by Marcus Chown
Hunting the invisible planet
In 1841, John Couch Adams, an autistic mathematical genius from Cornwall in England, set out to deduce where in the sky the new planet must be in order to have the observed effect on Uranus.6 His calculation was horrendously complicated. But, four years later, in 1845, he was able to take his result to the Astronomer Royal, George Challis. Challis did not take him seriously. It did not help Adams’ credibility that he kept refining his calculations and presenting Challis with new and slightly different predictions of where to look for the new planet.
Meanwhile in France, and unknown to Adams, Le Verrier was carrying out similar computations. In order to simplify the intimidating calculations, he made a number of educated guesses. He assumed, for instance, that the unknown planet was a long way from the Sun or else it would have been spotted already by astronomers; he assumed it had a comparable mass to Uranus, which is about fifteen times as massive as the Earth; and he assumed that it orbited the Sun in the same plane as the other planets.7
Coincidentally, Le Verrier faced similar obstacles to Adams in being taken seriously. The director of the Paris Observatory, Françoise Arago, did not see a search for a new planet as at all urgent. Unable to pin down Arago on a date to start looking, Le Verrier lost patience, and, on 18 September 1846, sent his estimate of the rough location of the new planet to Berlin. Five days later, the only man to trust Le Verrier, Johann Galle, entered the history books as the discoverer of Neptune.
Like Uranus, Neptune had been seen before but not recognised as a planet. It was actually just about visible with the naked eye. In fact, there is some evidence that, as early as December 1612, Galileo, in Padua, had seen it through his new-fangled telescope, mistaking it for a star.
The discovery of Neptune triggered a priority dispute between France and England. Remarkably, it did not spill over into the relationship between Adams and Le Verrier, even though Le Verrier had a reputation for arrogant and bullying behaviour. Perhaps because they appreciated each other’s mathematical wizardry and because they had faced similar obstacles in getting mere mortals to believe them, they became firm friends as soon as they met. Nowadays, often as not, the discovery of Neptune is attributed jointly to Adams and Le Verrier.
The discovery of Uranus had been a sensation. It was the first planet discovered in the age of the telescope, the first in the age of science. It was twice as far from the Sun as Saturn, so overnight it doubled the size of the Solar System. The discovery of Neptune was a sensation of an entirely different order. Whereas Uranus had been stumbled on by accident, Neptune’s existence – its mass, its appearance, its very location – had been predicted. Science had given men the power of gods. Not only did Newton’s law explain what we could see, it also predicted what we could not see.
And in the twenty-first century history may be repeating itself.
Planet Nine
At the start of 2016, two planetary scientists in the US stunned the scientific world by claiming that a hitherto undetected planet about ten times the mass of the Earth is orbiting the Sun far beyond the outermost planet. For want of a better name, Konstantin Batygin and Mike Brown of the California Institute of Technology in Pasadena have christened the new world ‘Planet Nine’. Pluto was of course the ninth planet until its ignominious demotion to a dwarf planet in 2006.8
The evidence cited by Batygin and Brown is not the anomalous motion of a planet but the anomalous motion of Kuiper Belt Objects. As mentioned earlier, these bodies – icy builder’s rubble left over from the birth of the planets – orbit in their tens of thousands just beyond the outermost planet, Neptune.9 Batygin and Brown note that six of the most distant KBOs have highly elongated orbits which are roughly aligned. Rather than pointing in random directions, as might be expected, they all point more or less in the same direction. The orbits are also tilted in the same way, pointing about 30 degrees downwards from the plane of the eight known planets. The best explanation for the anomalous motion of these KBOs, according to Batygin and Brown, is that all are being herded by the gravity of the distant and unseen planet.10
Not only must such a planet be extraordinarily big, it must also be extraordinarily far away: on average about twenty times as far from the Sun as Neptune. Batygin and Brown estimate that Planet Nine follows a highly elongated orbit, swinging in to about seven times Neptune’s distance from the Sun before flying out to about thirty times the distance of the outermost planet. Travelling in such a huge orbit, it takes not 165 years to go around the Sun, as does Neptune, but about 15,000 years.
