by Alex Boese
That was an important detail! If our solar system had a second sun, you would think that someone might have noticed it by now. But not necessarily, they argued. Nemesis could be a red-dwarf star. These are the most common type of star in the galaxy, but they’re small and very dim, a mere fraction of the size of our more familiar sun. This would explain why it had never been seen. It had got lost in the background, blending in with all the other stars. Although, now that the possibility of its existence had been realized, the hunt for it could begin.
The arguments for Nemesis were perfectly legitimate. If this second sun existed, it would neatly explain the puzzling periodic repetition of mass extinctions. So, the scientific community duly took the hypothesis under consideration. There was, nevertheless, a bit of eye rolling among sceptics. This wasn’t just because of general wariness at the idea of a death star; it was because they had heard similar claims before. By the 1980s, there was already a firmly established but slightly fringe tradition in astronomy of suspecting that some kind of massive object, often referred to as Planet X, lay undiscovered at the edge of the solar system.
There’s a definite mystique about searching for hidden things. It stirs the imagination, and quite a few disciplines feature this kind of hunt. Biology has an active subculture of cryptozoologists, who are absolutely convinced that nature is full of creatures yet to be found, the most famous of these elusive critters being Bigfoot and the Loch Ness Monster. Archaeology similarly boasts a long history of explorers consumed by quests for lost cities, such as El Dorado. These hunts can easily acquire a fanatical, obsessive tinge, and the search for Planet X was no different.
The astronomer William Herschel planted the initial seed of Planet X back in 1781, when he discovered Uranus. Until then, it hadn’t occurred to anyone that there might be planets yet to discover in our solar system. Everyone had assumed that its roll call was complete. With the realization that it might not be, the hunt was on.
The search soon bore spectacular fruit when the French astronomer Urbain Le Verrier, led by irregularities in Uranus’s orbit, found the planet Neptune in 1846. But, instead of satisfying the appetite for lost planets, his discovery only amplified it. Planets, moons and asteroids were all soon being examined for irregularities in their orbits. If any were found, these were claimed to be evidence of the existence of yet another planet.
The wealthy American businessman-astronomer Percival Lowell coined the term Planet X in the early twentieth century. It was his name for a massive planet that he believed lay beyond Neptune. He spent the last decade of his life trying to find it and, though he died unsuccessful, the astronomer Clyde Tombaugh took up his quest and, in 1930, found Pluto. However, as scientists became aware that Pluto was a mere dwarf planet, not even as big as our moon, the Planet X enthusiasts grew restless. That wasn’t the Jupiter-sized behemoth they were looking for. So, the hunt continued.
With this history in mind, when Muller proposed the Nemesis hypothesis in 1984, sceptics couldn’t help but wonder if it wasn’t the latest and greatest incarnation of the hunt for Planet X. Except, now, instead of being a mere planet, the mystery missing object had swelled into a sun.
But just because Planet X had achieved some notoriety, that didn’t mean it wasn’t actually out there. Astronomers conceded this. The same was true for Nemesis. It could exist. The only way to know for sure was to look for it. This, however, was easier said than done. It was a bit like looking for a tiny needle in a haystack the size of Texas, and to do so in extremely dim lighting.
The best hope was that Nemesis might be detected by one of the all-sky surveys that are periodically conducted, in which astronomers use a high-powered telescope to systematically search for and catalogue every visible object in the sky. As luck would have it, in the mid-1980s NASA launched its Infrared Astronomical Satellite (or IRAS) survey, capable of detecting extremely faint objects, but it didn’t find Nemesis. Nor did the even more sensitive Two Micron All-Sky Survey (2MASS), conducted from 1997 to 2001. When NASA’s Wide-field Infrared Survey Explorer (WISE) space telescope launched in 2009, many astronomers viewed it as the final chance for Nemesis. When this too failed to find it, the general consensus was that this meant our sun doesn’t have a companion star.
This has been the central problem for the hypothesis. Absence of evidence isn’t necessarily evidence of absence, but the more time has passed without anyone finding Nemesis, the less willing astronomers have been to believe that it exists. And most of them were pretty sceptical about it from the start.
There’s been another problem for the hypothesis. In 2010, a pair of researchers re-examined the fossil evidence to make sure that the periodicity found by Raup and Sepkoski really existed. They concluded that it did. In fact, using a larger fossil database, they were able to extend the periodicity back over 500 million years, slightly revising it to a twenty-seven-million- rather than a twenty-six-million-year interval. This might sound like confirmation of Nemesis, but instead they argued that their findings actually suggested a companion star couldn’t be the cause of the extinctions. Their reasoning was that an object such as Nemesis, with a very large orbit, would inevitably be affected by the gravity of passing stars and the galactic tidal field. This would cause variations in its orbital period, preventing it from maintaining clockwork periodicity. Since these variations weren’t seen in the fossil record, Nemesis must not have been the culprit.
