The sun is the center of our solar system. It is the largest body in our solar system. Nine planets follow paths called orbits around the sun. The shape of each orbit is called an ellipse. An ellipse is a flattened circle. The nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. You can make project Solar system drawing as given picture and planets below the second picture.
Moons, asteroids, comets, and meteoroids are also part of our solar system. Moons orbit planets. Asteroids, comets and meteoroids orbit the sun. You will learn about these objects as you read Our Solar System. The sun is the only object in our solar system to shine with its own light. All other objects in our solar system reflect the light of the sun.
Scientists have studied our solar system from telescopes on Earth and from telescopes in orbit around Earth. Astronauts have visited the moon. Spacecraft have been sent to study almost all of the planets in the solar system.
Giant dust storms, freezing temperatures, colorful clouds, and beautiful rings can be found throughout the solar system.
The Sun
The sun is a ball of hot, glowing gases. It is much hotter than any of the nine planets. The outermost layer of the sun that we can see is about 10,000°F (5,600°C). The hottest the oven in your kitchen gets is about 500°F (260°C)! The sun is the most important part of our solar system. It gives us warmth and light. Without the sun, our Earth would be very cold. If there were no sun, there would be no life on Earth.
The sun is a star. It is the closest star to Earth. At night we can see many stars in the dark sky. During the day when we can see our star (the sun) shining, its light is so bright we cannot see the other stars. Some stars are hotter than our sun, others are cooler. Some stars are bigger than our sun and other stars are smaller, but they are so far away from Earth that they look like tiny points of light. Our sun is 10 times larger than the largest planet, Jupiter.
The Mercury
Mercury is the closest planet to the sun. Because it is so close to the sun, Mercury gets very hot. During the day the temperature on Mercury can get as high as 800°F (430°C). The hottest it has ever gotten on Earth is about 135°F (60°C)!
At night when it is dark Mercury can also get very cold, as cold as -280°F (-175°C). This happens because there are no clouds and very little air surrounding the planet. The atmosphere helps to keep a planet warm when the sun is not shining. Mercury's very thin atmosphere cannot keep the planet warm at night.
The surface of Mercury is hard and rocky. Mercury has cliffs and valleys just as Earth does. The surface of Mercury is covered with craters. There is no liquid water on Mercury.
An American spacecraft flew past Mercury and took pictures of one half of this rocky, hot-and-cold planet.
The Venus
Venus is the second planet from the sun. We can call Venus our neigh- bor because it is the closest planet to our Earth.
Venus is the hottest planet in the solar system, even though it is farther from the sun than Mercury. It can get as hot as 900°F (480°C) on Venus. The temperature can get this high because Venus has a thick atmosphere. The air around the planet is mostly a gas called carbon dioxide. The carbon dioxide traps the heat from the sun on the planet's surface. This is called the greenhouse effect. A greenhouse on Earth is designed to trap heat to help plants grow.
Venus is a very dry planet. It is covered by thick clouds. Earth's clouds contain water, but Venus' clouds contain sulfuric acid. These clouds are so thick that astronomers on Earth cannot see the surface of the planet with their telescopes.
Soviet spacecraft landed on Venus and sent back pictures of its sur- face. In 1990 the American spacecraft Magellan began orbiting Venus, us- ing radar to "see" through the clouds. These radar pictures show craters, mountains, volcanoes, and valleys on the surface of Venus.
The Earth
The third planet from the sun is Earth, our home. Earth does not get as hot as Venus. The highest temperature ever recorded on Earth is about 135°F (60°C). The lowest recorded temperature is about -125°F (-85°C).
The surface of Earth is similar to the surfaces of Mercury and Venus. Earth is a hard, rocky planet. There are high mountains, valleys, volcanoes, and even some craters. Earth is different in some very important ways. Most of the planet is covered with water. Also, the air is made of nitrogen, oxygen, and carbon dioxide. It is just right for us to breathe! Earth is home to people, plants, and animals because it has both water and the right kind of atmosphere.
American astronauts took the first pictures of the whole Earth from a spacecraft traveling to our moon in 1968. Earth is our home. It has air for us to breathe and is warm enough for us to live.
Earth's Moon
Earth has one moon. The moon is our nearest neighbor in the solar system. It follows a path or orbit around Earth, just as Earth follows a path around the sun.
Our moon has mountains and valleys. It is covered with craters. The surface of the moon is rocky and covered with dust. The moon's atmosphere is even thinner than Mercury's! The temperature on the moon can reach 265°F (130°C). Because there is almost no atmosphere, the temperature can drop to -170°F (-110°C) at night. There is no water on the moon. There is no life on the moon because it has no water and no air.
Twelve American astronauts walked on the moon between 1969 and 1972. They took pictures and collected lunar rocks and dirt. Scientists studied the rocks and dirt to learn more about the moon. They learned that the surface of the moon is much older than the surface of Earth.
The Mars
Mars is the fourth planet from the sun. Mars can get very cold. The temperature can get as low as -200°F (-130°C). In a few places it can be a pleasant 80°F (30°C) during the day. In most places it never gets above freezing.
Mars is a hard, rocky planet. The soil on Mars contains iron oxide (rust), which makes the ground look red. This is why Mars is often called the red planet. Sometimes the red dirt is stirred up by strong winds. These huge dust storms can last for months. Mars has mountains, canyons, volcanoes, and craters. Scientists think that the large canyons were formed long ago by water. There is no liquid water on the surface of Mars now. There may be frozen water under the surface.
There is ice on the surface. at the coldest places. Mars has an atmosphere made almost entirely of carbon dioxide and traces of nitrogen and other gases. American and Soviet spacecraft were sent to Mars to study the planet. Two spacecraft landed on Mars. They studied the atmosphere and collected soil samples to look for signs of life, but no life was found. Orbiting spacecraft also studied the two small, rocky moons of Mars.
