Our solar system consists of various bodies of different sizes which orbit the sun. The planets are the most important of these orbiting objects, and are arrayed in this order (first is closest to the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.

The SUN, which is actually just an ordinary star, lies at the center of our solar system. What makes the sun so special to us is simply the fact that it is so close to the Earth, only 93 million (92,960,000) miles away. This distance may seem huge, but when you realize that the closest star to us besides the sun is 25.3 trillion (25,300,000,000,000) miles away, you can see how giant space really is. Besides the obvious effects the sun has on our lives, solar activity also contributes to weather patterns on Earth. For instance, between the years 1640 and 1710, there were almost no sunspots on the surface of the sun. This coincided with a period of very cold weather on Earth known as the "Little Ice Age." Solar eclipses are particularly fascinating to watch. During a solar eclipse, the moon, the Earth, and the sun line up so that the moon lies between the sun and Earth. From our point of view on Earth, the sun is blocked completely by the moon for a few minutes. Another beautiful sight visible from Earth are the aurorae, or northern lights. These brightly colored bands usually appear in the sky in locations near the north and south poles. The bright colors occur when high-energy particles from the sun are swept into our atmosphere and make the atoms there glow. The sun gets its energy from nuclear fusion, which is a nuclear process where four hydrogen atoms are converted into one helium atom. Since the total atomic mass of four hydrogen atoms is 0.7 percent more than the atomic mass of the helium atom, there is a little bit of extra mass left over at the end of this conversion process. In his famous E = mc^2 equation, Einstein proved that mass and energy are transferable. The extra mass created in the nuclear process described above is converted into energy. This nuclear fusion occurs in the central core of the sun, where the temperature is about 15 million degrees Fahrenheit and the pressure is 250 billion times the pressure of the Earth's atmosphere.

MERCURY is the closest planet to the sun, and so it is also the fastest moving, orbiting the sun at 107,136 mph. This fast speed led the Romans to name it after Mercury, who was the speedy messenger of the Roman gods. Since Mercury is so close to the sun, it is difficult to see from Earth. It is visible only right after sunset or right before sunrise. Mercury has no moons. Nearly 4,000 of the best photographs of Mercury were taken by the Mariner 10 space mission in 1974. Those pictures show that Mercury is a small rock that resembles the moon. However, it has long cliffs and shallow craters instead of the deep craters found on the moon. At the center of the planet is a large iron core. One year on Mercury is only 88 Earth days long, while one day on Mercury is equal to 59 days on Earth! This means that since Mercury has no atmosphere, and the sun shines continuously on the surface for 59 Earth days in a row, the daytime temperatures on Mercury can reach as high as 800 degrees Fahrenheit, under which conditions even metals such as lead would be melted! When it is nighttime on the surface, the temperature is -250 degrees Fahrenheit!

VENUS is the closest planet to Earth, and is the third brightest object in our sky after the sun and moon. Although it is similar to our planet in size, mass, and density, its surface environment is quite different. The atmosphere is made up of 96% carbon dioxide and small amounts of acids. One feature of the atmosphere is the greenhouse effect, in which heat (infrared radiation) is trapped near the surface because it gets reflected back down by the clouds. Venus is therefore very hot, approximately 900 degrees Fahrenheit. The surface has been studied by the Pioneer Venus 1 and Magellan spacecrafts, which used radar to peer through the clouds and make maps. They found that most of the surface is covered by craters and hills. There are also indications that Venus had a lot of volcanic activity in the past. The atmospheric pressure at the surface is 90 times Earth's pressure! Venus does not have any moons.

Our planet, EARTH, is very unique. The environment is rich enough to support life, and many life forms thrive. Unlike any of the other planets, Earth is covered by vast quantities of water. In fact, more than 70% of the surface is under the ocean. Temperatures on Earth are very moderate compared to the other planets. Humans can live relatively comfortably at almost any location on the surface. Our only moon is a quarter the size of Earth. It rotates on its axis at exactly the same rate that it orbits the Earth, and so we always see the same side of the moon. There is no air or water on the surface of the moon, and there is also no geological activity. The moon is the only other place in the universe that people have traveled to.

