Our solar system is made up of the sun and all the bodies that travel around the sun. Nine planets, including the earth, move around the sun in orbit. Some planets have one or more satellites, rings, along with smaller debris, including asteroids, comets, meteorites, and dust. Our solar system is dominated by one body-the sun, and not the earth. The sun is a typical star, a great sphere of luminous gas, making it impossible for the earth to get but so close.
The sun and its nine planets form the solar system. But smaller bodies, satellites (moons) orbit the planets and asteroids and comets orbit the sun. If the paths the planets follow around the sun were visible, we would see that the solar system is like a huge set of elliptical rings, centered approximately on the sun and extending about four-billion miles outward to Pluto's orbit. It is hard to imagine such immense distances measured in miles. In fact, it is as foolish to use miles to measure the size of the solar system as it is to use inches to measure the distance between New York and China. Whenever possible, one should use units of measure appropriate to the scale of what one seeks to measure.
The earth's radius makes a convenient unit scale for measuring the size of the other planets. The earth's distance from the sun makes another acceptable unit for measuring the scale of the solar system.
The astronomical unit, abbreviated, AU, is the unit of measuring the distance from the planets. The distance from earth to the sun is about ninety-three trillion miles (150 million kilometers). Using AU to measure the scale of the solar system, Mercury turns out to be 0.4 AU from the sun, while Pluto is about 40 AU.
The solar system is the limit of our exploration of the universe with space craft. But telescopes, such as the Hubble, have extended our view far beyond the solar system to reveal that the Earth is one of many planets orbiting the Sun, and the Sun is but one of a vast swarm of stars orbiting the center of our galaxy, the Milky Way.
