Most people know about the solar system : 4 smaller rock-like planets orbiting the sun at small distance, and 4 outer gas giants. An asteroid belt between Mars and Jupiter is dividing those 2 categories of planets. Pluto, a small planet, circling the outer region of our solar system, was allways considered to be a special case.
Gerard Kuiper, had suggested in the 1940s and 1950s that perhaps Pluto was not a world without context but the brightest of a vast ensemble of objects orbiting in the\r\nsame region. This concept, which came to be known as the Kuiper Belt. Extrapolating from the small fraction of the sky that has been surveyed so far, investigators estimate that the Kuiper Belt contains approximately 100,000 objects larger than 100 kilometers across. As a result, the Kuiper Belt has turned out to be the big brother to the asteroid belt, with far more mass and far more objects (especially of large sizes).
It is now clear that Pluto is not an anomaly. Instead it lies within a vast swarm of smaller bodies orbiting between about five billion and at least eight billion kilometers from the sun.