|MadSci Network: Astronomy|
You are right about our inability to see Pluto with the naked eye. Pluto is the smallest of the nine planets and is at the greatest distance from the sun. If you imaged Pluto and the Earth to be balls, you could place 5 balls the size of Pluto inside the Earth and still have room to spare.
It is the only planet that has never been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.
Pluto was discovered in 1930 by a fortunate accident. Calculations, which later turned out to be erroneous, predicted a planet beyond Neptune that exerted a gravitational pull on Neptune and Uranus. Unaware of the error, Clyde Tombaugh did careful observations with a telescope which positively identified the ninth and last of the planets. The way he worked was by taking two pictures of the same spot on the sky at different times and blinking them. He would then blink the pictures against one another to check if any of the "stars" on the picture had moved between the two exposures. Eventually, he found Pluto.
For more information on Pluto, try this site!
For the story of Pluto's discovery try: http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Pluto
Try the links in the MadSci Library for more information on Astronomy.