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25 February 2022

The Tortuous Journey to Pluto, the Dwarf Planet at the Edge of the Solar System

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An old joke goes that it has been a busy year for Pluto: from being discovered and hailed as the ninth planet in the Solar System, to being demoted to the status of a dwarf planet, and finally becoming front page news thanks to spectacular photos sent back by a space probe. And all this in less than a year… a Plutonian year, since Pluto takes just over 247 Earth years to complete a full orbit around the Sun. From its accidental discovery to the surprises revealed by its recent scientific study, Pluto seems to exert an attraction that, even though it hasn’t been considered a planet since 2006, has nevertheless turned it into a bona fide star.

Clyde Tombaugh was a 23-year-old Kansas farmer who had built his own telescope from automobile and farm machinery parts. Image: Wikimedia

When in the 1840s the perturbations observed in the orbit of Uranus led to the discovery of Neptune, the calculations of some astronomers suggested that this was not all, and that there were still other planets to be discovered beyond the eighth. In the early 20th century, mathematician and entrepreneur Percival Lowell was determined that the ultimate legacy of his life would be the discovery of what he called “Planet X”, after his unsuccessful attempt to convince the scientific community of the existence of artificial canals on Mars. From his observatory in Flagstaff (Arizona) and with the collaboration of astronomer William Henry Pickering, Lowell put at the head of a team of “human computers” the physicist Elizabeth Williams, one of the first women graduates of the Massachusetts Institute of Technology whose contribution was key, but whose name has been almost forgotten.

Tombaugh’s achievement and the blink comparator

Lowell died in 1916 without his discovery, and the search for Planet X was sidelined for more than a decade. In 1929, the director of the Lowell Observatory, Vesto Melvin Slipher, revived the project by assigning it to a 23-year-old Kansas farmer who had built his own telescope from automobile and farm machinery parts, and whose astronomical drawings had impressed him. The assignment Clyde Tombaugh received was not particularly inspiring: take countless photographs of sections of the sky two weeks apart and compare them with a blink comparator to detect objects that had changed position, thereby distinguishing stars from planets. But after two million stars photographed, Tombaugh finally succeeded: on 18 February 1930 he found his planet.

It was then discovered that the new planet had actually appeared in earlier astronomical photographs, but had been overlooked. All that was missing was a name for the new member of the Solar System. As the story goes, it was 11-year-old British girl Venetia Burney who suggested the name Pluto—the Roman god of the underworld in classical mythology—to her grandfather, a librarian at Oxford University, who in turn passed the suggestion on to the astronomers. And although the story of Venetia is true, it seems that the scientists had already been using this name beforehand, partly as a tribute to Lowell because of the match of his initials with the first letters of the name. With the arrival of the new planet, a star was born: the name Pluto inspired the christening of a new radioactive element, plutonium, and perhaps also that of Mickey Mouse’s dog, although this has never been confirmed.

BBVA-OpenMind-El tortuoso camino a Pluton-planeta enano en los confines del Sistema Sola 2 La sonda New Horizons se aproximó en 2015 a Plutón y realizó las mediciones e imágenes de mayor precisión que se han tenido nunca. Imagen: Unsplash
In 2015, the probe New Horizons flew above the surface of Pluto and provided the best images of Pluto to date. Image: Unsplash

Except that, in reality, Pluto was not Planet X. After being recognised and embraced by earthlings as the ninth planet of the Solar System, in the decades that followed, observations and calculations gradually stripped Pluto of its initial glory: far from Lowell’s prediction of a planet as large as seven Earths, it was too tiny to justify the perturbations in the orbits of Uranus and Neptune. In fact, these anomalies disappeared when more accurate models were applied to the planetary orbits. Thus, the reason Pluto was designated as the ninth planet was because it appeared at the right time to fill a gap that was expected to be filled. It was a fortunate finding that was prompted by a mistaken hypothesis: in reality, Planet X as proposed by Lowell did not exist, although in recent years there has been a renewed search for possible giant planets beyond Neptune that could explain the unusual orbits of certain trans-Neptunian objects (TNOs).

The controversy over Pluto’s status

With that, the controversy over Pluto’s status began to rage, especially when similar objects in the Kuiper belt beyond Neptune began to be detected as early as 1992. The final nail in the coffin came from Caltech astronomer Mike Brown, when in 2005 he discovered Eris, another TNO estimated to be larger than Pluto (it is actually slightly smaller). NASA announced the discovery of the “tenth planet”, but some astronomers had a different idea. On 24 August 2006, the International Astronomical Union (IAU) met in Prague to approve a formal definition of a planet, which until then had not existed. The outcome downgraded Pluto to a dwarf planet for failing to meet one condition—namely, to clear its orbit of other objects, since its path intersects with that of Neptune; in fact, from 1979 to 1999, Pluto was closer to the Sun than the eighth planet.

Rarely has the outcome of a scientific meeting provoked so much backlash, and so much vitriol, not only among much of the public, outraged by Pluto’s demotion, but also among the many scientists who rejected the IAU’s definition. One of the most disgruntled was Alan Stern, who seven months before the meeting had launched NASA’s New Horizons probe to Pluto; Stern saw his brand new mission to the ninth planet relegated to the exploration of a dwarf planet.  

Pluto’s diameter is 1/6 the diameter of the Earth. Image: Wikimedia

However, even if the debate about Pluto’s status has never died down, this has in no way dampened the New Horizons mission, whose results have fascinated the world. On 14 July 2015, after a space journey of more than nine years, the probe flew above the surface of Pluto, providing stunningly detailed images of what was until then just a blurry, pixelated disc of colours. The image of its large heart-shaped blob, christened Tombaugh Regio and made up of nitrogen, carbon monoxide and methane ices, was received by Pluto enthusiasts almost as a metaphor—an affectionate wink from the ex-planet to its terrestrial defenders.

After completing its work on Pluto, New Horizons set off for other destinations in the far reaches of the Solar System. But its data and images of Pluto and its five moons—Charon, Nix, Hydra, Kerberos and Styx—have revealed to scientists the secrets of a world whose mass is about one sixth that of our Moon, but geologically active, with mountains of frozen water, ice volcanoes and perhaps a liquid ocean beneath the surface. What’s more, the world once thought of as just a cold, dark, distant rock may actually have had a warmer start than previously suspected, according to recent research based on New Horizons data. Stern and his collaborators, responsible for this study, suggest possible implications for even its past habitability. Planet or not, it seems that Pluto will continue not only to reveal surprises to scientists, but also to occupy a place in our popular culture that no other dwarf planet will be able to dethrone.

Javier Yanes


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