planets to read

jupiter

Because of Jupiter's massive size, it has an enormous impact on the objects around it. One of the most dramatic examples can be found on its moon Io. Because Io orbits so close to its parent planet, the gravitational tidal forces actually cause the moon to stretch. This creates a great deal of friction within the small moon, which generates a great deal of heat. This is the power source behind the active volcanoes that have been discovered on Io's surface. Jupiter's gravity also had a strong impact on its other moons. On Europa, the gravitational forces crack and shift its icy crust. It most likely has a heating effect on the moon's core. Because of this, most astronomers believe that an ocean of liquid water may exist beneath Europa's frozen crust. A source of heat could provide the right conditions for life, similar to the communities of organisms found around the deep sea hydrothermal vents in the oceans on Earth. Exploring the Jovian System To date, the Jovian system has been visited by a total of eight spacecraft. Pioneer 10 was the first to explore this previously uncharted territory in 1973. It was followed by Pioneer 11 about a year later. These spacecraft provided us with our first close-up views of the planet and discovered its magnetic field and radiation belts. They gathered valuable information about some of the moons and paved the way for future missions. In 1979, the first of two new spacecraft encountered the Jovian system. Voyager 1 made it closest approach to the system in March, followed soon by Voyager 2 in July. These two spacecraft sent back amazing new images of Jupiter and its moons. Many new moons were discovered, and the four Galilean moons were discovered to be much more dynamic and unique than previously thought. Evidence of of volcanic eruptions was discovered on Io, and features on Europa hinted at a possible ocean of water beneath the ice. Two other spacecraft flew past the Jovian system for a gravity boost on their way to other destinations. The Ulysses probe shot past Jupiter in 1992 on its way to study the Sun. NASA's New Horizons probe encountered the Jovian system in 2007 on its way to study Pluto and the Kuiper belt. These two spacecraft sent back new images and new information that helped better our understanding of Jupiter and its moons. The biggest contribution to our understanding of the Jovian system has come from the Galileo mission. On December 7, 1995, it became the first spacecraft to go into orbit around Jupiter. On that same day, a special probe released from Galileo entered Jupiter's atmosphere and send back valuable information about its chemistry. Galileo sent back hundreds of high resolution images of Jupiter and many of its moons. A technical problem with the high gain antenna prevented it from sending more, but what we did get was a treasure trove of new information and new views of the Jovian system. NASA is planning a new mission to study Jupiter in even more detail. Known as Juno, the spacecraft is scheduled to be launched in August 2011. It will enter a polar orbit around Jupiter where it will study the planet's composition, magnetic field, and other aspects to try to determine how it was formed and exactly what it is made of. t is the fifth planet from the Sun and is the largest. It is also 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. It was Galileo who discovered Jupiter's four largest moons, Callisto, Europa, Ganymede, and Io, in 1610. We now know of 64 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. Perhaps the most famous feature of Jupiter is its giant red spot. This spot is actually a gigantic storm system. This large blemish 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.

saturn

The sixth planet from the Sun is a large, bright gas giant surrounded by thousands of delicate, glistening rings. As we get closer to the planet, we can see even more rings. What originally appears a two rings now reveals itself to be a complex system composed of thousands of smaller ringlets. There are rings and there are rings inside of rings. Surrounding all of this stunning beauty is a system of 62 moons, ranging from the tiny to the truly gigantic. Seven of these natural satellites are large enough to warrant further investigation 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 phenomenon occurrs because the tilt of Saturn's rings changes as it orbits the Sun. Because the rings are less than a mile in thickness, they seem to disappear completely every few years when they appear on edge as seen from Earth. 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 known 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. 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 container 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.

mars

a beautiful, bright red world. 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 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 process would involve using various chemical and biological means to make the planet warmer and the atmosphere more hospitable for human life. 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 riverbeds 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.

sun

a gigantic ball of gas so massive that the immense pressure has ignited a fusion reaction. It is an average-sized yellow star. It is one of the most common type of star in the universe. 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. 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. lthough the light from the Sun seems constant to us, the Sun is far from stable. Magnetic storms and anomalies cause a number of unique phenomena. Dark spots can sometimes been seen on its surface when viewing through a solar filter to reduce the light. Known as sunspots, they are actually parts of the surface that are several hundred degrees cooler than the surrounding areas. Tremendous flames of hot gas called solar flares and prominences shoot out several hundred thousand miles into space. A stream of ionized particles known as the solar wind blows out into the far reaches of the Solar System at speeds of up to a million miles per hour. The Sun provides us with a unique opportunity to study what makes a star work. It is the only star in the universe that is close enough for us to see its surface.

