The Discovery of Pluto

01 The Wobble in Neptune

The story of Pluto begins not with the planet itself but with Neptune. After its discovery in 1846, Neptune’s orbit was tracked with increasing precision over the following decades. By the late nineteenth century, astronomers noticed that Neptune was not quite where it should be. Its observed positions deviated slightly from the predictions derived from Newtonian gravitational theory, even after accounting for the gravitational pull of all known planets. The discrepancy was small, a matter of arcseconds, but it was persistent and it suggested that something beyond Neptune was exerting a gravitational tug.

The logic was the same that had led to Neptune’s discovery: irregularities in the orbit of one planet implied the gravitational influence of another, as-yet-unseen body. In the 1840s, Alexis Bouvard had noticed that Uranus was deviating from its predicted path; that led Urbain Le Verrier and John Couch Adams to calculate the position of an eighth planet, and Johann Galle found Neptune exactly where the math said it should be. The method worked. Now astronomers applied it one step further out.

The hypothetical ninth planet was dubbed Planet X. The challenge was formidable. The perturbations in Neptune’s orbit were tiny, and the number of unknowns, mass, distance, orbital inclination, was large. Any calculation would be underdetermined, a system of equations with more unknowns than constraints. But one man was convinced it could be done, and he had the resources to build an observatory specifically for the purpose.

02 Percival Lowell and Planet X

Percival Lowell was a Boston patrician, a mathematician trained at Harvard, and an astronomy enthusiast whose earlier career had been devoted to the study of Mars. Lowell had famously argued that the canals visible on Mars through telescopes were artificial structures built by a dying civilization. This claim was wrong, and it had damaged his scientific reputation, but it did not diminish his energy or his wealth. In 1894, Lowell had founded the Lowell Observatory in Flagstaff, Arizona, at an elevation of 2,100 meters, specifically chosen for its dark, dry skies and steady air. He turned the institution to the Planet X problem in 1905.

Lowell’s approach was computational and observational in parallel. He and his staff attempted to calculate the expected position of Planet X from Neptune’s residuals, and they launched a photographic survey of the ecliptic, taking repeated exposures of the same regions weeks apart and comparing them for any object that moved. The method, called blink comparison, required photographing a field of stars, then photographing the same field again later, and then rapidly switching between the two images in a specially designed device. A star stayed put. A planet shifted its position.

The search was tedious and error-prone. Lowell’s calculations produced predicted positions that shifted with each revision, and the photographic plates covered enormous fields containing hundreds of stars, any one of which might be the quarry. Lowell himself died in 1916 without finding Planet X. His observatory continued sporadically, and a legal dispute over his estate froze the endowment for years. The Planet X search languished. But the infrastructure and the method were still in place when, in 1929, the observatory’s director Vesto Slipher decided to revive the program with a new instrument and a new observer.

03 The Farm Boy From Kansas

Clyde Tombaugh was 22 years old when he arrived at Lowell Observatory in January 1929. He had grown up on a farm in Burdett, Kansas, where the dark prairie skies had inspired an early interest in astronomy. Unable to afford college, Tombaugh had built his own telescopes, grinding mirrors from glass blanks and mounting them on parts salvaged from farm machinery. He sent drawings of Jupiter and Mars, made with his homemade 9-inch reflector, to Lowell Observatory, asking for advice. Slipher was impressed enough to offer him a job operating the new 13-inch Lawrence Abbott telescope, a purpose-built astrograph designed for the Planet X survey.

The work was grueling. Tombaugh would expose photographic plates for intervals of up to three hours, guiding the telescope by hand to compensate for the sky’s rotation. On a good night he might capture three or four plates. Each plate, when developed, covered a field about 13 degrees on a side and contained thousands of star images. The blink comparator allowed him to view two plates of the same field taken on different nights, alternating rapidly between them. A moving object would appear to jump back and forth; a star would remain stationary.

Tombaugh’s discipline was extraordinary. He systematically covered the zodiacal band, working month after month. He had to distinguish moving objects from defects on the plates, variable stars, and asteroids that shifted from night to night by amounts comparable to a distant planet. He eliminated thousands of false candidates. By the time he had been at the observatory for nearly a year, he had examined millions of star images without finding Planet X.

04 February 18, 1930

On the afternoon of February 18, 1930, Tombaugh was blinking a pair of plates taken on January 23 and January 29, 1930, of a region in the constellation Gemini. The plates were taken six nights apart. At about four o’clock in the afternoon, after hours at the comparator, he saw it. A faint point of light shifted by approximately 3.5 millimeters between the two exposures, a movement consistent with a trans-Neptunian object. He checked for plate defects and found none. He re-examined the region with a third plate taken on January 21 and confirmed the object was real, moving in the right direction at the right rate.

The object was roughly at the position Lowell had predicted, close enough to validate the search strategy in the eyes of the astronomical community. The Lowell Observatory announced the discovery on March 13, 1930, timed to coincide with the 75th anniversary of Percival Lowell’s birth and the 149th anniversary of William Herschel’s discovery of Uranus. The news made headlines worldwide. A new ninth planet had been found, the first planet discovered in nearly 85 years.

