Pluto Day

 On this day in 1930, American astronomer Clyde Tombaugh discovered Pluto at Lowell Observatory in Flagstaff, Arizona.

The heart shape there is named the Tombaugh Regio in his honor.

The western lobe, called Sputnik Planitia, is a plain of nitrogen and other ices lying within a basin.  The eastern lobe consists of reflective uplands thought to be coated in nitrogen transported through the atmosphere from the west.  Some of this nitrogen returns to the basin via glacial flow.

Percival Lowell, who founded the observatory, had always believed there was a 9th planet in our solar system.  He died 14 years before it was found.

Before him, in the 1840's, Urbain Le Verrier had predicted the existence of Neptune through mathematics.  To explain discrepancies in Uranus' orbit, he provided calculations to Johann Gottfried Galle, who was able to locate the planet.  But then they realized there had to be something else disturbing Uranus as well....

It's been almost 20 years since the International Astronomical Union reclassified Pluto as a dwarf planet.