Pluto is the farthest known planet in our solar system and was discovered only in 1930. Very little is known about this icy world. Its one year equals to 248 of our years. Unlike the outer giants- Jupiter, Saturn, Uranus and Neptune- Pluto is a dwarf, not much bigger than Mercury.
After it's meeting with Neptune, Voyager II will leave the solar system without meeting Pluto. So it will be quite a few decades before we know more about this remote little member of our sun's family.