Since its discovery and classification as the ninth planet in our solar system in 1930, Pluto has been the subject of much controversy in the scientific community.  Its small size and extreme distance from Earth have made gathering specific data about its characteristics difficult, and no real consensus exists amongst astronomers about the information that is known about Pluto.  In 2006, the International Astronomical Union created an official definition for the term "planet" which listed three criteria for classification:

1. The object must be in orbit around the sun.
2. The object must be massive enough to be rounded into a sphere by its own gravity.
3. The object must have "cleared the neighborhood" around its orbit.

Because Pluto is much smaller than the other objects in its orbit, it fails to meet the third condition and has since been known as a "dwarf planet".  Some scientists have gone so far as to suggest that Pluto may actually be one of the many moons of its neighboring planet, Neptune.

When Pluto was first discovered in 1930, astronomers estimated that it may be as large as earth and thus were confident that it was, in fact, a planet.  As our ability to gather information about outer space continues to improve through more powerful telescopes and space probes, scientists are now able to use the new, more accurate information they receive to accurately classify objects in space.  While some still argue that Pluto meets the accepted criteria to be known as a planet, for the time being, conventional scientific thinking will hold that our solar system only has eight planets.

Passage adapted from "The Significance of the Frontier in American History" written and delivered by Frederick Jackson Turner during the 1893 World's Columbian Exposition.

The American frontier is sharply distinguished from the European frontier  a fortified boundary line running through dense populations. The most significant thing about the American frontier is, that it lies at the hither edge of free land. In the census reports it is treated as the margin of that settlement which has a density of two or more to the square mile. The term is an elastic one, and for our purposes does not need sharp definition. We shall consider the whole frontier belt, including the Indian country and the outer margin of the settled area of the census reports. This paper will make no attempt to treat the subject exhaustively; its aim is simply to call attention to the frontier as a fertile field for investigation, and to suggest some of the problems which arise in connection with it.

In the settlement of America we have to observe how European life entered the continent, and how America modified and developed that life and reacted on Europe. Our early history is the study of European germs developing in an American environment. Too exclusive attention has been paid by institutional students to the Germanic origins, too little to the American factors. The frontier is the line of most rapid and effective Americanization. The wilderness masters the colonist. It finds him a European in dress, industries, tools, modes of travel, and thought. It takes him from the railroad car and puts him in the birch canoe. It strips off the garments of civilization and arrays him in the hunting shirt and the moccasin. It puts him in the log cabin of the Cherokee and Iroquois and runs an Indian palisade around him. Before long he has gone to planting Indian corn and plowing with a sharp stick, he shouts the war cry and takes the scalp in orthodox Indian fashion. In short, at the frontier the environment is at first too strong for the man. He must accept the conditions which it furnishes, or perish, and so he fits himself into the Indian clearings and follows the Indian trails. Little by little he transforms the wilderness, but the outcome is not the old Europe, not simply the development of Germanic germs, any more than the first phenomenon was a case of reversion to the Germanic mark. The fact is, that here is a new product that is American. At first, the frontier was the Atlantic coast. It was the frontier of Europe in a very real sense. Moving westward, the frontier became more and more American. As successive terminal moraines result from successive glaciations, so each frontier leaves its traces behind it, and when it becomes a settled area the region still partakes of the frontier characteristics. Thus the advance of the frontier has meant a steady movement away from the influence of Europe, a steady growth of independence on American lines. And to study this advance, the men who grew up under these conditions, and the political, economic, and social results of it, is to study the really American part of our history. . .