future manned mission to the planet is in the works. Only then will we be able to tell if the red planet is really a dead planet.

Jupiter

This is the planet Jupiter. It is the fifth planet from the Sun and is the largest. In fact, Jupiter is almost twice the size of all of the other planets combined. Over one thousand Earths could fit inside if it were hollow. Because of its gigantic size, Jupiter was named after the king of the gods in Roman mythology. In Greek mythology, he was known as Zeus, and was famous for hurling lightning bolts at earthbound mortals. Jupiter is the fourth brightest object in the sky, so it has been known to exist since prehistoric times. But it was Galileo who discovered Jupiter's four largest moons, Callisto, Europa, Ganymede, and Io, in 1610. We now know of 61 moons in orbit around Jupiter, ranging in size from just a few miles across to nearly Earth-sized. The Jovian system is like a miniature solar system within a solar system. With so many satellites tugging on Jupiter with their tidal forces, these moons are actually causing Jupiter's rotation to slow down. These same tidal forces are also pushing the moons farther away from the massive planet.

The Famous Red Spot

Jupiter is probably most famous for its giant red spot. This spot is actually a gigantic storm system. It is a high pressure region where the winds blow in a counter-clockwise direction at over 250 miles (402 km) per hour. The cloud tops are much higher and colder than the surrounding clouds. This accounts for the spot's darker color. With a diameter of more than 16,000 miles (25,000 km), this storm is twice the size of the Earth!  Many spots and colorful bands appear and disappear in Jupiter's turbulent atmosphere, but the great red spot has been know to exist for more than 300 years. Astronomers do not yet know what forces have kept this storm together for so long. And nobody knows just how much longer it will last. Spots of a different kind were seen in 1994 when a comet named Shoemaker-Levy 9 smashed into the planet. The comet had broken into 21 separate fragments as a result of a close encounter with Jupiter's massive gravitational forces. On July 16 1994, for seven days, these fragments bombarded the giant planet. The impacts left monstrous black scars on the Jupiter's surface. Explosive plumes several times the size of Earth were observed, and the dark scars remained visible on the planet for nearly a year. It was one of the most watched astronomical events in history, and it proved once and for all that comets do hit planets.

Features of Jupiter

Jupiter has amazed and intrigued astronomers for centuries. It has been visited by a total of six spacecraft, including the current Galileo mission. In 1995, Galileo dropped a probe into Jupiter's atmosphere. The probe took measurements and gave us our first clues as to what was inside Jupiter's thick atmosphere. With this data, and data collected on previous missions, we now know that Jupiter is essentially a gigantic ball of gas. The planet is composed mainly of hydrogen, with helium and traces amounts of other gases. One surprising find was that there is much less water in Jupiter's atmosphere than previously expected. Astronomers believe that Jupiter has no solid surface. Instead the gases get thicker and and denser as they get closer to the planet's center. The extreme pressure at Jupiter's core compresses the hydrogen into a rare liquid metallic state. The planet may also have a small, rocky core about 10 times the size of Earth. Jupiter's outer atmosphere is extremely dynamic. Colorful bands and spots can be see rotating and changing regularly. The colors in the bands are believed to be the result of chemical reactions in the atmosphere. The Galileo probe measured wind speeds exceeding 400 miles (643 km) per hour. A big surprise revealed by the Voyager 1 spacecraft was that Jupiter has rings, much like Saturn. But unlike Saturn's rings, Jupiter's rings are very delicate and simple. These rings are divided into three main segments, and were not known to exist prior to Voyager's discovery because they are too thin to be seen from Earth. One other item in Jupiter's long list of oddities is its magnetic field. It is so strong that is actually extends past the orbit of Saturn. In fact, it is 4000 times stronger than Earth's magnetic field. Even stranger still is the fact that Jupiter's rotation is so fast - only 9.8 Earth hours, that the planet actually bulges out in the center. A close examination of any Jupiter photo will reveal that the planet is more oval shaped than round.

