The Planet That Wasn’t There

Last​ January, a pair of scientists at the California Institute of Technology, Konstantin Batygin and Michael Brown, announced that they had discovered compelling evidence of an as-yet-unseen giant planet – Planet X – orbiting the Sun, seven times further out than Neptune.1 This isn’t the first time that astronomers have believed there may be nine planets in the solar system. From its discovery in 1930 until 2006, Pluto – smaller than our moon, and nearly forty times as far from the Sun as the Earth is – was considered the ninth. And between 1859 and 1915 it was widely believed, for perfectly sound reasons, that a small planet known as Vulcan lurked invisibly close to the Sun, inside the orbit of Mercury. How that belief came about, and how Einstein came to demolish it, is the subject of Thomas Levenson’s eye-opening book.

Pluto lost its planetary status ten years ago after a group of astronomers, Brown among them, discovered another object of roughly the same size orbiting the Sun beyond Neptune.2 If Pluto was a planet, then so was Eris (as it’s now known): either a tenth planet had been discovered, or Pluto wasn’t a planet after all. The International Astronomical Union made its decision public in August 2006. Since then, a planet has been defined as a ‘celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit’. It’s the last of these conditions that Pluto and Eris don’t satisfy, so the IAU invented a new category for them: dwarf planets. A full-blown planet has enough mass to exert gravitational control over anything in its path; a dwarf planet doesn’t. (There are five known dwarf planets in the solar system: along with Pluto and Eris, there are two more beyond Neptune – Haumea and Makemake – as well as Ceres, the largest body in the asteroid belt between Mars and Jupiter. It has a diameter of nearly six hundred miles, roughly the length of Britain from Dunnet Head to Lizard Point. Discovered in 1801, and at first thought to be a comet, it had a brief life as a planet before sightings of other nearby objects led it to be classified as just one among many asteroids, until Pluto’s relegation gave it a place among the newly defined dwarf planets.)

Clyde Tombaugh, a 24-year-old researcher at the Lowell Observatory in Arizona, spotted Pluto on 18 February 1930. The observatory had been established in 1894 by Percival Lowell. As well as doing much to popularise the idea that there might be life on Mars (his many books include Mars and Its Canals and Mars as the Abode of Life), Lowell initiated a systematic search for a new planet beyond the orbit of Neptune, which he called – you’ve guessed it – Planet X. The body that Tombaugh discovered wasn’t exactly what Lowell had been expecting to find, but never mind. The name Pluto was suggested by Venetia Burney, an 11-year-old girl from Oxford whose grandfather passed the idea on to the Lowell Observatory.

In the years following Pluto’s discovery, several astronomers argued that there might be other objects out there beyond Neptune. In 1951, Gerard Kuiper, a Dutch-born astronomer then at the University of Chicago, whom Nasa describes as ‘the father of modern planetary science’, suggested that a disc of myriad smaller bodies had formed on the far side of Neptune during the solar system’s infancy, but had later been dispersed by Pluto. Kuiper died in 1973. The discovery during the 1970s of increasing numbers of ‘short-period’ comets implied that Kuiper’s speculative disc might still be there, as the Uruguayan astronomer Julio Fernández proposed in 1980. A comet is a small, icy solar system object that warms up and releases dust and gas (water, mostly) when it passes close to the Sun. It’s this cloud of exhaust that can make comets visible to the naked eye, and gives them their distinctive appearance. Short-period comets – those that take less than two hundred years to orbit the Sun – tend to run out of steam within a hundred thousand years or so. As the solar system is more than four billion years old, comets formed at the same time as the planets would have evaporated a very long time ago. The short-period comets we see now must have come from somewhere relatively nearby, relatively recently. That somewhere is now known as the Kuiper Belt.

The first Kuiper Belt objects to be discovered (apart from Pluto) were spotted in the early 1990s. In 2003, a team led by Michael Brown found Sedna, slightly smaller than Pluto and Eris, and then the most distant known object in the solar system. Sedna’s highly eccentric orbit was a puzzle. At its perihelion (the point on its orbit at which it’s closest to the Sun) it’s around 76 astronomical units away, too far to be influenced by Neptune’s gravity (one AU is the distance from the Earth to the Sun); at its aphelion (the furthest point of its orbit) it’s around 936 AU away. Something massive must have pulled Sedna out into that orbit, but what? Brown and Batygin, taking into account the eccentric orbits of five other remote objects discovered between 2004 and 2013, think that by far the most likely contender is their hypothetical Planet X. They’re looking for it using the Subaru telescope on Hawaii; it will take five years to search the area of sky where it may be.