The Jupiter
Jupiter is the fifth planet from the sun. Because it is so far from the sun, its temperature is only -220°F (-140°C) at the cloud tops.
All that astronomers can see when they look at Jupiter through a telescope are the tops of the clouds in its atmosphere. These clouds are made of frozen gases such as ammonia and water. These colorful clouds cover the entire planet, making it look white, brown, red, and orange. Jupiter's Great Red Spot is a storm which has been going on for over 300 years!
The atmosphere is mainly made of two gases, hydrogen and helium. Deep in the atmosphere the gases are pressed together so much that they turn into liquid. It would be impossible to land a spacecraft on Jupiter. The pressure deep in the atmosphere is so great that a spacecraft would be crushed. Scientists think that at the very center of Jupiter there may be a core of ice and rock about the size of Earth.
The United States sent several spacecraft to fly by Jupiter. These spacecraft took pictures of the planet and its icy and rocky moons. They discovered thin rings around the planet.
Jupiter's four largest moons are called the Galilean moons. They were discovered by Galileo, the first person to use a telescope to look at the heavens. These four moons are very large. Some are even larger than the smallest planets. Jupiter's moon, Ganymede, is the largest moon in our solar system. (The chart on page 34 shows the sizes of the nine planets and some of the moons in our solar system.) Jupiter's smaller moons are probably made of rock.
Three of the Galilean moons are thought to be made of ice and rock. The fourth moon, Io, is covered with sulfur. Spacecraft flying by Io took pictures of active volcanoes on this moon. Volcanoes on Earth erupt lava, but the volcanoes on Io seem to erupt liquid sulfur.
The Saturn
Saturn is the sixth planet from the sun. It is a lot like Jupiter. The temperature at Saturn's cloud tops is -285°F (-175°C). When astronomers look at Saturn through a telescope they see the cloud tops. These clouds are made of frozen gases such as ammonia and water. Saturn's clouds are not as colorful as those covering Jupiter.
Saturn's atmosphere is similar to Jupiter's atmosphere. It is mainly made of two gases, hydrogen and helium. Deep in the atmosphere the gases are pressed together so much that they turn into liquid. A spacecraft traveling down through the atmosphere of Saturn would have the same problem as a spacecraft traveling through the atmosphere of Jupiter. The pressure is so great that a spacecraft would be crushed. Scientists think that at the very center of Saturn there may be a core of ice and rock about the size of Earth.
The United States sent several spacecraft to fly by Saturn. These spacecraft took pictures of the planet, its moons, and its rings. Saturn has the most spectacular rings in the solar system. These rings are made of billions of small chunks of ice and rock. Each of these pieces is like a tiny "moonlet" that orbits Saturn just as the big moons do. Even though these wide rings stretch out far beyond Saturn's cloud tops, they are probably less than 100 feet (30 meters) thick!
Saturn's largest moon, Titan, is the second largest moon in the solar system. It is larger than some planets. compare Saturn and Titan to other objects in the solar system.) Titan has an atmosphere of nitrogen and methane. We have never seen the surface of Titan because its sky is filled with a haze similar to smog. Scientists believe that the surface of Titan may be covered by an ocean of liquid methane. The next six largest moons appear to be made mostly of ice. The smaller moons are probably made of rock or ice and rock.
The Uranus
Uranus is the seventh planet from the sun. When astronomers look at Uranus through a telescope they see some clouds and the atmosphere above the clouds. These clouds are made of frozen methane. Methane is a gas which we use for cooking and heating on Earth. The temperature at the top of the clouds is -370°F (-220°C). The clouds of Uranus appear bluish-green because of the methane gas in the atmosphere above them.
The atmosphere below the clouds is mainly made of hydrogen and helium. Deep in the atmosphere the gases are pressed together so much that they turn into liquid. Scientists think that at the very center of Uranus there may be a core of ice and rock.
The United States sent a spacecraft to fly by Uranus. It took pictures of the planet and its rings and moons. The moons are probably made of ice and rock.
The Naptune
Neptune is usually the eighth planet from the sun. Sometimes Pluto moves closer to the sun. Then Neptune is the ninth planet from the sun for a while. The temperature on Neptune above the clouds tops is -355°F (-215°C).
The clouds of Neptune are made of frozen methane. These clouds appear blue because of the methane in the atmosphere above the clouds. The atmosphere below the clouds is mainly made of hydrogen and helium. Neptune has a Great Dark Spot. This is probably a storm similar to the Great Red Spot on Jupiter. The very center of Neptune may be a core of ice and rock.
The United States sent a spacecraft to fly by Neptune. The spacecraft took pictures of the planet and its moons. Neptune's largest moon, Triton, has a surface of frozen nitrogen and methane. Pictures of Triton show features that appear to be volcanoes erupting liquid water and ammonia. The smaller moons are probably made of rock or ice and rock.
The Pluto
Pluto is usually the ninth planet from the sun, making it the farthest known planet from the sun. Sometimes Pluto's orbit crosses the orbit of Neptune. When it does, Neptune becomes the farthest planet from the sun. From 1979-1999 Pluto is the eighth planet from the sun and Neptune is the ninth.
The temperature on the surface of Pluto is about -370°F (-220°C). Pluto has a thin atmosphere of methane and perhaps some nitrogen. The planet's surface is methane frost. Scientists think that Pluto is made of methane ice, water ice, and some rock. Pluto is so far away from the sun that a day on Pluto is only as dusk on Earth!
Scientists must study Pluto from Earth. It is the only planet in our solar system that has not been visited by a spacecraft.