Saturn

This is the planet Saturn. It is the sixth planet from the Sun and is the second largest in the solar system. Saturn was named after the Roman god of agriculture and the harvest. His Greek counterpart was Chronus, the youngest of the Titans. Many consider Saturn to be the most beautiful object in the solar system. It is one of the more bright objects in the night sky, and has been known since prehistoric times. Galileo was the first person to observe Saturn through a telescope in 1610. He noticed the planet's odd shape, but was unable to figure out the significance of what he had seen. In 1659, Christiaan Huygens was able to correctly identify the odd shape as Saturn's rings. Since then, Saturn has been visited by three Spacecraft including Pioneer 11 and Voyagers 1 and 2. An new space probe named Cassini is on its way to the giant planet, and will arrive in 2004. Saturn is surrounded by a small army of 31 known moons second only to Jupiter in number. Most of these moons are small, but seven of them are large enough to warrant further investigation. We visit them later in the tour.

The Ringed Planet

Without a doubt the most striking feature of Saturn is its rings. Unlike the rings of Jupiter, which were not discovered until the Voyager 1 encounter in 1979, Saturn's rings have been known to exist since 1659. This is due to the fact that the ring system is quite large and bright. It can easily be seen from Earth with a small telescope. The view of Saturn's rings from Earth actually changes from year to year. This is caused by the Earth passing through the plane of Saturn's rings. Every few years, the rings are seen edge-on and actually seem to disappear completely. This is because the rings are only slightly more than a mile thick. This change in the ring system's orientation causes the brightness, or apparent magnitude of the planet to change as well. Saturn has three main ring bands. They are identified as the A, B, and C-rings. A large gap can be seen between the A and B rings. This is know as the Cassini division, and was named after Giovanni Cassini, the discoverer the gap as well as four of Saturn's largest moons. It wasn't until the Voyager encounters with Saturn in 1980 and 1981 that the true mystery of Saturn's rings was unveiled. The ring system is really composed of seven rings with extremely complex structures. The larger rings are actually composed of many smaller ringlets. The Voyager probes also found strange radial, spoke-like structures in the rings. These are believed to be caused by dust particles. Saturn's innermost ring, the F-ring, appears to be knotted or braided. Astronomers are not sure yet what causes this strange phenomenon. The Voyager probes revealed the existence of small moons in and near the rings. They are called "shepherding satellites" because they are believed to help keep the rings in place. Without the gravitation of these moons, the ring particles might fly off into space and the rings would be lost. The origin of Saturn's rings is not well understood. They appear to be composed of millions of tiny particles of ice and rock, each in its own orbit around the planet. The size of these particles ranges from as small as a dust grain to as large as an iceberg. Astronomers believe that the rings could be the debris of one or more moons that were pulverized by asteroid or comet impacts.

Features of Saturn

Even though the ring system is Saturn's most striking feature, the planet is also interesting in a number of other ways. Saturn is a gas giant with a chemical composition very similar to that of Jupiter. It is composed of about 75% hydrogen and 25% helium, with traces of methane, water, and ammonia. Astronomers believe that Saturn's interior is also similar to that of Jupiter, with a core of rock and ice, surrounded by a layer of liquid metallic hydrogen. There is no solid surface on Saturn. Instead, the atmospheric gases get denser and thicker as they approach the planet's center. Saturn is also the least dense of all the planets. Its specific gravity is only 0.7, which is less than that of water. If Saturn could be placed into a giant sea of water, it would actually float!  Saturn shares many other features with its close cousin, Jupiter, such as high winds and cloud banding. Winds at the equator can reach an unbelievable 1,100 miles per hour. The horizontal bands on Saturn are much fainter than those on Jupiter. The Hubble space telescope has also revealed that storms occasionally erupt in Saturn's turbulent atmosphere. Like most of the other gas giant planets, Saturn has a strong magnetic field. Like Jupiter, Saturn is visibly flattened at its poles. This is caused by the planet's rapid rotation of only 10 hours, 39 minutes. In July of 2004, the Cassini probe is scheduled to arrive at Saturn. It will orbit the planet and send back detailed images and data. It will also drop the Huygens probe into the atmosphere of Saturn's moon Titan. There it will land and sample the composition of Titan's thick atmosphere and send back images of the surface.