moon

a large, cratered object orbiting only 238,800 miles (384,400 km) away. This is Earth's moon, and it is the planet's only natural satellite. Although we call it simply "the Moon", it is associated with the Roman goddess Luna, who was goddess of the hunt and of the Moon. It is the sixth largest moon in the Solar System, and has been Earth's partner for most of the planet's known history. Although it has been lighting our dark nights for so long, its origins are mostly unknown. Some believe it was formed when a gigantic asteroid smashed into the Earth. The resulting impact flung molten material far out into space where it cooled and formed the Moon we see today. Others believe it was a wandering planetoid captured by the Earth's gravitational pull. Wherever it came from, it has fascinated mankind for centuries. The Moon is the only planetary body whose surface can be seen from Earth with the naked eye. ur fascination with the Moon came to an exciting climax in 1969. On July 20, Neil Armstrong became the first human being to step onto an alien world. The crew of Apollo 11 left their footsteps behind as a tribute to the spirit of human exploration. Five other Apollo missions soon followed. By the time these missions had ended, a total of ten men had walked on the Moon's surface. Their tasks included surveying, mapping, setting up experiments, and collecting samples of rock and sand. The Moon's surface is now littered with evidence of our visits. The remains of 6 lunar landers can be found along with various scientific experiment packages and several American flags. We now know a great deal about our partner in space. Recent unmanned missions have continued mapping the surface and have even detected the presence of water ice on the surface. The presence of water is important if we are to ever establish a permanent base on the Moon. Perhaps the most prominent features of the Moon are its numerous craters. Its surface has been bombarded continually throughout its history. Since the Moon has no measurable atmosphere and no liquid water, there is no erosion taking place. This has preserved the many thousands of craters on its surface. Another prominent feature on the Moon are the smooth areas called maria. The maria are believed to have been formed by more recent lava flows which have covered over the older craters. There are also many mountain ranges and rift valleys on the Moon's surface. All of these features have combined to form a unique facial feature known as the "Man in the Moon" that can be seen if one knows what to look for. This feature is visible every night of the year because the Moon always keeps its same face towards the Earth. This is because its period of rotation and revolution are the same; 29 days, 12 hours and 44 minutes. This cycle is known as the lunar month As the Moon orbits the Earth, it appears to change shape as more or less of its sunlit side is visible. When the Moon is on the opposite side of the Earth as the Sun, it appears as a full moon. When it is on the same side, it is invisible, also known as a new moon. Since it is so close to the Earth, the Moon's gravity pulls on our oceans and creates the tides.

venus

a planet completely covered by wispy 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. Contrary to its outward appearance, Venus is a very inhospitable world. The planet is covered by a thick atmosphere composed primarily of carbon dioxide. It contains almost no water vapor. The atmosphere 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 wispy cloud tops, however, Venus could easily be mistaken for Hades, the underworld. 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. Thousands of volcanoes and numerous impact craters dot the surface. Venus is a very active planet undergoing constant change.

neptune

early the same size as Uranus, and its composition is extremely similar. It is also the outermost of the two ice 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 13 known moons, most of which were not discovered until after 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 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 named "scooter". 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. Astronomers believe this is caused by internal forces deep within the planet.

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 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. Mercury was first 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 steep cliffs, also known as rupes, which rise as much as 6300 feet (1920 m) above the valley floors. In 2008, a second spacecraft passed close to Mercury. This spacecraft was named MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. MESSENGER brought high resolution cameras to the planet and revealed many new features and new terrain not previously seen on the Mariner mission. MESSENGER is scheduled to enter orbit around Mercury in March 2011, where it will spend a full year mapping the planet's surface. New images from MESSENGER show that Mercury's surface is more homogeneous, ancient, and more heavily cratered, unlike the surfaces of the Moon and Mars.

earth

most beautiful sight in the entire solar system. Sparkling before us is a bright blue gem of dazzling complexity. Delicate white clouds dance above its surface of brilliant blues, greens, and browns. This is 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 inner planets in the Solar System. It is also the densest of the four. 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! 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. Earth is a rocky world that is still very much alive and active. Volcanic activity continually creates new land while earthquakes 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 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.