If Planet X does turn out to be out there, it won’t be the first planet to have been discovered in this way. In 1846, Urbain-Jean-Joseph Le Verrier predicted the existence of another planet in the solar system based on perturbations in the orbit of Uranus. He said how big it would appear to be, and which part of the sky it was likely to be found in. But his colleagues at the Paris Observatory weren’t interested, so Le Verrier wrote to Johann Gottfried Galle in Berlin. Galle received Le Verrier’s letter on 23 September, and began looking for the planet that evening. Between midnight and 1 a.m., he and his assistant found it. ‘Galle’s sighting,’ Levenson writes, ‘was the climax of what was almost immediately understood to be the popular triumph of Newtonian science.’ Le Verrier had predicted the existence and position of the planet he called Neptune using Newton’s laws. The discovery provided definitive experimental proof of the theory of gravitation set out in the Principia. ‘With Neptune in hand,’ Levenson says, ‘one matter was settled. No working astronomer, no physicist, had any residual doubt about gravity. As Newton had described it, so it was: a universal force, at play throughout the cosmos, depending only on the masses contained within a system and on the inverse of the square of the distance between any two objects (in the simplest case).’

Le Verrier was made director of the Paris Observatory in 1854. He wasn’t the world’s easiest boss. ‘The personnel records,’ Levenson writes, ‘reveal a monstrous casualty list: during Le Verrier’s first 13 years in the job, 17 astronomers and 46 assistants abandoned the Observatory.’ He appears to have managed perfectly well without them. Among his achievements in the 1850s (helped by his ‘luckless and overstretched’ assistants) were refining the best estimate of the distance between the Earth and the Sun, and producing much more accurate tables for plotting the orbits of the four terrestrial planets (Mercury, Venus, Earth and Mars). But Mercury was intransigent: Le Verrier couldn’t get the numbers to fit the planet’s observable behaviour.

Specifically, he had no way to explain ‘the precession of the perihelion of Mercury’s orbit’. The point on its elliptical orbit at which Mercury is closest to the Sun moves forward at a rate of 565 arcseconds (roughly one sixth of a degree) every hundred years. Nearly half of that movement can be explained by the gravitational effect of Venus, pulling Mercury along. Jupiter accounts for just over a quarter of it, Earth more than a sixth, and the other planets cause what Levenson calls ‘scraps of motion’. But that still leaves 38 arcseconds per century that Le Verrier was unable to account for. Either Newton’s laws were wrong – or at least incomplete; but they couldn’t be, because Le Verrier had used them to predict so precisely, and so spectacularly, the existence of Neptune – or there was something else out there, ‘orbiting between the Sun and Mercury’.

Le Verrier thought a group of asteroids more likely than another planet, but a total eclipse on 16 July 1860 would give observers the chance to look for whatever might be there – hidden, most of the time, by the ‘brutal glare of the Sun’. As it happened, they didn’t have to wait even that long. In December 1859, Le Verrier received a letter from Edmond Lescarbault, a doctor and amateur astronomer in the small town of Orgères-en-Beauce, between Chartres and Orléans. Nine months earlier, Lescarbault had watched an unknown object pass across the face of the Sun. Le Verrier rushed to Orgères, where Lescarbault ‘permitted us to examine his instruments closely and gave us the most detailed explanations of his work’, which ‘gave us total conviction that the detailed observation he had completed must be admitted to science’. Within two months the planet had a name: Vulcan, after the Roman god of fire and volcanos, Venus’ husband, the Olympian blacksmith.

In the years that followed there were countless more alleged sightings of the planet, but they didn’t add up to a consistent, predictable pattern. It didn’t appear when and where it was expected to, and some of the dark spots that people saw moving across the Sun had to be just sunspots. And even if what Lescarbault had seen was the elusive planet, it wasn’t big enough to ‘account for all of the perihelion advance Le Verrier had discovered’.

Le Verrier died in 1877. He ‘left the solar system larger than he found it’, Levenson writes, ‘one both better and less completely understood’. There was a total solar eclipse the following year, on 29 July 1878. The path of the eclipse crossed the route of the recently completed transcontinental railroad in Wyoming; dozens of enthusiasts from the East Coast and Europe descended on the town of Rawlins, fifty miles north of the Colorado state line. James Craig Watson, the director of the Ann Arbor Observatory, and Simon Newcomb, from the Naval Observatory in Washington, were among those hoping to catch a first definitive glimpse of Vulcan, though others had other purposes: Thomas Edison wanted to test one of his inventions, an infrared measuring device he called a tasimeter. During the three minutes of total eclipse, Watson spotted ‘a ruddy star’ between Theta Cancri and the Sun that wasn’t on any of the charts. The New York Times reported on 16 August that ‘the planet Vulcan, after so long eluding the hunters … appears at last to have been fair run down and captured.’ But Newcomb and the others hadn’t seen it, and after Watson died in 1880 it was generally agreed that his sighting had been a mistake. ‘Vulcan, whether imagined as a single planet or a flock of asteroids, was no longer plausible as the source of Mercury’s anomaly.’

