In 2006 the International Astronomical Union defined the term “planet” for the first time. Pluto and seven other objects are now called “dwarf planets.” One of these five, Eris, is 27% more massive than Pluto.

Einstein in his General Theory of Relativity showed that the gravitational field was actually a distortion of space-time caused by the mass of the object. Because space-time has four dimensions that are very difficult to visualize, the distortion is best seen with a two-dimensional model. Often the model consists of a rubber sheet in which a bowling ball is placed (see illustration on page 39). The sheet is pulled down by the ball, which represents the sun. Earth is a tiny ball that is placed on the sheet and given a push perpendicular to the direction of the sun. This ball “orbits” the sun a few times until friction causes it to speed up and spiral into the sun. In the words of physicist John Wheeler: