Prior to 2006, a planet was defined to be any natural body orbiting the Sun not emitting light. However, in the years leading up to 2006 many objects similar in size and composition to Pluto were being discovered beyond the orbit of Neptune. With the discovery of these so-called trans-Neptunian objects the scientific community was faced with the decision to either continue classifying Pluto as a planet, and thus be forced to classify all of the new-found objects as planets as well, or to form a more rigid definition for a planet, thereby excluding Pluto and the other trans-Neptunian from being classified as planets. Simply put, either a new definition for planet had to be created or the number of planets in the Solar System was going to grow very rapidly.
With this dilemma in mind, the International Astronomical Union formulated a new definition of planet in 2006. According to the IAU, a planet is a celestial body that is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and has "cleared the neighbourhood" around its orbit.
Given this definition, Pluto and all trans-Neptunian objects discovered so far are excluded from being classified as planets. As a result, there are only eight known planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The eight planets were formed early in the life of our solar system. Aproximately 4.5 billion years ago the Solar System was merely a cloud of gas and dust. For reasons that may never be fully determined, this cloud started to collapse and rotate. As the center of the cloud became increasingly dense and hot, nuclear fusion began to take place and the Sun was born. At the same time the Sun was forming, the gas and dust found in the outer part of the cloud began to collide and stick together, forming larger and larger objects. Eventually, bodies large enough to exert gravitational pull on surrounding objects formed. These objects, known as planetesimals, in turn collided and fused together into young planets, or protoplanets as they are often called. Over time these protoplanets evolved into the planets we know today.