Imagine you were on a starship deep within interstellar space, heading toward the solar system where Earth resides. As you approached from tens of billions of miles away, the Sun would appear to grow ever brighter. Eventually you would detect Earth as a faint pinpoint of light. If you observed for long enough, you would notice that Earth follows a wide path around the Sun. You would also see, at various distances from the Sun, eight other objects of various sizes. You might detect that many of these planets are circled by still smaller objects—their moons. In the space between the orbits of two of the planets, Mars and Jupiter, you would see thousands of very small "planets," or asteroids, also revolving around the Sun. You might even spot a few comets, their long, streaming tails slicing across the planetary orbits.


The Sun

The Sun lies at the very heart of our solar system. It is a typical star, one of the 150 billion in the Milky Way galaxy. Because the Sun is much closer to us than is any star, it seems many, many times larger than the more distant bodies. Its disk appears about the size of a full Moon.

Compared with the other stars of our galaxy, the Sun is an average-size star. But it is giant compared with even the largest planets. Its diameter of 865,278 miles (1.39 million kilometers) is more than 100 times greater than that of Earth. Even though it is of gaseous composition, the Sun weighs more than 300,000 times as much as Earth. Its surface temperature is 9,945° F (5,507° C). At its center, the temperature may reach as high as 25,000,000° F (14,000,000° C)—hot enough to smash atoms and generate energy through a process called nuclear fusion. Each second, the Sun converts 661 billion tons of hydrogen into 657 billion tons of helium. In the process, 4 billion tons of matter are converted to pure energy. This energy initially takes the form of deadly gamma rays, but by the time it bubbles to the surface of the Sun, the energy has been transformed into a torrent of light, illuminating the planets and nurturing the many forms of life on Earth.

Children of the Sun

The nine planets, in order of their distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The planets all lie in about the same orbital plane, and they orbit the Sun in the same direction. This suggests that the solar system is a relic of a vast disk of dust and gas that surrounded the Sun as it formed 4.6 billion years ago. In the first few million years after the Sun ignited, major planets, ranging from several thousand to tens of thousands of miles across, formed within this gaseous disk. The largest chunks of leftover debris became trapped in the gravitational fields of the newly formed planets, and began orbiting them as moons. Gravity pulled the smaller chunks to the surfaces of the planets and moons. Many of the craters that pepper the surfaces of these bodies are relics of this early period of intensive bombardment by interplanetary debris.

The solar system has two types of planets. The tiny rock, or terrestrial, planets all lie close to the Sun, like campers huddled around a bonfire. The immense outer planets—Jupiter, Saturn, Uranus, and Neptune—lie in the colder reaches of the solar system. They consist mostly of liquid and gas. The farthest planet from the Sun, Pluto, though still called a planet, may not be one after all. Recent evidence indicates that it may be the last remaining "fossil" of a population of thousands of "icy dwarf" bodies that once inhabited the solar system. These icy dwarfs were either absorbed into the major planets or tossed out of the solar system altogether.

We have used robot spacecraft to fly by, orbit, and even land on, eight of the nine planets of our solar system. Probes have transmitted spectacular close-up pictures of all the planets (except Pluto), and in the process have revolutionized our understanding of how the celestial objects in our solar system evolved. The manned Apollo expeditions to the Moon, in the late 1960s and early 1970s, returned with the very first samples of rock collected from another world. Some of the Moon rocks were found to be 4.5 billion years old, providing additional evidence that the formation of the planets and moons accompanied that of the Sun.


A moon is any natural body that orbits a planet. There are more than 90 known moons in our solar system. The majority of them orbit the giant planets Jupiter and Saturn, and are little more than huge, airless balls of ice, ranging from hundreds to more than a thousand miles across. One of the largest moons, Saturn's Titan, is so big (3,449 miles, or 5,550 kilometers, in diameter) that it retains its own atmosphere of nitrogen. Mars has some of the smallest moons, a pair called Deimos and Phobos, each no bigger than an asteroid, which indeed they may have been at one time.