pluto

or many decades, Pluto was thought to be alone in its dark corner of our solar system. But in 1978, astronomer Jim Christy discovered that Pluto had a companion. This moon, known as Charon, is more than half the size of its parent planet. In fact, many astronomers once 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. The orbit of Pluto 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. Because Pluto crosses Neptune's orbit, there is a very real possibility that the two worlds may collide at some point in the future. In 2005, two new moons were discovered by the Hubble Space Telescope Pluto Companion Search Team on. Once the orbits were confirmed, the moons were assigned the names Nix and Hyrda. The names were chosen in part because the initials (NH) refer to the New Horizons mission, which explored Pluto in 2015. In Greek mythology, Hydra was the 9 headed serpent who guarded the waters of the underworld and Nix was the goddess of the night. Two additional moons were discovered in 2011 and in 2012, bringing the total to five. In keeping with the underworld theme, these new moons were named Styx and Kerberos. Features of Pluto Until recently, very little was known about the surface of Pluto. It is so tiny and so far away that even the Hubble Space Telescope was unable to resolve any surface features. With a diameter of only 1,473 miles (2,370 km), Pluto is not considered to be a true planet. Instead, it belongs to a relatively new category of objects known as dwarf planets. Until recently, no one really knew why this small, rocky object could be found among the giant gas planets of the outer solar system. But recent discoveries of many similar objects have answered that question. There may be hundreds or even thousands of Pluto-like objects lurking in the outer Solar System in the Kuiper belt. In 2015, NASA's New Horizons spacecraft reached the Plutonian system after a voyage of over 9 years and 4.6 billion miles. Th spacecraft made its closest approach at only 7,750 miles (12,472 km) from the surface of the dwarf planet and captures amazing high-resolution images of Pluto and its largest moon Charon. New data showed that Pluto is about 50 miles (80 km) larger than previously thought and that it has a nitrogen-rich atmosphere extending tens of thousands of miles out into space. Images showed that Pluto's surface is quite varied, with large differences in both brightness and color. In fact, Pluto is one of the most contrastive bodies in the Solar System. The color varies between charcoal black, dark orange and white. The most famous geographical feature on Pluto is a gigantic heart-shaped plain known as Tombaugh Regio. Other notable features include Cthulhu Regio, a large dark area on the trailing hemisphere resembling a whale, and the Brass Knuckles, a series of equatorial dark areas on the leading hemisphere. Other images showed large mountain ranges made entirely of water ice. One range known as Sputnik Planum rises 11,000 feet (3,500 meters) high. The Lost Planet For years, many astronomers questioned the status of Pluto as one of the nine planets of the Solar System. They believed that Pluto and Charon could be asteroids or comets that somehow got locked into an orbit around the Sun. Soon after the discovery of the Kuiper belt, most astronomers began to think that these two bodies were actually rogue Kuiper objects and should not longer be classified as planets. The debate raged on for decades, supported by the public's fondness for their favorite planet. Finally, on August 24, 2006, the International Astronomical Union (IAU) decided to redefine the definition of a planet. This decision officially reclassified Pluto and Charon as dwarf planets. Only 76 years after its discovery, Pluto was quietly added to the list of minor planets and given the number 134340. It was the end of an era as the Solar System's ninth planet surrendered its title. Today, we know that many similar objects can be found in this region of space. In fact, over a dozen other similar objects have been discovered beyond the orbit of Pluto. Over 100,000 smaller objects have been found in the Kuiper Belt. The future holds the possibility of many exciting new discoveries as we continue to probe the darkness in search of Pluto's cousins.

uranus

seventh planet from the Sun. It is a beautiful yet nearly featureless giant, blue green sphere. The outer layer of thick, blue gas gives no hint as to what might lie beneath. We have left the realm of the gas giants and entered the domain of the ice giants. This planet is a little bit smaller than its close neighbor, Saturn, and is surrounded by a very thin layer of small delicate rings. On closer inspection, we find this mysterious world to be tilted on its side for some strange reason. In orbit around the planet, we find 27 moons of varying sizes and shapes. Five of these natural satellites are large enough to explore in detail. only planet that rotates on its side. The planet has a tilt of 97.77 degrees. Compare this to the tilt of the Earth, which is only 23.5 degrees. This extreme tilt gives the planet seasons unlike anything else in the Solar System. As the planet orbits the Sun, each of the poles and the equator face the Sun at different times in its long trip around its parent star. Needless to say, this gives the planet some very unusual weather patterns that cause drastic changes throughout the Uranian year. Astronomers are divided as to the cause of this unusual tilt. Some believe that a large object, possibly an asteroid or an errant moon, may have collided with the planet knocking it on its side. Others think the extreme tilt may be caused by complex gravitational interactions between Uranus and its neighbors, Saturn and Neptune. 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 Sun shines alternately on the poles and equatorial regions of the planet rather than constantly shining on 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. When Voyager 2 arrived at Uranus, astronomers were expecting to find complex cloud structures similar to those on Jupiter and Saturn. But when 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 planets' molten core. Since Uranus has no molten core, astronomers are not sure what is responsible for creating the magnetic field.