Planet Nine could have formed along with the other planets 4.55 billion years ago and then been catapulted out into the cold by a close encounter with an embryonic giant planet such as Jupiter or Saturn. Or there is a small possibility that it is a planet captured from another star. In the stellar nursery where the Sun was born there would have been hundreds of other suns in close proximity, and encounters between them could conceivably have switched planets. The possibility that there exists an alien planet in our Solar System is a reminder that science is often stranger than science fiction.
At its predicted distance from the Sun, Planet Nine is expected to reflect little sunlight, making it very difficult to find even with the biggest telescopes. If the planet happens to be at its closest point to the Sun in its orbit, it should be possible to spot it in existing celestial images, captured by previous surveys of the night sky. If it is in the most distant part of its orbit, spotting it will require the world’s largest telescopes such as the twin 10-metre telescopes at the Keck Observatory on Mauna Kea in Hawaii. But Planet Nine – which is estimated to be about 3.7 times the diameter of the Earth with a chilly surface temperature of -226 degrees Celsius – might be easier to detect with infrared telescopes sensitive to its meagre heat output.
If Planet Nine exists, it may make our Solar System more like the 2,000 or so planetary systems so far found around other stars. One of the most common types of extrasolar planet has a mass between that of the Earth and Neptune, which is seventeen times heavier than the Earth. If such a ‘Super Earth’ once existed but got kicked out into the cold, it would explain why our Solar System apparently lacks such a world.
Ironically, Brown played a key role in the demotion of what was the ninth planet, Pluto, to its dwarf planet status. It was his 2005 discovery of Eris, a remote icy world nearly the same size as Pluto, that showed that what had been considered the outermost planet since 1930 is merely the biggest of many bodies in the Kuiper Belt. By proposing a possible replacement for Pluto, Brown is making amends for his murder of a planet.
Of course, Planet Nine may turn out to be an illusion. And some astronomers remain sceptical. Nevertheless, the power of Newton’s law of gravity to predict what we cannot see – to make a map of the invisible world – has continued to reap dividends.
Exoplanets
Currently, we know of several thousand worlds in orbit around other stars. Hardly any of these ‘exoplanets’ have been seen directly. Instead, the existence of many of them has been inferred indirectly from the effect they exert on their parent suns. It is all down to the mutuality of Newton’s law of gravity. A sun pulls on a planet with exactly the same force that a planet pulls on its sun. Of course, the sun, being hugely more massive and more difficult to budge, moves relatively little in response. Nevertheless, it moves.
What this means is that, strictly speaking, a planet does not orbit a static sun – that’s another one of the approximations used by Newton in order to squeeze out predictions. Instead, both a planet and its sun orbit their common centre of mass. Since a sun is a lot more massive than a planet, this centre is close to the centre of the sun, and usually well inside its fiery body.11 While the planet orbits the centre of mass of the system in a big orbit, the sun orbits the centre of mass in a tiny orbit.
Another way to think of it is that a planet tugs on its sun in one direction, then half an orbit later, tugs it in the opposite direction. This causes the sun to
move a little, or ‘wobble’, and this is detectable from Earth with sensitive instruments. In exactly the same way that the frequency, or ‘pitch’, of a police siren gets higher as it approaches and lower as it recedes, the frequency of light emitted by atoms in a star becomes higher or lower depending on whether a star is approaching or receding from the Earth. By measuring the magnitude of this ‘Doppler shift’ for common atoms of elements such as hydrogen, astronomers can determine the velocity of a star towards us and away from us.
