When the stamp was designed as part of a planetary set, Pluto was the only planet that lacked a robotic companion -- no spacecraft had ever been sent to explore it, so there was no American robot to show off in the stamp illustration.
Some say that the words on the stamp actually fueled scientists' efforts to make them untrue.
“It was my idea to send it," New Horizons principal investigator Alan Stern told Astronomy Magazine of the stamp. "For many years, people had waved that stamp around as sort of a call to arms — as a motivating graphic — ‘Not yet explored.' That stamp had been in so many presentations by that point, I knew it would please people to have it go along.”
"We're honored to have the 'Not Yet Explored' stamp travel the billion of miles to Pluto," Mary-Anne Penner, the U.S. Postal Service's acting director of stamp services, told collectSPACE.com. "We are hoping that the New Horizons mission provides Pluto with a 'stamp of approval,' regardless of its status in the solar system."
Now that the Pluto stamp is about to be quite untrue, perhaps we'll finally see an update. In 2012, the New Horizons team petitioned for a stamp commemorating the mission, but USPS has yet to design one.