Difference Between Satellites And Planets
Imagine looking up into the night sky and seeing several objects that looked like our moon. That is what you would see if you lived on Jupiter or Saturn, because those planets have many moons, or satellites, instead of just one. Any natural object that orbits a planet is called a satellite.
Spacecraft and other objects launched from Earth to orbit a planet or the sun are called artificial satellites. These are discussed in the article Satellites, Artificial .
Of the eight planets in the solar system, only Mercury and Venus have no known satellites. The Earth has one satellite, called the moon. Mars has two satellites, Phobos and Deimos. Each of the four remaining planets have many satellites. Saturn has 1 large satellite, Titan, and more than 40 small ones. Jupiter has more than 50 small satellites and 4 large ones: Io, Europa, Callisto, and Ganymede, which is the largest satellite in the solar system. Many of the known satellites were discovered by space probes, such as Voyager I and Voyager 2. Others have been discovered with Earth-based telescopes.
Pluto, which is no longer considered a true planet, has three satellites. Their names are Charon, Hydra, and Nix.
How Planets Got Their Satellites
Astronomers think that the satellites were formed billions of years ago, at the same time as the sun and planets were formed. At that time, the solar system was a vast rotating disk of dust particles, ice, and gases. Over time, these materials condensed and accumulated into larger and larger bodies of matter, eventually becoming the planets. The gravitational force of each planet attracted smaller bodies of matter, which crashed into the planet and added to its size. However, some of the small bodies of matter did not crash, but began to circle the planet instead. These smaller objects became the planet’s satellites.
In ancient times, sky watchers observed that there were five special points of light in the night sky. All the other points of light always kept their same positions relative to one another. These were the stars. The constellations they formed remained the same year after year, lifetime after lifetime. The five points of light were different. They slowly moved among the stars. The ancient Greeks called these points of light the planetai, a word that means “wandering stars.” And the Greeks named them after their gods: Hermes, Aphrodite, Ares, Zeus, and Chronos.
Today we know that these points of light, which we call planets, wander slowly in the sky because they are other “worlds” traveling around our sun. Instead of using the names of the Greek gods, we call these planets by the names of the equivalent Roman gods: Mercury, Venus, Mars, Jupiter, and Saturn.
Our own Earth is also a planet. If you could observe Earth from outer space, you would see that it, too, seems always to be moving among the stars. Planets give off no light of their own. They shine brightly in the night sky because they reflect the sun’s light.
What Is a Planet?
Until recently, defining a planet was simple. A planet was any large, rounded object that revolved around a star, such as our sun, in a path called an orbit. However, as astronomers have learned more about our solar system and all the different kinds of objects it contains, this simple definition has proved insufficient.