Pluto

Pluto was found on 18 February 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona, during a search for a further body thought to be perturbing Uranus and Neptune; the perturbations turned out to be errors in the mass of Neptune, so the discovery was in a real sense accidental. The name was suggested by Venetia Burney, an eleven-year-old in Oxford. In August 2006 the International Astronomical Union adopted Resolution B5, which defined a planet as a body that orbits the Sun, has sufficient mass for its self-gravity to make it nearly round, and has cleared the neighbourhood of its orbit; Pluto meets the first two conditions and not the third, and was reclassified as a dwarf planet. The decision followed the discovery of Eris and other large trans-Neptunian objects, which made it clear that Pluto is one member of a populous Kuiper belt rather than a ninth planet of the old kind.

Three further facts fill out why Pluto sits awkwardly among the planets. Its orbit is inclined about 17 degrees to the ecliptic, far more than any planet's, and eccentric enough that it runs from 29.7 astronomical units at perihelion to 49.3 at aphelion, so that between 1979 and 1999 it was inside Neptune's orbit. The two never approach, because they are locked in a 3:2 resonance: Pluto completes two orbits for Neptune's three. And Pluto's largest moon, Charon, is more than half its diameter, so the pair orbit a common centre of mass that lies outside Pluto's surface, a configuration no planet and moon share.

Astrologers assigned Pluto the significations of transformation, death and rebirth, power, compulsion, the hidden, the buried and the collective unconscious, and commonly gave it the rulership of Scorpio in place of or alongside Mars. Its orbit is long, about two hundred and forty-eight years, and eccentric enough that it spends very different lengths of time in different signs, from about twelve years in Scorpio to over thirty in Taurus. Most astrologers who use it continued to do so after 2006, on the reasonable ground that astrological practice is not bound by the IAU's definitional decisions, which concern nomenclature within astronomy. What the reclassification does establish is a plain matter of fact that any honest account should include: Pluto is a small icy body, smaller than the Moon, and one of many such in the outer solar system. Its astrological meaning, less than a century old, is a modern construction.

The unequal sign durations have a specific cause and a specific consequence. The cause is an orbital eccentricity of about 0.25, which carries Pluto from 29.7 astronomical units at perihelion out to 49.3 at aphelion; it moves fastest through the signs it passes near perihelion and slowest at the far end. The consequence is that a Pluto sign is a generational marker and nothing more personal. Everyone born across a span of twelve to thirty years shares it, so whatever is said of a Pluto placement individually has to come from the house and the aspects. The New Horizons flyby of 14 July 2015 returned the first close images, showing nitrogen ice plains and mountains of water ice on a body that is geologically active despite its size.

Pluto was discovered by Clyde Tombaugh in 1930. Its glyph is a monogram of the letters P and L, which also stand for the initials of the astronomer Percival Lowell, whose observatory sought it. The double meaning was almost certainly intentional: Lowell had predicted a ninth planet from irregularities he believed he saw in the orbits of Uranus and Neptune and spent years searching for it before his death, and the monogram let the new planet's sign honour both its mythological namesake and the astronomer whose observatory, and whose initials, made the discovery possible. Since the 2006 reclassification, many astrologers and even NASA itself have shifted toward a different glyph, a bident topped by a small circle or orb, distancing Pluto's symbol from the original PL monogram without abandoning the planet's place in practice. Unicode encoded the original monogram with the rest of the astrological set in 1991 and has since fielded several later proposals for alternative Pluto glyphs reflecting this divided modern usage, though none of the alternatives has yet been formally added to the standard alongside the original sign. Pluto's short, crowded symbolic history, a schoolgirl's suggested name, an astronomer's memorialised initials, and now a second, unofficial glyph competing with the first, compresses into under a century a dispute over meaning and representation that took the seven classical planets the better part of two millennia to work through.