Terran system

the earth/the moon; As we approach the third planet from the Sun, we see a shiny blue sphere of unimaginable beauty, partnered with a singular, large rocky Moon. A delicate veil of thin white clouds encompasses this blue globe. The blue color of this unusual world is caused by liquid water, an rare sight in the Solar System. In fact, this truly unique gift is known to exist only on this one planet. The blue spaces are broken by rocky shapes covered with a color we have never seen before. The beautiful browns of the land masses are painted with various shades of green. These are life forms. This planet is home to living organisms, making it potentially unique in the entire universe. This is the Terran system, comprised of the planet Earth and its lonely Moon. The Wonder of life The Terran system is home to the only planet in the universe known to contain the miracle of life. And life on the Earth exists in a myriad of forms and varieties. It is believed that life first started in the Earth about 3.5 billion years ago. These early life forms were simple, single-celled organisms that lived in water. Early life forms were able to capture the energy of the sun through a process known as photosynthesis. Through the miracle of cellular mutation and environmental change, life has evolved in to unimaginably complex forms. Today there are millions of species of animals and plants inhabiting the Earth. One of these creatures even developed self awareness, the intelligence to question its own existence, and the driving curiosity to explore its environment. That creature is man. The road to modern life forms has been a rocky one. Because our Solar System is populated by a variety of small bodies such as asteroids and comets, there have been many times in Earth's past when life almost didn't survive. In fact, many scientists believe that there have been five mass extinction events since life first exploded on Earth. The last of these events occurred about 65 million years ago when an asteroid larger than mount Everest smashed in to the planet. The impact occurred in an area known as Chicxulub, near the eastern coast of Mexico. That impact and the resulting environmental changes are believed to have wiped out more than half of all species on the planet, including the mighty dinosaurs. Some scientists believe that this event paved the way for the mammals to take control of the planet, which eventually led to the development of modern humans. The Goldilocks Zone The Terran system is locates in an area of the Solar System that is not too hot and not too cold. Astronomers refer to this area as the Goldilocks zone, or the habitable zone. This thin ring around the Sun begins just beyond the orbit of Venus and extends to just before the orbit of Mars. This is the area where water can exist in its liquid form. And water is the most important factor in supporting life as we know it. If the Earth was too close to the sun, the water and oceans on the planet would boil away. If it were too far, all of the liquid water on the planet would freeze solid. Fortunately, the Terran system exists in this zone where everything is just right. It is because of this that our planet has been blessed with liquid water and life. The Double Planets The Earth's neighbor in space is the Moon. It it the planet's only natural satellite. It is very large in comparison to its parent planet, about a quarter as large. This leads some astronomers to consider the Terran system a double planet system. However, since the common center of mass around which both bodies revolve is located beneath the surface of the Earth, the Terran system is officially labeled as a planet-satellite system. Leading theories suggests that the Moon was formed when a small planet the size of Mars smashed into the early Earth about 4.5 billion years ago. The iron cores of the two planets merged, giving the Earth a larger metal core than the other terrestrial planets. A large portion of the crust and mantles of the two planets was thrown out into space, where it formed a ring of material. This ring of material eventually coalesced to form the Moon. When the Moon first formed, it was much closer to the Earth. It only took 20 days to orbit the planet instead of the 28 days it takes today. At this time, a day on Earth was only 18 hours long. Since then, the Earth's rotation has slowed down and the Moon has been slowly drifting away. Today, the Moon moves about 1.6 inches farther away each year. The Moon plays a very important role in the Terran system. Its gravity helps to keep the Earth stable on its axis. Without it, the axis of the Earth would wobble wildly, making it difficult for life to exist. The Moon is also responsible for the ocean tides on Earth. Its gravity pulls on the water, causing a slight elliptical bulge. As the Earth rotates through this bulge, the sea levels rise and fall by several feet. Many scientists believe that life may not have been possible on Earth if there was no Moon.