The Sun's family of worlds is called the solar system. At its center lies the Sun, the star nearest to Earth. Many types of objects circle nearby. These include planets, moons, dwarf planets, asteroids, and other bodies.

Scientists use telescopes and robot spacecraft to study these worlds up close. They have found that our family of worlds is not alone. Many other stars have solar systems. To understand them, we must first learn more about our own.

Our solar system was discovered long ago. Ancient stargazers saw five strange lights moving through the stars from night to night. Some people thought that these lights were gods. Eventually, they were identified as planets. For many centuries, people thought that Earth was the center of the universe.

In 1543 the Polish astronomer Nicolaus Copernicus wrote that the planets circle the Sun. In the 1600s, Italian physicist Galileo Galilei used a telescope and saw that Copernicus was right. Other astronomers soon built larger telescopes. They began finding new planets, moons, and asteroids that also orbit the Sun.

The Sun is a star. It is our most important star because nothing could live on Earth without it.

The Sun weighs more than 332,000 Earths. Its gravity holds the solar system together. On the inside, its temperature is about 25,000,000°F (14,000,000°C). Our star is made mostly of hydrogen gas. Its strong magnetic field can sometimes become twisted and form dark sunspots and powerful explosions. The Sun's light and heat keep us alive.

Astronomers have found two different kinds of planets in our solar system. The four nearest the Sun are known as the inner planets. They are Mercury, Venus, Earth, and Mars. Each is a small world made of rock. They have thin atmospheres and very few moons.

The largest planets lie farther away; they are known as the outer planets. Jupiter, Saturn, Uranus, and Neptune are made mostly of gas. They have many moons and rings around them.

Pluto is a tiny ice world; it has about the same surface area as Mexico. Pluto used to be considered a planet. Recently it was reclassified as a dwarf planet.

Circling the planets are dozens of natural satellites, or moons. They come in many different colors, shapes, and sizes. Earth has one moon. Mars has two tiny moons known as Deimos and Phobos. Jupiter and Saturn each have more than 60 satellites. Astronomers have discovered that Uranus and Neptune together have more than 40 moons.

Some moons have layers of ice. Some may even have lakes of liquid chemicals. And others have geysers and volcanoes that often erupt.

Thousands of smaller bodies also orbit the Sun. These are called asteroids and meteoroids.

Asteroids are made of rock and metal. Many are the size of mountains or boulders, but some are nearly as large as planets.

Most asteroids orbit between Mars and Jupiter. Some come close enough to Earth that astronomers worry about a collision. Others lie farther out than the most distant planets.

Meteoroids are tiny pebbles of rock and ice that sometimes fall into Earth's atmosphere. When one burns up as it falls, we see a meteor, or "shooting star."

Comets are huge chunks of ice that orbit the Sun. The Sun's gravity sometimes pulls comets toward it. When this happens, a comet heats up and forms a long tail. Some comets circle the Sun many times. Others appear only once and then vanish forever.

In 1950 the Dutch astronomer Jan Oort discovered that many comets came from a cold cloud of ice near the edge of the solar system. Today that area is known as the Oort cloud.

Tiny specks of dust also fill the solar system. Most are left behind by comets or colliding asteroids. Stargazers in a very dark place can sometimes see this dust. Around the start of spring and fall, sunlight hits the dust just right and makes it glow. It looks like a faint cone of light just before sunrise or just after sunset. Astronomers call this the zodiacal light.

Astronomers have always wondered where our solar system came from. Long ago some believed that the planets were thrown outward when the Sun was formed. Others thought that the planets were born from the same cloud as the Sun.

Modern telescopes and robot journeys to the planets are helping to solve the mystery. Astronomers now believe that our solar system came from the material left over from the Sun's birth. They now study distant stars and watch other systems being born.

The Sun is not standing still. It speeds along toward a point in the constellation Hercules. This point is called the solar apex.

The Sun is also carrying our solar system on its journey. Together we travel at 43,000 miles (69,000 kilometers) per hour. It will take millions of years to reach the solar apex.