In the 18th century, astronomers calculated astrophysical laws that predicted they would find an as-yet-unseen planet between Mars and Jupiter. And they eagerly searched the skies for it. On the night of January 1, 1801, the Italian astronomer Giuseppe Piazzi discovered a small celestial body, which he took to be a planet, in the space between the orbits of Mars and Jupiter. This body, which was later called Ceres, was found to have a diameter of only 623 miles (1,002 kilometers). Over the years, many more small, planetlike bodies were found in the gap between Mars and Jupiter—a region of space dubbed the Asteroid Belt. Today, some 30,000 of these small bodies are discovered every year, and more than 210,000 are known to exist in our solar system. Some even have moons of their own.

These small bodies are now known as minor planets, or asteroids. The orbits of some extend beyond the Mars-Jupiter gap. But their combined mass is only a fraction of Earth's.

Astronomers once thought that asteroids were the fragments of a big planet that once orbited the Sun in a path between Mars and Jupiter and then broke apart for unknown reason. But in recent years, scientists have come to believe that asteroids are probably debris from throughout the solar system that never coalesced to form a planet.


Comets are among the strangest members of the solar system. Instead of moving as the planets do, in nearly circular orbits in the same direction, comets revolve around the Sun in very elongated ellipses, and from every conceivable direction. Much of the time they are so far away from the Sun that they are invisible even to our largest, most powerful telescopes.

Today, astronomers know that comets are members of the Sun's family. Many "long-period comets" may originate in a vast shell of icy debris called the Oort cloud, 50,000 times farther from the Sun than is Earth. Others, particularly those known as "short-period comets," come from the Kuiper Belt, a region 30 to 100 times farther from the Sun than is Earth. Both clouds contain trillions of icy comet bodies that are gravitationally bound to the Sun.

When astronomers first discover a comet, it usually appears as a faint, diffused, fuzzy star, with a dense, starlike center and a veil-like region, known as its coma. As the comet approaches the Sun, its coma becomes brighter, as more and more material vaporizes off the surface of the comet's solid, icy nucleus. When they are some 100 million miles (160 million kilometers) from the Sun, some comets begin to show a tail streaming behind them, pointing directly away from the Sun. Comet tails consist of very thin gases that fluoresce, or glow, under sunlight, as well as a fine stream of dust particles. This material is forced away from the Sun by the pressure of sunlight and the solar wind.


Comets eventually break up into particles, which are sometimes seen entering Earth's atmosphere as meteors. Meteors — some of which originate in comets, and others as chunks from asteroids, moons, or other planets—range in size from specks the size of a pinhead to huge stones weighing many tons. We become aware of meteors only through the bright light produced when they collide with air molecules in our atmosphere. Most meteors disintegrate once they strike the atmosphere. Those that reach the ground are called meteorites. Most meteorites are fragments of asteroids, but a small number of them may have come from the Moon or Mars.

Early Ideas of the Solar System

The ancient Greek philosophers did not realize that Earth itself is a planet, or wanderer in the heavens (which, incidentally, is what planet means). Earth, they thought, hung motionless at the very center of the universe. They believed that each of the five planets they had seen (Mercury, Venus, Mars, Jupiter, and Saturn) were attached to concentric, invisible crystal spheres. The Moon and the Sun were attached to other spheres. These crystalline spheres, set one within the other, revolved around Earth, carrying with them the heavenly bodies. This theory could not explain certain phenomena, however.

For one thing, the planets do not move at an even rate across the sky. At certain times, they move more rapidly than at others. An even greater mystery was the observation that a planet such as Mars occasionally ceases its apparent eastward motion among the stars and reverses itself to head westward for a time. To explain this "retrograde motion" of the planets, early astronomers invented a complicated system of "epicycles." They held that each planet traveled along the circumference of a small circle, the center of which traveled along the circumference of a larger circle. Earth, it was maintained, was at the center of the larger circle.

