02/18/1930 • 7 views
Clyde Tombaugh’s 1930 Discovery of Pluto
On February 18, 1930, American astronomer Clyde Tombaugh identified a ninth planet—later named Pluto—using photographic plates at Lowell Observatory, marking a major milestone in 20th-century planetary astronomy.
By the 1920s, Percival Lowell’s search for a trans-Neptunian planet—often called "Planet X"—had motivated systematic sky surveys at Lowell Observatory in Flagstaff, Arizona. Lowell had died in 1916, but the observatory continued his program, seeking perturbations in the orbits of the known outer planets that might indicate an unseen world.
Tombaugh’s method and the discovery
Clyde W. Tombaugh, a young technician and amateur astronomer hired by Lowell Observatory in 1929, used a blink comparator to examine pairs of photographic plates taken weeks apart. The blink comparator—an instrument that rapidly alternates between two images—lets an observer spot objects that move relative to the background stars. On plates taken in January 1930, Tombaugh noticed a faint object that shifted position night to night. Further exposures confirmed a slow-moving body beyond Neptune. The discovery was announced by Lowell Observatory on March 13, 1930; Tombaugh’s initial detection date is recorded as February 18, 1930.
Naming and early characterization
Following the announcement, an international appeal among schoolchildren and the public produced many name suggestions. The name "Pluto," proposed by Venetia Burney, an 11-year-old in England, was quickly adopted—partly because the initials PL honored Percival Lowell—and formally accepted by the astronomical community. Early observations established Pluto as a distant, faint object; however, its size and mass remained uncertain for decades, leading to overestimates of its significance as a major planet.
Scientific developments after discovery
For much of the 20th century, Pluto was considered the ninth planet. Subsequent advances—particularly the discovery in the 1990s of many small icy bodies in the Kuiper Belt, a region beyond Neptune populated by objects similar to Pluto—challenged that categorization. Improved measurements in the late 20th and early 21st centuries showed Pluto to be much smaller and less massive than Earth’s Moon, and the discovery of comparable bodies, notably Eris in 2005, prompted a redefinition of what constitutes a planet. In 2006 the International Astronomical Union (IAU) reclassified Pluto as a "dwarf planet."
Legacy and continued importance
Tombaugh’s discovery catalyzed decades of observational and theoretical work on the outer Solar System. Pluto has remained of scientific and public interest: it became the target of NASA’s New Horizons mission, which performed a close flyby in 2015, revealing a complex world with mountains of water ice, nitrogen-glacier flows, and a layered atmosphere. Tombaugh’s methods—meticulous photographic surveying and careful visual inspection—reflect an era of astronomy transitioning from photographic plates to digital detectors, and his discovery remains a notable example of patient, systematic observational work.
Notes on attribution and chronology
The discovery date commonly cited for Tombaugh’s first identification on plates is February 18, 1930; the public announcement followed on March 13, 1930. Historical accounts vary in emphasis on Lowell Observatory’s role, Tombaugh’s contributions, and how much the Lowell search guided later interpretations; those nuances are reflected in primary archival records and subsequent histories of the observatory and planetary astronomy.