Pluto Not Yet Explored . ... Credit Copyright: NASA , JPL , Michael W. Carroll Explanation: Cold, distant, Pluto is the only planet in our Solar System which has not been visited by a spacecraft from Earth. The story goes that the legend " Pluto Not Yet Explored" on a US postal stamp depicting the tiny, mysterious world inspired a JPL employee to develop plans for a Pluto flyby. These plans evolved into the current "Pluto Express" mission intended for launch early in the next decade. ...