MARS is a rocky red planet about half the size of Earth. It orbits the sun every 1.88 Earth years, and a day on Mars lasts about as long as one on Earth does. The planet's axis of rotation is also similar to the Earth's, which means that Mars experiences seasons as well. One unique feature is the polar ice caps, which are a combination of water ice and frozen carbon dioxide. The ice caps melt and rebuild according to the seasons, just as they do on Earth. The surface features on Mars are huge: the Valles Marinaris is a valley longer than the whole United States, and Olympus Mons is a volcano larger than Arizona and three times higher than the Hawaiian Island volcanoes. Three spacecraft have landed on Mars, and they have sent back pictures of a relatively flat surface covered with red rocks. Mars has two moons, Phobos and Deimos, which are probably asteroids captured by Mars' orbit long ago.

The ASTEROID BELT lies between Mars and Jupiter. The first asteroids, or minor planets, were discovered in the very early 1800s. Eventually over 4,000 of them were found. Astronomers had calculated that another planet should lie between Mars and Jupiter, and the asteroid belt seemed to solve this problem. Asteroids are composed of raw material from the creation of our solar system. For some reason, this material did not come together to form one planet. Asteroids are the source of most meteors that have hit the Earth.

JUPITER is named after the king of the Roman gods. It is the largest planet, yet it spins very quickly on its axis- one day is about 10 Earth hours. The atmosphere is composed mainly of hydrogen and helium, although it is the small amounts of gases like ammonia and methane which make the bands on Jupiter look so colorful. It is very turbulent in the atmosphere, with huge storms frequently lasting years. The Great Red Spot is a storm which has been in existence for 300 years, and covers an area larger than Earth! Jupiter is a gas giant with a very faint ring system. The atmosphere is so thick that it is impossible to see the rocky core that is probably at the center of this huge body. There are 16 moons orbiting Jupiter. The four brightest ones- Io, Europa, Ganymede, and Callisto- are visible from Earth through a small telescope. They were first discovered in 1610 by Galileo, who was using a telescope that he built. These four moons are therefore called the Galilean satellites.

SATURN is one of the most beautiful and unusual sights in the sky. It is a giant gas planet about ten times the size of Earth. Its density is so low that it would float in a (very) big dish of water. Like Jupiter, Saturn has an atmosphere of hydrogen and helium and is often very stormy. The most unique feature of Saturn is its bright and colorful ring system, which is easily visible from Earth with a small telescope. The rings are composed of billions of small rock and ice particles which reflect the sunlight and are held together by gravity and a few of Saturn's moons. The dark separation between the two main rings is called the Cassini division. Saturn has 18 moons. The largest is Titan, which is actually larger than Mercury and Pluto.

URANUS was found in 1781 by the astronomer William Herschel. Like Saturn and Jupiter, Uranus is a gas giant with a helium and hydrogen atmosphere. Uranus has a faint ring system that cannot be seen from Earth except with a very large telescope. The rings were discovered only 23 years ago, when Uranus passed in front of a star, which appeared to blink as it passed the rings. Uranus has at least 15 moons, some of which are probably captured asteroids.

NEPTUNE was discovered in 1846, after astronomers calculated where its orbit should be in order to cause strange motions in Uranus' orbit. Neptune's atmosphere is also composed of hydrogen and helium, and it has a very faint ring system that cannot be seen from Earth. For 20 years out of every 248 years (one orbit of Pluto), Neptune is actually the farthest planet from the sun. This was the case from 1979 until 1999. Pluto is now once again the last planet in the solar system. Neptune has eight moons.

PLUTO is usually (see 'Neptune' above) the farthest planet from the sun. It was found in 1930. Pluto is quite different from the other gas giants in the outer reaches of the solar system. In fact, it is simply a rock composed of frozen methane. There is an on-going debate about whether Pluto should be considered a planet, since it more closely resembles an asteroid or moon. Some astronomers believe that it is an escaped moon of Neptune, and others believe that it could be the remains of a large comet. Pluto is extremely cold, with a surface temperature of only about -360 degrees Fahrenheit. Pluto's only moon, Charon, was discovered in 1978.

REFERENCES FOR THIS PAGE:

M. Chartrand, The Audubon Society Field Guide to the Night Sky, Knopf, 1991.

K. Kuhn, In Quest of the Universe, West Publishing Company, 1994.

The photographs used on this page are reproduced for educational purposes from the NASA, STScI, and Hubble Space Telescope Picture Galleries.