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

In 1991, the US Postal Service issued stamps of the then-nine planets. Pluto - which has since been demoted to dwarf planet - was the only one that read "not yet explored."

When NASA launched its New Horizons in 2006, the lead scientist stuck that stamp onto the spacecraft.

Now, since New Horizons flew by Pluto last July, the USPS is issuing "Pluto -Explored!" stamps to celebrate.

The Post Office is also releasing stamps in honor of the solar system's eight planets. You can buy them all online starting May 31.

Here they are: