Pluto

• 1. Introduction

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  Pluto sits roughly 39.5 AU from the Sun, so far away that sunlight takes 5.5 hours just to reach it. When Clyde Tombaugh spotted a faint moving speck on photographic plates taken on January 23 and 29, 1930, he had no idea he was about to ignite one of astronomy's longest-running arguments. Was this tiny, icy world a planet? A comet? Something else entirely? For most of the 20th century, Pluto wore the title of ninth planet, beloved by schoolchildren and textbook writers alike. Then, in 2006, the International Astronomical Union changed the definition of "planet," and Pluto found itself reclassified as something new: a dwarf planet. The backlash was immediate. State legislatures passed resolutions in protest. The American Dialect Society coined a new verb. And scientists are still arguing today. How did a small body of rock and ice become the center of such a fierce debate? And what did NASA's New Horizons spacecraft reveal when it finally flew past on the 14th of July 2015, after a journey of 3,462 days? Those are the questions this documentary will answer.

• 2. The Hunt for Planet X

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  In 1906, Percival Lowell, a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894, launched an ambitious search for a body he called "Planet X." The idea had roots in the 1840s, when Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered Neptune by studying wobbles in Uranus's orbit. Later observations of Neptune suggested yet another unseen planet was tugging on Uranus from beyond.

  Lowell and William H. Pickering proposed possible celestial coordinates for Planet X in 1909. The search relied on mathematical calculations made by Elizabeth Williams, and it continued until Lowell's death in 1916 without success. What Lowell never knew was that his survey had captured two faint images of Pluto on March 19 and the 7th of April 1915, without anyone recognizing them for what they were. Fourteen other precovery observations exist, with the earliest made by the Yerkes Observatory on the 20th of August 1909.

  After Lowell's death, his widow Constance entered a ten-year legal battle with the observatory over his estate, halting the search until 1929. Observatory director Vesto Melvin Slipher then handed the task to a 23-year-old newcomer named Clyde Tombaugh, who had impressed Slipher with a sample of his astronomical drawings.

  Tombaugh's method was painstaking. He photographed the night sky in pairs, then used a blink comparator to rapidly alternate between the two images, making any object that had shifted position appear to jump. On the 18th of February 1930, after nearly a year of searching, he found it: a moving object on plates taken days apart in January. News of the discovery was telegraphed to the Harvard College Observatory on the 13th of March 1930. One complete orbit of Pluto around the Sun takes 247.94 Earth years, meaning Pluto will not complete its first full orbit since its discovery until 2178.

• 3. A Name from Myth and a Schoolgirl

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  Lowell Observatory received over a thousand name suggestions following the discovery announcement. Three rose to the top: Minerva, Pluto, and Cronus. Minerva was the staff's first choice, but that name had already been assigned to an asteroid. Cronus was ruled out because its chief advocate was Thomas Jefferson Jackson See, an astronomer described in the source as unpopular and egocentric. A vote was held, and Pluto won unanimously.

  The first person to suggest "Pluto" was Venetia Burney, an eleven-year-old schoolgirl in Oxford, England, born in 1918, who had an interest in classical mythology. She proposed the name to her grandfather Falconer Madan when he read the discovery news to the family over breakfast. Madan passed the idea to astronomy professor Herbert Hall Turner, who cabled it to colleagues at Lowell on March 16, three days after the announcement. The name was published officially on the 1st of May 1930.

  The choice carried multiple layers of meaning. The god Pluto was one of six surviving children of Saturn, and his siblings had already lent their names to planets: brothers Jupiter and Neptune, and sisters Ceres, Juno, and Vesta. Both the god and the newly named body inhabited dark, remote regions, and the god possessed the power of invisibility, much as the distant world had hidden from astronomers for decades. The first two letters of Pluto also happened to match the initials of Percival Lowell, adding a quiet tribute to the man who spent years searching for it.

  The name quickly spread beyond astronomy. In 1930, Walt Disney apparently drew inspiration from it when he gave Mickey Mouse a canine companion named Pluto, though Disney animator Ben Sharpsteen could not confirm why the name was chosen. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, following a tradition that had already produced uranium (named after Uranus) and neptunium (named after Neptune).

• 4. The Shrinking Planet

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  Almost from the moment of its discovery, Pluto's mass was in dispute. Lowell had predicted Planet X would be roughly seven times the mass of Earth. In 1931, astronomers calculated Pluto at about one Earth mass. By 1948 that estimate had fallen to roughly one-tenth of Earth. In 1976, Dale Cruikshank, Carl Pilcher, and David Morrison of the University of Hawaii found evidence that Pluto's surface was covered in methane ice. That finding allowed them to calculate its reflectivity for the first time, and they concluded the object could not exceed one percent the mass of Earth.

  Cruikshank and colleagues also pointed out that Pluto's mass was far too small to have any meaningful effect on the orbits of Uranus or Neptune, meaning Tombaugh's discovery was the product of a thorough search, not a prediction. The real nail in Planet X's coffin came in 1992, when Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised Neptune's estimated mass downward by 0.5 percent, an amount comparable to the mass of Mars. Once the corrected figures were applied to the orbital calculations, the discrepancies that had made Planet X seem necessary simply disappeared.

  The discovery of Pluto's moon Charon in 1978 by astronomer James Christy allowed the first direct measurement of Pluto's mass: roughly 0.2 percent that of Earth. To put that in context, Pluto's mass is 0.07 times the combined mass of all other objects sharing its orbital zone, whereas Earth's mass is 1.7 million times the remaining mass in its own orbital neighborhood. That single ratio would later become central to Pluto's demotion.

• 5. The 2006 Reclassification

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  From 1992 onward, astronomers began finding more and more small icy bodies occupying the same region of space as Pluto. These discoveries raised an obvious question: if Pluto shared its neighborhood with hundreds of similar objects, what made it special enough to be called a planet? The debate moved from academic journals into public view. In February 2000 the Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.

  The pressure became impossible to ignore in July 2005, when a team from Caltech announced the discovery of Eris, a scattered-disk object substantially more massive than Pluto and the most massive body found in the Solar System since Triton in 1846. The press briefly called Eris the tenth planet. For many astronomers, it was the clearest possible argument for reclassification.

  At the IAU's triennial meeting in August 2006, Uruguayan astronomers Julio Angel Fernandez and Gonzalo Tancredi proposed a new three-part definition of "planet": an object must orbit the Sun, be massive enough for gravity to pull it into a roughly spherical shape, and have cleared the neighborhood around its orbit. Pluto meets the first two conditions but fails the third. The IAU voted to adopt the definition, and Pluto became a dwarf planet. In September 2006 it was entered into the Minor Planet Catalogue as (134340) Pluto.

  Resistance was swift and vocal. Alan Stern, principal investigator of NASA's New Horizons mission, pointed out that fewer than five percent of IAU astronomers voted for the resolution. Marc W. Buie, then at Lowell Observatory, petitioned against it. Several U.S. state legislatures pushed back: New Mexico declared Pluto a planet within its skies and named the 13th of March 2007, Pluto Planet Day; Illinois passed a similar resolution in 2009, noting that Tombaugh was born there; and in April 2024, Arizona, where Pluto was discovered, passed a law naming it the official state planet. The American Dialect Society voted "plutoed" its word of the year in 2006, defining it as "to demote or devalue someone or something."

• 6. An Orbit Unlike Any Other

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  Pluto's orbital period runs about 248 years, and its path around the Sun is dramatically different from those of the eight recognized planets. Where the planets follow nearly circular paths close to the flat reference plane called the ecliptic, Pluto's orbit is inclined more than 17 degrees and is moderately elliptical. That eccentricity means a small portion of Pluto's orbit actually passes closer to the Sun than Neptune does.

  Yet the two bodies never collide. Pluto is locked in a 2:3 orbital resonance with Neptune: for every two orbits Pluto completes, Neptune completes exactly three. The cycle of their gravitational interaction lasts about 495 years. At perihelion, the point in Pluto's orbit closest to the Sun, Pluto's path passes about 8 AU north of Neptune's, keeping the bodies safely apart. The Pluto-Charon barycenter reached perihelion on the 5th of September 1989, and Pluto was last closer to the Sun than Neptune during the period from the 7th of February 1979, to the 11th of February 1999.

  Additional layers of gravitational choreography further protect the pair. Pluto's argument of perihelion, a measure of where in its orbit it comes closest to the Sun, hovers around 90 degrees, which means that when Pluto is at its closest solar approach it is also farthest north of the plane of the Solar System, away from Neptune's path. This behavior, called the Kozai mechanism, creates an angular separation of always more than 52 degrees between Pluto's perihelion and Neptune's orbit. A so-called 1:1 superresonance, involving all four of Jupiter, Saturn, Uranus, and Neptune, reinforces this arrangement and has kept it stable over millions of years.

• 7. A Surprising World Up Close

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  New Horizons captured its first distant images of Pluto in late September 2006 from roughly 4.2 billion kilometers away, just to confirm the spacecraft's ability to track its target. The close encounter came years later, and what the probe found was far more complex than anyone had predicted.

  The plains of Sputnik Planitia, the western lobe of a feature nicknamed the "Heart," are a 1,000-kilometer-wide basin of nitrogen and carbon monoxide ices divided into polygonal convection cells. No craters were visible to New Horizons there, indicating a surface less than 10 million years old; later studies refined that age to about 180,000 years. In the western parts of the basin, winds blowing outward from its center have sculpted fields of transverse dunes whose wavelengths fall in the range of 0.4 to 1 kilometer, likely formed from methane particles 200 to 300 micrometers in size.

  The New Horizons science team found evidence for glaciation, possible cryovolcanism, tectonics, and mass-wasting processes. Pluto's surface is one of the most contrastive in the Solar System, rivaling Saturn's moon Iapetus, with colors ranging from charcoal black to dark orange and white. Named features include Belton Regio, a large dark zone called the "Whale," and a series of equatorial dark patches known as the "Brass Knuckles."

  Inside the planet, radioactive decay is expected to have heated the ices enough to separate rock from ice, leaving a dense rocky core of about 1,700 kilometers in diameter surrounded by a mantle of water ice. Scientists now believe a subsurface liquid water ocean, potentially 100 to 180 kilometers thick, may exist at the core-mantle boundary. In June 2020, astronomers reported evidence that this ocean may have existed even when Pluto first formed, raising the possibility that the world was once habitable. On the 25th of October 2016, the final bit of data from the encounter, the last of 50 billion bits in total, was received on Earth, completing a transmission that had been trickling back since the July 2015 flyby.

• 8. What Comes Next

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  Since the New Horizons flyby, planetary scientists have mapped Pluto's northern hemisphere and equatorial regions down to about 30 degrees south latitude. Large portions of the southern hemisphere remain poorly observed, captured only at low resolution from Earth or glimpsed faintly through light reflected off Charon.

  Scientists have proposed an orbiter mission to Pluto that would map its surface at a resolution of 30 feet per pixel, study the smaller moons, track changes as Pluto rotates, and investigate the suspected subsurface ocean. Mapping Pluto's permanently darkened southern regions could be accomplished using laser pulses to build a complete topographic picture. A conceptual study funded by NASA's Innovative Advanced Concepts program envisions a fusion-enabled orbiter and lander based on a Princeton field-reversed configuration reactor.

  Alan Stern has proposed a Cassini-style orbiter that would launch around 2030, timed to mark the 100th anniversary of Pluto's discovery, and use Charon's gravity to adjust its orbit once it arrives. After completing its science objectives at Pluto, the same spacecraft could leverage a final Charon gravity assist to depart the system and go on to study more Kuiper belt objects farther out. Whether that mission ever flies, Pluto remains the largest known member of the Kuiper belt, a category of objects that planetary scientists believe are the frozen remnants of material from the Solar System's earliest days, bodies that never quite managed to coalesce into a full planet.