— Ch. 1 · Discovery And Naming —
Pluto.
~9 min read · Ch. 1 of 6
On the 18th of February 1930, Clyde Tombaugh spotted a faint moving point of light on photographic plates taken at Lowell Observatory in Flagstaff, Arizona. The object had been captured earlier that month on January 23 and 29, but only became visible when Tombaugh used his blink comparator to rapidly switch between the images. This mechanical device created an illusion of movement for any celestial body that had shifted position relative to the background stars. News of the discovery was telegraphed to Harvard College Observatory on the 13th of March 1930. Before this moment, Percival Lowell had spent years searching for a ninth planet he called Planet X without success. His widow Constance Lowell eventually resumed the search in 1929 after a legal battle over her husband's legacy. Vesto Melvin Slipher gave the job to the twenty-three-year-old Tombaugh who had impressed the observatory director with astronomical drawings. Over one thousand name suggestions arrived at Lowell Observatory following the announcement. Three names topped the list: Minerva, Pluto, and Cronus. Staff rejected Minerva because it was already assigned to an asteroid. They dismissed Cronus due to its association with an unpopular astronomer named Thomas Jefferson Jackson See. A vote then made Pluto the unanimous choice. An eleven-year-old schoolgirl named Venetia Burney from Oxford, England, had suggested the name to her grandfather Falconer Madan three days before the official announcement. Her grandfather passed the suggestion to astronomy professor Herbert Hall Turner who cabled it to colleagues at Lowell. The name Pluto came from the Roman god of the underworld. It also served as an epithet for Hades, the Greek equivalent. The first two letters matched the initials of Percival Lowell while the last two letters matched the beginning of Tombaugh's surname. The name appeared publicly on the 1st of May 1930.
Planetary Status Debate
Astronomers initially calculated Pluto's mass based on its presumed effect on Neptune and Uranus. In 1931 they estimated it weighed roughly the same as Earth. By 1948 that figure dropped to about the mass of Mars. Dale Cruikshank, Carl Pilcher, and David Morrison calculated Pluto's albedo in 1976 finding it matched methane ice. This meant Pluto could not exceed one percent of Earth's mass. The discovery of Charon in 1978 allowed scientists to measure Pluto's true mass for the first time. They found it was only 0.2% that of Earth. Robert Sutton Harrington searched for an alternative Planet X but failed. Myles Standish used data from Voyager 2's flyby of Neptune in 1989 to recalculate gravitational effects on Uranus. With new figures added in, the discrepancies vanished entirely. Ernest W. Brown concluded soon after Pluto's discovery that Percival Lowell's prediction had been a coincidence. From 1992 onward many bodies were discovered orbiting in the same volume as Pluto showing it belonged to a population called the Kuiper belt. Museum directors occasionally created controversy by omitting Pluto from planetary models. The Hayden Planetarium in New York City displayed a model with only eight planets in February 2000. Astronomers at Caltech led by Michael E. Brown found numerous Trans-Neptunian objects in the early 2000s. Many appeared larger than or equal in size to Pluto igniting a debate over whether they should be considered planets. Estimates later revised downward due to higher than expected albedos. In July 2005 these astronomers announced the discovery of Eris which was substantially more massive than Pluto. The press initially called it the tenth planet though no official consensus existed. The International Astronomical Union met in August 2006 to redefine the term planet. Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi proposed three conditions for planethood. An object must orbit the Sun. It must be massive enough to be rounded by its own gravity. It must have cleared the neighborhood around its orbit. Pluto failed the third condition because its mass is only 0.07 times the combined mass of other objects in its orbit. Earth's mass is 1.7 million times greater than remaining material in its orbit. Bodies meeting criteria one and two but failing criterion three became dwarf planets. The IAU included Pluto in their Minor Planet Catalogue in September 2006 giving it designation 134340. Alan Stern principal investigator with NASA's New Horizons mission derided the resolution stating less than five percent of astronomers voted for it. Marc W. Buie then at Lowell Observatory petitioned against the definition. Public reception remained mixed with resolutions passed in California Illinois and New Mexico declaring Pluto a planet within those jurisdictions.