When, in 1816, John Keats began reading an Elizabethan translation of Homer, he became so enthralled that he stayed up all night. By morning, he had written the famous sonnet that includes the couplet:
Then felt I like some watcher of the skies
When a new planet swims into his ken.
He was referring to the astronomer William Herschel, who had spotted the planet Uranus 35 years earlier. Keats imagined a moment of instantaneous, rapturous recognition – but in reality, prolonged controversy clouded the planet’s identification.
While Uranus is now firmly accepted in the astronomical lexicon, deciding what counts as a planet is not straightforward. Herschel regarded astronomy as celestial botany, explaining that: ‘The heavens … resemble a luxuriant garden, which contains the great variety of productions in different flourishing beds.’ Like a collector of rare plants, his job was to classify the heavenly bodies and place them into groups – but there is no single correct way of doing that.
Centuries of tradition
Although ancient astronomers lacked telescopes, they detected five bright objects regularly gliding through the myriad of stars scattered across the skies. Now known as planets from the Greek word for ‘wanderer’, they were given the names of Roman gods – Mercury, Venus, Mars, Jupiter, and Saturn. According to the Aristotelian geocentric system that prevailed in Europe until the early 17th century, seven heavenly bodies – the sun, the moon, and the five planets – revolve around Earth in circles, God’s perfect shape. That quasi-magical number meshed neatly with the days of the week, the alchemical stages of purification, and the steps up to King Solomon’s temple.
In the absence of light pollution the skies seemed black and close, so any celestial newcomer was more readily visible than now. While the planets circled serenely and eternally through the pure aether of the celestial realm, comets and other transient visitors occasionally disturbed the heavens. Collectively known as meteors, they belonged to the changeable sublunar region between the moon and Earth. In particular, the comets that flared across the skies were often interpreted as divine messengers reporting God’s displeasure with a sinful world.
After astronomical diagrams had been inverted to place the sun rather than Earth at the centre of the universe, the number of orbiting bodies dropped to six – five traditional planets plus Earth. In Joseph Wright of Derby’s luminous painting of an astronomical demonstration, six small balls on sticks circle around the central lamp, which represents the sun and illuminates the faces of the attentive spectators. By then, Isaac Newton had proved that comets orbit elliptically around the sun like planets, yet they were still imbued with symbolic portent: Mary Shelley boasted that her birth had been heralded by the first comet to be identified by a woman – Herschel’s sister and collaborator, Caroline.
Restoring the number seven
Seeing is not the same as understanding: well before Herschel, other observers had already spotted Uranus, but failed to appreciate the evidence. The earliest candidate is the Greek astronomer Hipparchus, who may have seen the planet around 2,000 years earlier. After powerful telescopes were invented, sightings became more frequent – but Herschel was the first to identify the mysterious star as a planet.
Even so, Herschel was slow to claim his prize. At that stage, he was a relatively unknown immigrant from Hanover, a musician who obliged his sister Caroline to sacrifice her singing career and accommodate his growing obsession with astronomy. Unlike their contemporaries, the Herschels built reflecting telescopes with large mirrors instead of lenses: one of her more unpleasant tasks was sieving horse manure to make a smooth bed for casting the metal. On 13 March 1781, after gazing at the stars through a home-made telescope installed in their back garden, Herschel noted tentatively that he might have found a comet. Two weeks later, he felt more confident: ‘Saw the Diameter of the Comet extremely well defined … a glorious sight.’ He tracked it for weeks, but it was only months later that he acknowledged his error: in the absence of any radiant tail, this must be a planet.
Herschel refused to give his new planet a mythological name, behaving instead like an imperial explorer. When James Cook landed at Tahiti, he appropriated it as British territory by naming it King George’s Island – and correspondingly, since Herschel relied heavily on royal patronage, he dubbed his new planet King George’s star, Georgium Sidus. As he explained to his friend Sir Joseph Banks, president of the Royal Society and a royal confidant, they lived in a ‘more philosophical era’ than the ancients, and should choose a label that placed the discovery in its right chronological setting. Sycophancy worked: Herschel was appointed court astronomer and even secured a pension for Caroline, thus making her the first salaried scientific woman.
Unsurprisingly, astronomers from other countries disagreed, and bitter disputes dragged on for almost 70 years. Several Roman deities were proposed, including Neptune and Minerva, but the strongest voice belonged to the German Johann Bode. He pointed out that since Saturn was the father of Jupiter, it would be appropriate to choose Uranus, the father of Saturn, for the next planet. Uranus is Greek rather than Roman (which would be Caelus) but the name stuck – except in British almanacs, which clung on to Georgium Sidus until 1850.
Seven planets and counting
According to ideological accounts, scientists are engaged in a disinterested search for truth. Various undignified tussles over planets tell a different story: in repeated episodes, rival astronomers competed for personal and national glory. Another international row soon erupted – this time between France and Britain – after it became clear that Uranus was not behaving as predicted. Either the equations were wrong, or it was being pulled out of its expected orbit by an unknown large body, later identified as Neptune.
Galileo’s original drawings reveal that he had spotted Neptune, but failed to pick up the fact that it was slowly moving. More surprisingly, even when astronomers knew what they were looking for, detecting the planet proved tricky. The British contender, John Couch Adams, began the race, ploughing through all the observations he could get hold of to predict the suspected planet’s location. Armed with Adams’ estimates, the director of the Cambridge Observatory, James Challis, tried but failed to pinpoint its location.
In the meantime, the French astronomer Urbain Le Verrier had carried out his own calculations, and although he was unable to rouse much local interest, he persuaded an observer in Berlin to take a look. This search was more successful: the planet was located, and Le Verrier duly claimed the accolade of discoverer. But then Challis reappeared, announcing that after analysing his data more thoroughly, he now realised that he had detected Neptune a year before Le Verrier’s ally in Berlin. So Adams and his supporters promptly announced that this was a British victory.
Eventually, an international consensus was reached: Le Verrier and Adams deserved equal credit. But by the early 20th century, it had become clear that Neptune was also deviating from its anticipated orbit – and a similar cycle of claims and counter-claims emerged as astronomers searched for a ninth planet still further away from the sun.
This time the action took place at the Lowell Observatory in Arizona. Its founder and director, Percy Lowell, launched a search for ‘Planet X’, but he died in 1916 believing his quest had failed. In a déjà vu scenario, it later emerged that there had apparently been earlier sightings, which had gone unnoticed. Lowell’s widow entered the fray, launching a ten-year legal battle with the Observatory about Lowell’s reputation. Eventually, the new director handed to a junior the unenviable task of comparing pairs of photographs to detect any possible movement.
The project took nearly a year – but success finally arrived in 1930 when a planet smaller than Earth’s moon was detected. Some 150 suggestions for the new planet’s name flooded in, but the prize was scooped by an 11-year-old English schoolgirl, who nominated Pluto, god of the underworld: conveniently, his name begins with Percy Lowell’s initials.
War of the other worlds
As soon as Pluto was identified, astronomers began searching for more planets. As instruments improved, more and more celestial objects were detected beyond Neptune, some even tinier than Pluto. Rather than calling each one a planet, astronomers grouped them into the Kuiper belt, a massive rotating disc of remnants from the formation of the solar system. In 2006, Pluto was relegated to the new category of ‘dwarf planet’. Unsurprisingly, some astronomers disagree: the planetary wars are not yet over.
Patricia Fara is an Emeritus Fellow of Clare College, Cambridge. Her most recent book is Life after Gravity: The London Career of Isaac Newton (Oxford University Press).