In the case of a star being tugged by the gravity of a planet, the maximum wobble velocity is only a few metres per second for a planet the mass of Jupiter and a mere 10 centimetres per second if it is the mass of the Earth. In other words, a ball of hot gas, typically more than a million kilometres across, approaches and recedes from us at the speed of a jogger and a tortoise, respectively. Despite the formidable technical challenge, astronomers are able to measure such velocities using a super-sensitive ‘spectrograph’ and infer the existence of an invisible planet.12 In this way, more than 2,000 have been discovered since the mid-1990s, and the hunt is on for a second Earth.13
The most dramatic example of the power of Newton’s law of gravity to predict what we cannot see is not in the realm of stars and planets but in the domain of the large-scale Universe. In the late twentieth century, astronomers discovered, to their astonishment, that the stars and galaxies, which they had believed were the sole cosmic building blocks, are in fact only a minor constituent of the cosmos. There is an awful lot more stuff out there than we ever imagined. And it is entirely invisible.
Invisible Universe
In the late 1960s and 1970s, astronomers Vera Rubin and Kent Ford at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington began studying spiral galaxies. These great whirlpools of stars account for about 15 per cent of all galaxies and of course include our own Milky Way. Rubin and Ford set out to measure how fast the stars in spiral galaxies are whirling around their massive central ‘bulges’.
The two astronomers picked spiral galaxies seen edge-on because the stars in such systems are moving along our line of sight. With a super-sensitive spectrograph, they were able to measure the velocity of the stars more accurately than anyone had ever done before.
At greater and greater distances from the centre of each galaxy, the stars should be experiencing ever weaker gravity. Consequently, Rubin and Ford expected to find the stars orbiting ever more slowly, exactly as the planets in our Solar System do at greater and greater distances from the Sun.
But this is not what they found.
As far out from the centre of each spiral galaxy as it was possible for the two astronomers to see stars, the orbital velocity of the stars remained constant. The stars were whirling round far too fast. Like children on a sped-up merry-go-round, they should be flung off their galactic carousel. They should be sailing out across intergalactic space. There was no way that the gravity of their parent galaxies could be holding on to them.
But it was.
Modern-day astronomers, in common with their nineteenth-century predecessors, have an unshakeable faith in Newton’s law of gravity, which has scored so many successes over so many centuries.14 So they have come up with an explanation for the anomalous motion of the stars in spiral galaxies that is not a million miles from Adams and Le Verrier’s explanation of the anomalous motion of Uranus. The reason the stars in spirals do not fly off into intergalactic space, maintain astronomers, is that they are in the grip of the gravity of more matter than is visible with telescopes. A lot more.
Remarkably, every spiral galaxy is embedded in a vast spherical cloud of ‘dark matter’, rather like a CD embedded in a swarm of bees. The dark matter gives out no light – or at least insufficient light to be detectable by the most sensitive instruments currently available – and outweighs the visible stars typically often by a factor of 10.
The discovery of Neptune merely showed that we had overlooked the existence of a planet in the Solar System. The discovery of dark matter is a bit more serious than that. It shows we have overlooked most of the matter in the Universe.
Actually, the first hints that there is more to the Universe than anyone suspected came in the 1930s. Fritz Zwicky, a Swiss-American astronomer at the California Institute of Technology in Pasadena, was observing galaxy clusters. To his surprise, he discovered that their constituent galaxies are being whirled around so fast they should be flung off to infinity. In Holland, at roughly the same time, Dutch astronomer Jan Oort discovered that stars in the vicinity of the Sun appear to be orbiting faster around the centre of our Milky Way than can be explained by the gravity of the visible matter inside the Sun’s orbit.
Zwicky concluded that there must be a lot more matter in galaxy clusters, and Oort that there must be a lot more matter in our own Galaxy, than shows up in telescopes. It is the extra gravity of this dark matter – a term actually coined by Zwicky (Dunkle Materie in the German) — which must hold onto their galaxies and stars, respectively.
The idea that there is ‘missing mass’ in the Universe somehow did not enter the mainstream of astronomy, maybe because it was so hard to believe. But everything changed with Rubin and Ford’s super-precise observations of stars orbiting in spiral galaxies.15 Here were anomalous measurements of the velocities of stars – lots of them – which could not be swept under the carpet.