 Uranus

This is the planet Uranus, and it is the seventh planet from the Sun. Uranus was named after the god of the heavens in Greek mythology. According to myth, Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans. The planet Uranus was the first planet to be discovered in modern times. The British astronomer Sir William Herschel found it with his telescope in 1781. Before this, it had been assumed to be a star. Herschel also discovered Uranus' two largest moons, Oberon and Titania. Uranus has only been visited by one spacecraft. This was the extended Voyager 2 mission. The encounter took place on January 24, 1986. Nearly everything we know about Uranus was learned at this time.

The Sideways Planet

Uranus is different from any other planet in the solar system in that its axis of rotation is tilted 98 degrees. It is nearly parallel to the ecliptic. Uranus appears to be tipped on its side. This has some interesting effects on the planet. The poles of Uranus point alternately at the Sun rather than its equatorial regions, as on Earth. Astronomers are not sure what accounts for this strange position. One theory suggests that a massive collision with another planet-sized body may have knocked Uranus on its side. This collision is believed to have taken place very early in the solar system's history. Another strange fact about Uranus is that its magnetic field is tilted at a 60 degree angle to its axis of rotation. This angle is only 12 degrees on Earth. An interesting effect of Uranus' sideways tilt is that the tail of the planet's magnetic field is twisted like a corkscrew.

Features of Uranus

When Voyager 2 arrived at Uranus, astronomers were expecting to find complex cloud structures similar to those on Jupiter and Saturn. But the images finally arrived, nothing was visible but a featureless blue-green sphere. False-color images of the planet later showed that banded structures of clouds were present but were extremely faint. They are partially obscured by the overlying layer of methane. Uranus is the third of the gas giants from the Sun. It is believed to be composed of about 83% hydrogen, 15% helium, 2% methane, and trace amounts of acetylene and other hydrocarbons. It is the methane in the planet's upper atmosphere that gives it a blue-green color. This is because the methane absorbs red light. High winds in the middle latitudes of Uranus blow in the direction of the planet's rotation at up to 360 miles (580 km) per hour. Studies have shown somewhat slower winds blowing in the opposite direction at the planet's equator. Astronomers once thought that beneath these layers of clouds was a massive ocean of super-pressurized liquid ammonia and water. But data sent back from Voyager 2 seem to indicate that this is probably not the case. The internal structure of Uranus is probably very similar to that of Jupiter and Saturn, but without the rocky core and liquid metallic hydrogen center. Instead, there is probably a less-dense collection of rocky material at the core. In 1977, Earth-based astronomers discovered that Uranus, like all of the other gas giant planets, has a system of rings. A total of nine rings were identified from Earth. When Voyager 2 visited Uranus in 1986, two more rings were discovered. Two of Uranus' moons, Cordelia and Ophelia, act as shepherding moons for one of the rings. Voyager 2 also discovered that Uranus has a magnetic field. It is tilted 60 degrees from the planet's rotational axis, and is about the same intensity as the magnetic field on Earth. The magnetic field on Earth and the other terrestrial planets is probably caused by electrical currents produced in the planet's molten core. Since Uranus has no molten core, astronomers are not sure what is responsible for creating the magnetic field.

 

Neptune

God of the Seas

Leaving Uranus and its moons behind us, we next encounter Uranus's sister planet, Neptune, and its system of moons. Neptune is nearly the same size as Uranus, and its composition is extremely similar. It is also the outermost of the great gas giants. Neptune was named after the Roman god of the seas. It was discovered in 1846 by Johann Gottfried Galle, of the Berlin Observatory. Neptune has eight known moons, six of which were not discovered until Voyager 2's encounter with the planet in 1989. Most of these moons are quite tiny and are probably captured asteroids. The largest of these moons however, has a diameter of 1,680 miles (2,700 kilometers).

The Big Blue Spot

Since Neptune was known to be so similar to Uranus, astronomers were expecting to find another bland and featureless world when Voyager 2 arrived there in August of 1989. Much to the surprise of everyone on the Voyager team, Neptune would turn out to be anything but bland. The most startling feature to be discovered was a giant blue spot in the planet's southern hemisphere. It was very similar in appearance to the famous red spot on Jupiter, and about half the size. In 1994, observations of Neptune by the Hubble space telescope have revealed that the giant blue spot has disappeared. Recently, a new blue spot was observed in the planet's northern hemisphere. Astronomers are not sure why the spot on Neptune was relatively short-lived, while the great red spot on Jupiter has bee present for hundreds of years. This indicates that the Neptune's atmosphere is highly active and prone to sudden and severe changes. Several smaller spots were also noted, as was a small, fast-moving, cloud that astronomers called "scooter".