‘Discovery​ commences with the awareness of anomaly,’ Thomas Kuhn says in The Structure of Scientific Revolutions, ‘i.e. with the recognition that nature has somehow violated the paradigm-induced expectations that govern normal science. It then continues with a more or less extended exploration of the area of anomaly. And it closes only when the paradigm theory has been adjusted so that the anomalous has become the expected.’ Either something was out there between Mercury and the Sun, or Newton’s laws were wrong. And there was nothing out there between Mercury and the Sun. And yet, Kuhn says, ‘no one seriously questioned Newtonian theory because of the long-recognised discrepancies between predictions from that theory … and the motion of Mercury.’ After the 1878 eclipse there were a few ‘attempts to tweak Newton’, Levenson writes, but ‘by the turn of the 20th century, most researchers had given up. There was still no explanation for Mercury’s behaviour – but no one seemed to care.’

It was Einstein who finally explained it, not by discovering something that made Mercury’s behaviour fit with Newton’s laws, but by replacing the Newtonian paradigm with the theory of general relativity. The anomaly in Mercury’s orbit wasn’t the problem Einstein was trying to solve, however. He ‘seems not to have anticipated that general relativity would account with precision for the well-known anomaly in the motion of Mercury’s perihelion’, Kuhn says, but ‘he experienced a corresponding triumph when it did so.’ And when he spoke to the Prussian Academy on 18 November 1915, delivering the third of four lectures on his new theory, it was the example he gave to demonstrate that ‘this theory … agrees completely with the observations.’

A week later, he concluded his fourth lecture with the words: ‘Thus, the general theory of relativity as a logical edifice has finally been completed.’ It was an advance on Newton in two ways: not only did it describe the observable universe more accurately; it also provided an explanation of what gravity is. Newton, as Levenson says, ‘refused to propose any specific notion to explain why one hunk of matter pulls on another’. In the third edition of the Principia, he wrote: ‘I have not as yet been able to deduce from phenomena the reason for these properties of gravity and I do not feign hypotheses.’ According to Einstein, objects warp the geometry of space-time, affecting the way other objects move through it. The more massive the object, the greater the distortion. There’s a rough analogy, varieties of which are often used to illustrate the theory: imagine a bowling ball (the Sun) on a trampoline (spacetime) and a billiard ball (Mercury) rolling around it.

With Einstein’s revelations, ‘decades of attempts to save the Newtonian worldview were at an end,’ Levenson writes. ‘Vulcan was gone, dead, utterly unnecessary.’ But it has now served one more useful purpose, in the long twilight of its afterlife, in providing Levenson with a thread along which to trace the history of the science of gravity across 250 years. He begins his book with Newton (after a brief preface starring Einstein), then proceeds biographically from one great man of science to another: William Herschel, who discovered Uranus in 1781; Pierre-Simon Laplace, who showed that ‘the dynamics of the solar system … were governed by the law of gravitation as Newton had first stated it’; Le Verrier; Edison; Einstein.

The Hunt for Vulcan includes several thrillerish elements in its style: date stamps, short sentences, the present tense. ‘November 18, 1915, Berlin’, the preface begins: ‘A man is on the move, coming into the centre of town from the western suburbs.’ And there’s a twist to Levenson’s enthralling story: the Whiggish progression from one genius to the next is an illusion, which can seriously impede the advancement of science. For one thing, none of Levenson’s heroes could have achieved what he did without the help of collaborators (and, to a lesser extent, pressure from competitors). The Principia wouldn’t have been published without the help of Newton’s friend Edmond Halley; even Einstein, whose results Levenson in his spine-tingling preface describes as ‘the triumph of a lone thinker’ and ‘the greatest individual intellectual accomplishment of the 20th century’, couldn’t have done it without a little help from his friends: unable to do the maths himself, he turned to Marcel Grossman for assistance. Second, Einstein’s great breakthrough was to see, as none of his predecessors had, that Newton was wrong. The next step wasn’t to build on what had gone before, but to re-lay the foundations. ‘The resulting conceptual transformation’, as Kuhn puts it, was ‘decisively destructive’ not of a non-existent planet, but of the ‘previously established paradigm’.

There is a final irony in Levenson’s subtitle. Einstein ‘destroyed’ Vulcan in the sense of proving that it didn’t exist. But he came to fear that his insights had made possible the destruction, in an all too literal sense, of another, very real planet. In a 1946 article for Science Illustrated, ostensibly written to explain the law of the equivalence of energy and mass – as expressed by the formula E=mc² – to a popular audience, he said that the amount of energy released when an atom is split is ‘so enormously large that it brings with it a great threat of evil. Averting that threat has become the most urgent problem of our time.’ According to William Perry, the US secretary of defence from 1994 to 1997, the problem is as urgent now as ever.3 ‘Today, the danger of some sort of a nuclear catastrophe,’ Perry says in his memoir, ‘is greater than it was during the Cold War.’ And that was before Donald Trump was elected president.