I’m surprised that no one else jumped on this. While Percival Lowell founded the observatory it was discovered at and even funded the search by a provision in his will, he was quite dead by the time Clyde Tombaugh discovered Pluto. But the part about the origin of the symbol is correct.
As Spritle hints, the symbols for the planets were also used in alchemical books. This was because there was an equivalence set up between the 7 known metals and the 7 astrological planets. Indeed, when alchemists didn’t use the symbols, they used the planets name for the metals (Sol for gold, Luna for moon, etc.). Because of that, the name of the planet Mercury replaced quicksilver as the name of the metal.