Gravity can do more than reveal the presence of dark matter; it can be used to deduce its distribution as well. This is because the light from distant galaxies, on its journey to Earth, is bent, or ‘lensed’, by the gravity of the dark matter it passes. From the distortion, or ‘weak lensing’, of the images of the distant galaxies it is possible to deduce the distribution of that dark matter. Currently under construction on a mountaintop in Chile is a telescope that can exploit this effect. The Large Synoptic Survey Telescope is a kind of anti-telescope that turns the idea of a telescope on its head.16 By means of the light it collects, it images darkness.
The evidence for the existence of dark matter comes not only from spiral galaxies but also from another important place. The Universe was born 13.82 billion years ago in a titanic explosion – the big bang – and has been expanding and cooling ever since. Out of the cooling debris there congealed 100 billion or so galaxies, including our own Milky Way. One serious problem with this picture is that it fails to predict a rather important feature of the Universe: that we exist.
Galaxies formed because some regions of the big bang fireball were slightly denser than others. (The ‘density fluctuations’ are believed to have been imprinted on the Universe by ‘quantum’ processes in the first split-second of creation – but that is another story.)17 Because the slightly denser regions had slightly stronger gravity, they dragged in material faster than other regions, which boosted their gravity so they pulled in matter yet faster in a process akin to the rich getting ever richer. But the point is this: the process is too slow. The 13.82 billion years which have elapsed since the moment of the Universe’s birth is woefully too short a time to have built up galaxies as big as the Milky Way. Unless there exists a lot more matter than we can see with our telescopes – matter whose gravity speeded up galaxy formation. Dark matter.
The Universe’s dark matter outweighs the visible matter – the stuff made of atoms like you, me, the stars and galaxies — by a factor of between five and six. In fact, because of a European space telescope called ‘Planck’, which observed the ‘afterglow’ of the big bang fireball, we can be even more precise. Whereas 4.9 per cent of the mass-energy of the Universe is atoms, 26.8 per cent is dark matter. (The remaining 68.3 per cent, known as ‘dark energy’ and discovered only in 1998, is invisible, fills all of space and has repulsive gravity – but that too is another story.)18
As for the identity of the dark matter – what it actually is -your guess is as good as mine. One idea is it is made of hitherto undiscovered subatomic particles. Speculative theories of physics such as ‘supersymme
try’ postulate the existence of a host of new fundamental particles which do not ‘feel’ the electromagnetic force and so generate no electromagnetic waves, also known as light. Another idea is that the dark matter is made of fridgesized black holes each as massive as Jupiter which were created in the violent conditions that existed in the big bang fireball.19
If the dark matter is made of ‘primordial’ black holes, and assuming they are spread uniformly throughout the Universe, the nearest would be about 30 light years away, about ten times further away than the nearest star, Alpha Centauri. If the dark matter is made of subatomic particles, then they are flying through you at this very instant, as unaffected by the atoms of your body as bullets flying through fog. Only one thing is certain about dark matter: if you can discover its identity, there is a Nobel Prize waiting for you in Stockholm.
Using modern parlance we can now say that Neptune was the dark matter of its day. But, if we took a time machine back to the nineteenth century, we would discover that it was not the only dark matter planet. There was another, rather ghostly and slippery one. Its name was Vulcan.
Vulcan
You may be forgiven for thinking Vulcan is the ancestral home of the ultra-logical Mr Spock from Star Trek. But Gene Roddenberry, the creator of the 1960s American TV series, did not conjure the name from thin air. The planet already existed. Or at least it existed in the imagination of nineteenth-century astronomers, most notably Le Verrier.
After his triumphant prediction of the existence of Neptune, Le Verrier’s star rose in the scientific firmament, and, in 1854, he became director of the Paris Observatory. But nothing he did, nothing he achieved, came close to matching the sheer bloodpumping exhilaration he had felt at magically unveiling an unknown world at the edge of the Solar System. For his achievement, he had been courted by kings, revered as a god by scientists. Fame and adulation had intoxicated him and he craved that feeling again. If only he could repeat his success. If only he could make another god-like pronouncement that would stun the human world. And so his attention turned from the outer to the inner Solar System.
