You can now buy these awesome postage stamps of Pluto and the planets

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To celebrate New Horizons and its outdated postage flying by Pluto in July 2015, the USPS just issued two "Pluto —Explored!"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.

The New Horizons spacecraft, which flew by Pluto in 2015, sending us the first up close images of the dwarf planet.

Mercury, taken by NASA's Messenger spacecraft in 2011-12.