Although it was the last planet discovered in our solar system, tiny Pluto is not always the farthest planet from the Sun. Because of Pluto's unusual orbit, Neptune will be farther from the Sun until 1999. Pluto's orbit is more elliptical, or stretched out, than the orbit of any other planet. On average it lies 39 times farther from the Sun than Earth does.
Pluto is the smallest planet -- smaller even than our own Moon.
Astronomers searched for a ninth planet during the 1920s because they thought something was tugging at the giant planet Neptune. In 1930, Clyde Tombaugh, a young astronomer at Lowell Observatory in Arizona, found Pluto. But astronomers soon realized that Pluto is far too small to affect Neptune. Recent calculations, in fact, show that nothing is pulling at Neptune at all; astronomers had simply miscalculated Neptune's mass, so its orbit is slightly different from what they expected.
Because it is so unlike the other planets, some people wonder if Pluto should be considered a planet at all. It probably formed by a different process from the other planets, and was pushed into its peculiar orbit by Neptune.
Pluto consists of nitrogen ice and rock, although scientists are not sure which is more abundant. There is a small amount of frozen methane as well. Astronomers have also detected a very thin atmosphere of nitrogen, methane, and carbon monoxide. The atmosphere thickens when Pluto's orbit carries it closest to the Sun, and solar heat vaporizes some of the ice on its surface. As Pluto retreats from the Sun, the gas in its atmosphere will freeze out again, adding a layer of bright, fresh frost to the frozen planet.
Like Earth, Pluto has only one moon. But its moon, called Charon, is half the size of Pluto. Pluto and Charon are closer in size than any other planet-moon system in our solar system, and are sometimes called a double planet.
The search for new planets did not end with the discovery of Pluto, although recent work suggests it's unlikely that another large planet orbits the Sun. But some astronomers believe that several bodies the size of Pluto and Charon may hide on the outer fringe of the solar system. Recent discoveries indicate there are millions of smaller icy, rocky objects orbiting in the Kuiper Belt, a vast region that extends beyond Neptune. Perhaps other worlds lie undiscovered in the darkness beyond Pluto and Charon.