how Pluto was discovered

The Mechanism

*Clyde William Tombaugh* (born Streator, Illinois, *4 February 1906*; raised on a tenant farm near Burdett, Kansas; died Las Cruces, New Mexico, *17 January 1997*, aged 90) was an American astronomer who, on the afternoon of *18 February 1930*, working as a 24-year-old observer at the *Lowell Observatory* in Flagstaff, Arizona, identified the previously-unknown ninth planet of the solar system — *Pluto* — by comparing two photographic plates of the same star field taken six nights apart, on *23 January* and *29 January 1930*, on the observatory's *13-inch (33-cm) photographic refractor (astrograph)*. The search that produced the discovery had been instigated by the wealthy American businessman and amateur astronomer *Percival Lowell* (1855-1916), founder of the observatory in 1894 and the principal proponent of the *"Planet X"* hypothesis — the proposition, based on apparent perturbations in the orbital motions of *Uranus* and *Neptune*, that a then-undiscovered massive trans-Neptunian planet was perturbing the outer solar system. Lowell himself had searched for Planet X without success between 1905 and his death in 1916; the search had been suspended during a fifteen-year inheritance dispute over Lowell's estate (his widow contested the trust funding the observatory) and resumed in *1929* with the commissioning of the new 13-inch astrograph specifically designed for wide-field photographic planet-hunting. The observatory's director, *Vesto Slipher* (the same astronomer who had in the 1910s first measured the redshifts of distant galaxies that would later become the empirical basis of Hubble's law), had hired Tombaugh in *January 1929* for the search. Tombaugh had no university degree: he was the son of Kansas wheat farmers, had built his own telescopes in his teenage years out of crankshafts and grain-elevator parts, and had been hired by Slipher essentially on the strength of a portfolio of hand-drawn sketches of Mars and Jupiter that he had sent to the observatory unsolicited in late 1928. His starting salary was *$90 per month*. The 13-inch astrograph took *14 × 17-inch* glass photographic plates, each capturing a wide *12 × 15 degree* field of the night sky, with on average *300,000 stars* per plate. The procedure was: photograph the same star field on two or three separate nights spaced four to seven nights apart; develop and fix the plates; mount each pair side by side in a *Zeiss blink comparator*, an optical instrument that rapidly alternates the viewer's gaze between two photographic plates in such a way that any object that has changed position between the two exposures appears to "jump" while the unchanging background stars remain still. A trans-Neptunian planet, at the orbital distance Lowell had predicted, would move *roughly 2-3 millimetres* on the 14-inch plate between a pair of exposures six nights apart — a distance about one-fourteenth of an inch, visible against the background stars only as a jumping pinprick of light when blinked rapidly back and forth. Tombaugh's working hours, in an unheated upper room of the *Lowell Observatory administration building*, ran from *9:00 in the morning to 12 noon* and from *1:30 in the afternoon to 6:00 in the evening*, six days a week, with telescope observing duties at night. He had been blinking plates of the Gemini region — the constellation Percival Lowell had predicted would contain the planet — for *ten months* when, at approximately *4:00 pm* on the afternoon of *18 February 1930*, on the third pair of plates he examined that day (the *23 January* and *29 January* exposures of the region near the star *δ Geminorum*), he noticed a *very faint 15th-magnitude dot* that moved by about 3.5 mm between the two plates — exactly the displacement expected of an object at roughly 40 astronomical units from the Sun. Tombaugh blinked the pair for *forty-five minutes* to convince himself the object was real and that the motion was not an instrumental artefact. He examined a *14 January 1930* exposure of the same field (which he had taken as a routine control) and found the object on that plate too, in a position consistent with the *23 January* and *29 January* positions and intermediate between them. He walked down the corridor to the director's office, knocked, and said to Slipher: *"Dr. Slipher, I have found your Planet X."* Slipher, after examining the plates himself, instructed Tombaugh to keep the discovery confidential until the observatory could verify the motion over the following two weeks with additional plates. The discovery was formally announced on *13 March 1930* — the *149th anniversary of the discovery of Uranus* by William Herschel and the *75th birthday of Percival Lowell* — by *cabled telegram* to the *Harvard College Observatory*, the *Naval Observatory*, the *New York Times*, and the *Royal Astronomical Society* in London. The name *Pluto* was suggested in a letter to *Vesto Slipher* by *Venetia Burney*, an *11-year-old English schoolgirl* in Oxford, on *14 March 1930*; her grandfather, a librarian at Oxford's Bodleian, cabled the suggestion to Herbert Hall Turner, an astronomer in Oxford, who forwarded it to Slipher; the observatory adopted the name in a unanimous vote on *1 May 1930*. The name was thought particularly apt because the chemical element *plutonium*, named in turn after the planet, was synthesised in 1940 from uranium by Glenn Seaborg's group at Berkeley using the cyclotron — uranium having been named for Uranus, and neptunium (synthesised 1940) for Neptune. The astronomical situation of Pluto has changed substantially since 1930. The *perturbations of Uranus and Neptune that motivated Lowell's Planet X search* were, it later turned out, *spurious* — they were the result of a systematic error in the assumed mass of Neptune that was corrected by the *Voyager 2* flyby in 1989. Pluto is *too small* (mass approximately *0.00218 Earth masses*, or about *1/500th* of Earth's mass) to have caused the perturbations Lowell observed; the apparent agreement between Lowell's predicted position for Planet X and Tombaugh's actual discovery position of Pluto is a *coincidence*, made possible only because Pluto happened to be near its perihelion in 1930 and was, in the late 1920s, the brightest object of the right kind in the right region of sky. In 1992 *David Jewitt* and *Jane Luu* at the *University of Hawaii* photographed the first Kuiper Belt Object (KBO) other than Pluto — *1992 QB1* — and over the following decade thousands of additional KBOs were catalogued, several of them comparable in size to or larger than Pluto. In 2005 *Mike Brown* at Caltech announced the discovery of *Eris*, a trans-Neptunian object then thought to be more massive than Pluto (now known to be very nearly the same mass). On *24 August 2006* the General Assembly of the *International Astronomical Union*, meeting in Prague, adopted *Resolution 5A* defining a *"planet"* as a body that (a) orbits the Sun, (b) has sufficient mass for its gravity to overcome rigid-body forces and assume hydrostatic equilibrium (a roughly round shape), and (c) has *cleared the neighbourhood around its orbit* of other comparable-sized bodies — and demoted Pluto, which fails the third criterion (its orbital neighbourhood is densely populated with comparable Kuiper Belt Objects), to the new category of *dwarf planet*. The number of *planets* in the solar system was returned to *eight*. Tombaugh had died in 1997 and did not live to see the demotion; his widow, *Patricia Tombaugh*, remarked that her husband would have been irritated but unsurprised. On *14 July 2015* — *85 years* after Tombaugh's discovery — the *New Horizons* spacecraft flew within *12,500 km* of Pluto's surface and returned the first close-up images of the world Tombaugh had identified as a moving pinprick on a glass plate. *New Horizons* carried with it a small canister of *Clyde Tombaugh's ashes*, deposited inside its aluminium body at the request of the family. The canister bears the inscription: *"Interred herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's 'third zone.' Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997)."*