Background and context By the early 20th century, Percival Lowell and other astronomers had hypothesized the existence of a trans-Neptunian planet to explain perceived irregularities in the orbits of Uranus and Neptune. Lowell Observatory in Flagstaff, Arizona, continued a program of systematic photographic searches after Lowell’s death in 1916. The search relied on wide-field photographic plates taken on successive nights so moving objects could be detected. The discovery process Clyde W. Tombaugh, a young amateur astronomer from Kansas hired by Lowell Observatory in 1929, was assigned to examine photographic plates using a blink comparator, an optical device that rapidly alternates between two plates of the same star field taken on different nights. By blinking between plates, moving objects appear to jump while fixed stars remain stationary. On the plates taken in January 1930, Tombaugh noticed a faint object that shifted position relative to background stars, indicating it was within the solar system. Further observations confirmed consistent motion. Announcement and naming Lowell Observatory announced the discovery on March 13, 1930, citing observations made by Tombaugh on plates dated January 23 and 29, 1930. A search for a name attracted public and institutional suggestions; ultimately the name Pluto—proposed by Venetia Burney, an 11-year-old in Oxford, England—was selected. The name honored the Roman god of the underworld and conveniently began with the initials “PL,” echoing Percival Lowell’s initials. Initial reception and significance At the time, Pluto was hailed as the Solar System’s ninth planet. Its discovery was celebrated as the fulfillment of a decades-long search and as evidence of the effectiveness of photographic survey techniques and careful manual examination. Tombaugh’s methodical comparison of plates exemplified the observational practices of the era. Subsequent developments and revisions Early estimates of Pluto’s size and mass were uncertain because observations captured only its apparent brightness and motion. For decades astronomers overestimated Pluto’s mass; only with improved observations and the discovery of Pluto’s largest moon, Charon, in 1978 could its mass be measured more accurately, revealing Pluto to be far smaller than initially assumed. Later discoveries of other trans-Neptunian objects and advances in planetary classification led the International Astronomical Union (IAU) in 2006 to reclassify Pluto as a “dwarf planet.” This reclassification reflects evolving scientific criteria rather than a correction of Tombaugh’s observational discovery. Legacy Clyde Tombaugh’s discovery remains a landmark event in planetary astronomy and in the history of Lowell Observatory. Tombaugh continued to contribute to astronomy throughout his life, engaging in systematic searches for asteroids and variable stars and later participating in projects related to meteoritics and planetary observations. Pluto itself has remained an object of scientific interest: spacecraft flybys (notably NASA’s New Horizons mission in 2015) and ongoing telescopic study have revealed a complex surface, atmosphere, and a system of moons, deepening understanding of the outer Solar System. Historical accuracy and caveats The account above reflects documented chronology: Tombaugh’s detection in early 1930, the March announcement, and subsequent scientific revisions to Pluto’s estimated size and classification. Some early expectations about a large “Planet X” perturbing Neptune’s orbit proved incorrect; later analyses attributed the original orbital discrepancies to measurement errors. The discovery itself—Tombaugh’s identification of a moving object on photographic plates—is well-documented and not in dispute.