This model of the universe prevailed for more than 1,000 years. In the first half of the 16th century, however, Polish astronomer Nicolaus Copernicus revived an idea that had been first proposed by the Greek philosopher Aristarchus of Samos—that the Earth and other planets move around the Sun. This system was called the heliocentric theory, since it placed the Sun (helios, in Greek) at the center of the universe.

Motions of the Planets

It required a lifetime effort on the part of several great astronomers to prove the Copernician heliocentric system. The 16th-century Danish nobleman Tycho Brahe made a long and accurate series of observations of the planets. Johannes Kepler, a German disciple of Brahe, drew up three laws of planetary motion that still hold true today. Kepler also improved on the Copernician model, which maintained that the planets move in circular orbits around the Sun. This belief led to inaccuracies in predicting planetary positions. Kepler was able to show, instead, that orbits are ellipses, rather than true circles.

While Kepler was refining his theories, Italian inventor and scientist Galileo Galilei used the telescope, a recent Dutch invention, to gather additional evidence supporting Copernicus'theory. The telescope allowed Galileo to see the phases of Venus, which proved that it orbited the Sun, not Earth. Galileo also saw four tiny moons orbiting the distant planet Jupiter, in perfect accordance with Kepler's laws of motion.

The research of Kepler and Galileo clearly explained the nature of the planets'movements around the Sun, but neither scientist understood the force that governed these movements. This force was first revealed in 1687, when the great English scientist Isaac Newton presented his law of universal gravitation. This law states that every particle of matter in the universe attracts every other particle. This force of gravitation increases with the mass of an object, and depends on the distance between two objects. Newton showed mathematically that this is truly a universal law, since it applies not only to objects upon the Earth, but to heavenly bodies as well. The law of universal gravitation explains why planets, asteroids, and meteors keep orbiting the Sun, which is by far the most massive object in the solar system.

Using the law of universal gravitation, we can now analyze the motions of the planets with great accuracy. We can account for the small deviations that arise as one planet affects the orbit of another.

It was the study of such deviations that led directly to the discovery of the planet Neptune. After Uranus had been discovered by Sir William Herschel in 1781, careful studies showed that it did not follow the orbit predicted by the law of universal gravitation. This led young Englishman John Couch Adams and French astronomer Urbain-Jean-Joseph Leverrier to conclude that Uranus was being attracted by another planet even more distant from the Sun. Both men calculated the position in the sky of the unknown planet without ever having seen it. On September 23, 1846, on the basis of Leverrier's calculations, German astronomer Johann Gottfried Galle located Neptune.

Astronomers suspected the existence of Pluto because of the disturbances of motion they had seen in the orbits of Uranus and Neptune. Such deviations suggested that the two planets were being gravitationally tugged by yet another unseen body. Pluto was discovered in 1930 after a yearlong detailed search by astronomers at Flagstaff Observatory in Arizona.

In 1978 astronomers discovered that Pluto has at least one moon, which they called Charon. By plotting the moon's six-day orbit, astronomers were able to calculate the mass of Pluto, which turns out to be only 1/500 that of Earth. Pluto's orbit is rather unusual, taking a path some 17 degrees inclined to the plane taken by the other planets. Pluto's orbit is also highly elliptical, so much so that Pluto moves inside Neptune's path for about 20 years out of its 248-year orbit.

Because of Pluto's puny size, its solid icy surface, and its peculiar elongated orbit, many astronomers have begun to debate its status as an actual planet. It may, in actuality, be the largest of a new class of relatively large icy bodies beyond the orbit of Neptune, or maybe even a strange type of comet. In 1999, however, the International Astronomical Union tabled such debate and ruled that, until other evidence becomes available, Pluto will remain classified as the solar system's ninth planet.