Any of the nine largest objects that move around the sun are considered to be planets. Using the sun as the center and moving outward, the order of the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The sun, the planets and their satellites (moons), and smaller objects called asteroids, meteors, and comets make up the solar system. The planets are dark, solid bodies, much smaller than the sun and stars. All light and heat comes to them from the sun. They can be seen only because they reflect the light of the sun. In the night sky, planets and stars look alike but there are two ways to tell them apart. First, the planets shine with a steady light, but the stars seem to twinkle. Second, the planets change their positions in relation to the stars. All the planets move around the sun in the same direction. In addition, each planet rotates as it revolves. The planets' rotation periods (the time required to spin around once) range from less than 10 hours for Jupiter to 243 days for Venus. The earth rotates once every 24 hours, or one day. The conditions on the planets vary widely. They depend on three things: (1) the planet's distance from the sun, (2) the planet's atmosphere, and (3) the planet's rotation. The planets nearest the sun receive more heat than those far away from it. The temperature on the closet planet, Mercury, rises to about 625 degrees F. during the day. The temperature on a planet is estimated from measurements of infrared radiation and radio waves that the planet sends out. These measurements are difficult to make for objects with low temperatures. Astronomers determine the kinds of gases in a planet's atmosphere by studying the light, radio waves, and other radiation coming from the planet. The atmospheres of the terrestrial planets consist mainly of carbon dioxide and nitrogen. The atmospheres of the major planets consist mostly of helium, hydrogen, methane, and ammonia. The earth is the only planet with a large amount of oxygen in its atmosphere. The movement of the planets remained a puzzle until the 1600's. There were two theories which brought about a dispute. One theory was suggested by Ptolemy, a Greek astronomer (A.D. 150) who thought the sun and the planets traveled around the earth once a day. His theory explained what could be seen in the sky and guided people's thinking about the universe for over a thousand years. The other theory was suggested by Nicolaus Copernicus in 1543 who said that the earth and the other planets traveled around the sun. This theory made it easier to describe the motions of the planets and astronomers soon began to use it. Discoveries by other astronomers gradually convinced people that the Copernican theory was correct. During the space age, much progress has been made, however, there are still many questions about the planets that remain unanswered. One of the biggest questions is whether any form of life exists on any of the planets, in particular on Mars.