Features of Neptune

Nearly everything we know about Neptune was learned from the Voyager 2 encounter. Like Uranus, Neptune's blue color is the result of red light being absorbed by methane gas in the planet's upper atmosphere. Neptune's composition is believed to be very similar to that of Uranus. It probably has a small, rocky core about the size of the Earth. This core is thought to be surrounded by an extremely thick layer gases composed mainly of hydrogen and helium with a little methane. Besides the mysterious dark and light spots were other fascinating discoveries. Long, white clouds were seen floating in the planet's upper atmosphere. Neptune has the fastest winds in the solar system, reaching speeds of 1,200 miles (2,000 kilometers) per hour. Voyager 2 also revealed Neptune's system of delicate rings. There are 4 narrow rings which are extremely faint. Astronomers think they may be composed of dust particles formed from meteorites smashing into the planet's moons. Like Uranus, Neptune's magnetic field is tilted 47 degrees from its rotational axis. It is believed this is caused by internal forces deep within the planet.

 

Pluto

 

God of the Underworld

As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by a small, rocky world called Pluto, and its single moon, Charon. The darkness and isolation of these two worlds has likened them to the underworld. In fact, Pluto was named after the Roman god of the underworld. Charon was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time. Astronomers first theorized that a ninth planet might exist when they observed that something seemed to be exerting a gravitational pull on the planet Neptune. An in-depth sky survey finally turned up the tiny planet. Pluto is so far away from the Sun that it takes 248 Earth years to complete one orbit. The temperature of this inhospitable world averages -382° F (-230° C).

The Twin Planets

For many decades, Pluto was thought to be alone in its dark corner of our solar system. But in 1978, Jim Christy discovered that Pluto had a companion. This moon, Charon, is more than half the size of its parent planet. In fact, many astronomers have referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them. Pluto and Charon are also tidally locked in a synchronous orbit. Pluto's rotational period is 6.3 days. It takes 6.3 days for Charon to make one revolution around Pluto. Thus, the two bodies continuously face each other. But these are not the only strange characteristics of this remote planet. Pluto's orbit is so highly eccentric that is actually crosses inside the orbit of Neptune. because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17 degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune.

Features of Pluto and Charon

Pluto is the only planet that has not been visited by a spacecraft. It is so tiny, and so far away that even the Hubble space telescope has trouble resolving any surface features on the planet. With a diameter of only 1412 miles (2274 km, Pluto is the smallest planet in the solar system. Because of its small size, many astronomers believe that it should be classified as an asteroid instead of a planet. In fact, Pluto represents one of the great mysteries of our solar system. No one knows why this small, rocky  planet can be found among the giant gas planets of the outer solar system. Some believe that Pluto and Charon may be asteroids or comets that somehow got locked into an orbit around the Sun. Many astrononers now believe that they are actually part of the Kuiper belt and should not be classified as planets. The Kupier belt is a ring of rocky, icy objects located beyond the orbit of Pluto. Serious consideration has been given to removing Pluto from the list of planets, but the the public's fondness for this lonely wanderer will probably keep it there for some time to come. Between 1985 and 1990, the Earth was aligned with the orbits of Pluto and Charon in such a way that a series of eclipses would take place. Astronomers were able to take advantage of this situation to accurately determine the size of both Pluto and Charon, and to create the first image of Pluto showing some of its larger surface features. This revealed that Pluto is the second most contrasty body in the solar system. Astronomers are hoping that a future space probe may one day be able to show us the surface of this strange and distant world. But until then, every new discovery will bring with it a long list of new questions.

 

 

Some Web Sites to visit:

NASA for kids http://www.nasa.gov/audience/forkids/kidsclub/flash/index.html

Solar System Coloring Book to print out  http://www.windows.ucar.edu/cgi-bin/redirect.cgi/coloring_book/index.html

Star Child- NASA web site with children's games and more http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level1/solar_system.html

 Take a quiz on what you know about the planets http://www.siec.k12.in.us/~west/proj/space/quiz.htm

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