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Back in 1991, the US Postal Service issued stamps of the solar system's then-nine planets.
Although Pluto has since been demoted to dwarf planet, it had the only one that read "not yet explored."
So when NASA launched its New Horizons spacecraft in 2006, the lead scientist stuck that stamp onto the probe.
To celebrate New Horizons and its outdated postage flying by Pluto in July 2015, the USPS just issued two "Pluto —Explored!" souvenir postage stamps.
The Post Office is also releasing stamps to honor the solar system's eight official planets; you can buy them all online starting May 31.
Here's every one of the news stamps in their full philatelic glory:
Pluto, and its great big heart, as seen by the New Horizons spacecraft in 2015.
USPS
The New Horizons spacecraft, which flew by Pluto in 2015, sending us the first up close images of the dwarf planet.
USPS
Mercury, taken by NASA's Messenger spacecraft in 2011-12.
USPS
Venus, in simulated colors using data from the radar sensor NASA's Magellan spacecraft collected in 1991.
USPS
Earth, as seen from NOAA/NASA's Suomi NPP satellite in 2011.
USPS
Mars, in a composite photo of 24 images NASA's Mars Global Surveyor took one day in 1999.
USPS
Jupiter, as seen by NASA's Hubble telescope in infrared colors in 2004.
USPS
Saturn, as captured by Hubble in 1998.
USPS
Uranus, taken by Hubble in 2003.
USPS
Neptune, as seen by NASA's Voyager 2 space probe in 1989.
USPS
You can buy a souvenir sheet of all the planets. It's convenient that we have only eight now — they fit in neat rows on the sheet.
NASA/USPS
A souvenir sheet of the Pluto stamps is for sale, too.
USPS
