Planets are relatively large natural bodies that orbit around stars. Although there may be many planets in the universe, the most well-known are the ones that orbit around the Earth's sun. The collection of the sun and these planets is known as the solar system.

Planets in the solar system

The sun has nine planets in orbit around it. In order, from closest to the sun to farthest, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. They all vary widely in size, temperature and composition, but they can be put into rough groups in several different ways. Mercury, Venus, Earth and Mars are called terrestrial planets, since they all have roughly the same size and composition as Earth. Jupiter, Saturn, Uranus and Neptune are called Jovian planets. In general, they are larger than Earth and have thick, dense atmospheres. They are also known as gas giants because they are mostly composed of gases. Unlike the terrestrial planets they do not have solid surfaces. Pluto stands on its own. It is small, rocky, and far from the sun.

Mercury, Venus, Mars, Jupiter, and Saturn are visible with the naked eye, and in fact, apart from the moon, Venus is the brightest object in the night sky. When observed from Earth, the planets appear to rise in the East and set in the West, like the stars and the moon. However, their motion and the motion of the Earth make their paths much more irregular and complex than that of the stars. People have observed the planets carefully for thousands of years.

How the planets were formed

The planets in the solar system were probably formed about 4.5 billion years ago when a huge cloud of dust and gas began to clump together under the force of gravity. The largest clump of material compressed itself and became the Sun. The remaining material took the form of a huge disk rotating around the Sun. More and more dust and gas in this disk accumulated into clumps, forming the planets.

What planets are made of

Most planets have hot, often liquid cores of melted metals. They also have atmospheres, which vary widely in composition, thickness, and temperature. The planets themselves have numerous satellites and moons. The Earth has one moon. Jupiter, on the other hand, has 28, and Saturn's famous rings are composed of millions of small rocks and particles.

Planetary motion

Planets move in two ways. Each planet travels around the sun in a path known as its orbit. As first shown by German astronomer Johannes Kepler in the 1600s, the planets move in paths called ellipses, or flattened circles, around the sun. The ellipses are very nearly circles for all the planets, except for Pluto, whose orbit is irregular enough that sometimes it crosses over the path of Neptune. The time it takes for a planet to complete one orbit around the sun is equal to a year on that planet. A year on Earth, for example, is 365 days long because it takes that long for the planet to complete one trip around the sun. The planets all orbit counter clockwise around the sun, when viewed from above the Earth's North Pole.

Each planet also spins on its axis, which is an imaginary line that extends through the center of the planet from its north pole to its south pole. This causes the planet to have days and nights as different sides of the planet face the sun. The time it takes for a planet to make one rotation on its axis determines the length of a day on that planet. The Earth completes one rotation every 24 hours. Therefore, a day on Earth is 24 hours long.

Other planets in the universe

In the 20th century scientists began to discover planets orbiting around nearby stars. It is difficult to tell much about these planets, since they are so far away and give off no light of their own. They can only be detected by measuring tiny changes in the position and light of the stars around which they orbit.