Pluto, the planet, dwarf planet, or trans-Neptunian Kuiper belt object, has had an interesting life since it first came to prominence in 1930, after its discovery by Clyde Tombaugh. It was christened Pluto, after the Roman version of the Greek god of the Underworld (Hades), as suggested by 11 year old Venetia Burney, an English schoolgirl.
The choice of name was partly inspired by the fact that the first two letters are the initials of Percival Lowell, wealthy mathematician and astronomer who had started the search for the mysterious Planet X in 1906, but died in 1916, before it had been discovered.
The symbol used for the planet Pluto was the monogram for Lowell's initials, PL. (Perhaps he had used that monogram before, for cuff-links or handkerchieves, who knows?) You can find this symbol in a few fonts — it's stored in the Unicode table:
at hex location U+2647, nestling between the other planets and the zodiac signs. It's not the most attractive or interesting glyph. Here's Menlo:
and here's Arial:
Before the days of Unicode, you would probably have to buy a "pi font" to obtain the character in a font. Here it is in one of Linotype's Pi fonts called Astrology:
Unlike the other planets, whose astrological and astronomical symbols are the same, Pluto has another symbol, an astrological (rather than astronomical) version, which you won't currently find in the Unicode system. It is similar to Neptune's, but has a circle in place of the middle prong of the trident.
This makes some sense, because Hades (Pluto) was the brother of Poseidon (Neptune), and Hades was sometimes represented holding a trident (something for his three-headed dog Cerberus to play with?).
Here's Ralph Fiennes (taking a break from being Lord Voldemort) as a suitably miserable Hades, from the movie "Wrath of the Titans", with a two-pronged trident:
(It's more accurate than this image:
which is just bizarre.)
There's no shortage of explanations for the meaning of Pluto's astrological symbol on the various astrological sites:
it combines the circle (spirit) over the crescent (receptivity) and the cross (matter). This glyph suggests spiritual receptivity to "superworlds" or the superconscious, placed above the physical world. Pluto represents subconscious forces, ruling all that is "below the surface". On the up side, Pluto is associated with renewal and rebirth. It represents endings and new beginnings, as well as spiritual growth and rebirth. Negative expression of Pluto is an obsessive desire for power and control and general destructiveness.
You'll most likely find this glyph in one of the specialist astrological fonts. These often predate the Unicode-rich fonts we see today. Here’s the design from shareware font Astro.ttf by the aptly-named Cosmorama Enterprises (dated 1993):
The most pleasing versions of both Pluto symbols I've found in my font library are from the font called Astrological, designed by Peter Rempel in 1998:
I'm not sure whether New Horizons spacecraft, soon to arrive at Pluto, bears either of the Pluto symbols. But, in keeping with the theme of death and the underworld, there's a tiny sample of Clyde Tombaugh's ashes attached to the side:
2015-06-20 14:42:33