The naming of Pluto followed a familiar pattern. The Lowell Observatory solicited suggestions. The winning entry came from an 11-year-old Oxford schoolgirl named Venetia Burney, who, during a breakfast conversation with her grandfather, suggested the name Pluto, the Roman god of the underworld, appropriate for a dark and distant world. Her grandfather, a retired librarian, passed the suggestion to the astronomy professor at Oxford, who cabled it to Flagstaff. The name was adopted unanimously by the observatory staff. It carried an added resonance: the first two letters, PL, were Percival Lowell’s initials.

05 The Planet That Was Not

Almost from the beginning, Pluto was a puzzle. Initial estimates of its mass, derived from its assumed gravitational effect on Neptune, ranged from 1 to 10 Earth masses, values that would have made it a substantial planet. But as observations accumulated, Pluto kept shrinking. Its brightness was too faint for a large planet, suggesting it was small. Its disk could not be resolved even in the largest telescopes, placing an upper limit on its size well below Mercury’s. By the 1950s, estimates of Pluto’s mass had fallen to roughly 0.1 Earth masses. By the 1970s, it was clear that Pluto was smaller than Earth’s Moon.

In 1978, James Christy at the U.S. Naval Observatory discovered Pluto’s largest moon, Charon, by noticing that photographs of Pluto showed an elongation that appeared to rotate with a period of about 6.4 days. The discovery of a binary system allowed precise mass determination for the first time. Pluto’s mass turned out to be roughly 0.0022 Earth masses, about one-sixth the mass of the Moon. This was far too small to have produced the perturbations in Neptune’s orbit that Lowell had used to predict its position. The residuals that drove the Planet X search were later shown to be artifacts of inaccurate nineteenth-century measurements of Neptune’s position, not real gravitational effects. Pluto had been found in the right region more by persistence and luck than by accurate prediction.

The 1990s brought a more fundamental challenge to Pluto’s status. Astronomers began discovering numerous objects in the same region beyond Neptune, a zone now called the Kuiper belt. Several of these trans-Neptunian objects were sizable, and one, Eris, discovered in 2005, was slightly more massive than Pluto. If Pluto was a planet, then Eris should be one too, and so should many other Kuiper belt objects that were being identified at an accelerating pace. The alternative was that none of them were planets, including Pluto.

06 Reclassification and Resolution

The crisis came to a head in 2006. The International Astronomical Union, the body responsible for official astronomical nomenclature, convened a General Assembly in Prague to settle the question. After extensive debate, the IAU adopted a formal definition of the word “planet” for the first time in history. Under the new definition, a planet must orbit the Sun directly, be massive enough to be roughly spherical under its own gravity, and have cleared its orbital neighborhood of other debris. Pluto failed the third criterion: it shares its orbital zone with numerous Kuiper belt objects and has not gravitationally swept its neighborhood clean.

The IAU created the new category of dwarf planet for objects that meet the first two criteria but not the third. Pluto was reclassified as a dwarf planet, along with Ceres, the largest asteroid, and Eris. The decision was controversial and remains so among some astronomers and much of the public, who had grown up with nine planets. But it reflected a genuine scientific reality: the solar system’s outer reaches contained hundreds of Pluto-sized and smaller bodies in a disk of debris, and Pluto was simply the first-discovered member of that population.

The reclassification did not diminish Pluto’s scientific importance. If anything, it clarified it. Pluto was now understood as the archetype of a new class of worlds, the plutoids, and as a window into the early solar system. The Kuiper belt, of which Pluto is a part, preserves icy bodies that formed at the outer edge of the protoplanetary disk and were never incorporated into larger planets. Studying Pluto meant studying the raw material from which the outer solar system was built.

07 New Horizons

No spacecraft had visited Pluto until July 14, 2015, when NASA’s New Horizons probe, launched nine and a half years earlier, flew past the dwarf planet at a distance of approximately 12,500 kilometers. The flyby was the culmination of a decades-long campaign to send a mission to the outer solar system’s last unexplored world. New Horizons, traveling at over 14 kilometers per second, could not enter orbit around Pluto because the fuel required to brake into the system would have been prohibitive at that distance from the Sun. Instead, it executed a fast flyby, gathering data during a compressed observation window of roughly 48 hours.

The images and data returned by New Horizons transformed Pluto from a fuzzy point of light into a geological world. Pluto’s surface turned out to be far more diverse than anyone had predicted. The encounter revealed a vast nitrogen-ice plain, informally named Sputnik Planitia, that fills a major impact basin. Towering water-ice mountains rise along its western margin. Across the surface, mission scientists identified flowing nitrogen glaciers, dune-like features, and polygonal ice structures that suggest slow convective churning of the surface. The thin atmosphere, dominated by nitrogen with traces of methane and carbon monoxide, extends hundreds of kilometers above the surface and produces a blue haze visible in backlit images.

Perhaps the most surprising finding was Pluto’s geological youth. Sputnik Planitia appears to be no more than 10 million years old, and possibly much younger. The surface is being actively resurfaced by some internal heat source, possibly the slow freezing of a subsurface ocean. Pluto is not a dead, frozen ball but a geologically active world, and the data from New Horizons will occupy planetary scientists for decades. The probe continued past Pluto into the Kuiper belt, flying by another trans-Neptunian object, Arrokoth, on January 1, 2019. It is now deep in the Kuiper belt, still transmitting, heading outward into interstellar space.