Despite these setbacks, and after all these years, Muller hasn’t given up hope that Nemesis will eventually be found. He discounts the argument about the regularity of the extinctions, believing that the fossil record is ambiguous enough to allow for the kind of variability that the orbit of Nemesis should display. And, anyway, something has to be causing the extinctions. If it isn’t a second sun, what is it? This remains an unanswered question.
Muller has pinned his hopes on the Large Synoptic Survey Telescope, which is currently under construction in Chile and should start full operations in 2022. It’s been designed to have a very wide field of view, which will allow huge swathes of the sky to be examined. He says that if that doesn’t find anything, then maybe he’ll start to question whether Nemesis exists. Or maybe not. Space is vast enough for a star to hide. No matter how carefully you look, the possibility remains that it might be lurking somewhere out there, undetected, after all.
Weird became true: continental drift
Today, geologists accept it as a matter of fact that the continents are constantly moving around the globe at a rate of a few centimetres a year. Although slow, this translates into journeys of thousands of miles over millions of years. But when the young German meteorologist Alfred Wegener first proposed this idea in 1912, he faced a stone wall of resistance from geologists who were firmly committed to the belief that the continents are permanently fixed in place. They didn’t just reject his idea, they contemptuously dismissed it as ‘Germanic pseudoscience’. It took almost half a century before they finally admitted he had been right all along.
The idea that Africa, the Americas and the rest of the major land masses might be slowly wandering around the globe first formed in Wegener’s mind around 1910, when he was a young professor of meteorology at the University of Marburg. He had been admiring a friend’s new atlas, leafing through its pages, when he was struck by how the continents seemed to fit together like pieces of a jigsaw puzzle. This was particularly true for the Atlantic coasts of South America and Africa.
Others had noticed this fit before, as early as 1596, when Abraham Ortelius, the creator of the first modern atlas, remarked that the Americas looked like they had been somehow torn away from Europe and Africa. But Wegener was the first to develop the observation into a full-blown theory of geological change. He decided that the jigsaw-puzzle fit meant that the continents must once have been joined together as an enormous supercontinent, and that over the course of millions of years the land masses had drifted apart until they reached their current location.
He detailed t
his hypothesis in his book Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans), published in 1915. The timing of this with the onset of World War I wasn’t an accident. As a reserve officer in the German army, Wegener had been sent to the Western Front, where he got shot twice, and he wrote the book while recuperating. English speakers first learned of it in the early 1920s, with a full translation appearing in 1924.
The apparent fit of the continents was Wegener’s first and main argument, but he carefully gathered other evidence. He pointed out that fossils of identical reptiles and ferns had been found on either side of the Atlantic. The same was true of rock formations, and even living species such as earthworms on the different continents were strikingly similar. This only seemed to be possible if the continents had once been connected.
His theory, however, faced one serious problem. He didn’t know how this continental drift had happened. He speculated that the centrifugal force of the spinning planet might cause the movement, or perhaps the tidal forces of the sun and moon. But, basically, he had no idea.
When confronted with Wegener’s theory, geologists promptly ripped into its lack of a mechanism. The idea of continents chugging around the globe like giant cruise liners, ploughing through the solid rock of the ocean floor, struck them as self-evidently absurd. They pointed out that, even if the continents could somehow do this, the stress of doing so would surely cause them to fracture and break apart, leaving a trail of wreckage in their wake. No such thing was seen on the Earth’s surface.
This was a legitimate point. The idea of moving continents was a hard pill to swallow. It was what made Wegener’s theory so weird. Even so, it would have been possible to question the lack of a mechanism while still acknowledging that Wegener had gathered evidence that deserved to be taken seriously. Instead, geologists didn’t merely object to continental drift, they tried to completely destroy it.
They called into question Wegener’s competence, accusing him of being ignorant of the basics of geology. They criticized his methodology, sneering that he was cherry-picking evidence. They even dismissed the jigsaw-puzzle fit of the continents as an illusion, though they had to contradict the evidence of their own senses to do this, since anyone can look at a map and plainly see that they do, in fact, seem to fit.
But perhaps the most telling criticism geologists levelled against Wegener was that his theory contradicted everything they believed to be true about the Earth. In 1928, the geologist Rollin Chamberlin noted that, for Wegener to be correct, almost the entire edifice of geological knowledge constructed in the past century would have to be wrong. The discipline would need to start all over again. To him and most of his colleagues, this seemed absolutely incomprehensible.
To maintain their old beliefs, though, they had to introduce some weird theories of their own. In particular, there was that evidence of the similarity of species on different continents. It implied that these species had somehow been able to travel across the vast oceans, but how could this be? Had the ferns and reptiles built ships?
The answer the geologists conjured up was that the continents had once been connected by ‘land bridges’, narrow strips of land that criss-crossed the oceans. They imagined species obediently marching across these bridges at the appropriate times throughout the history of the Earth. These bridges, in turn, had the almost magical ability to rise and fall on command, as if they could be raised from the ocean floor on hydraulic jacks and then lowered back down again when not needed. There was no plausible mechanism for these land bridges, any more than there was for moving continents. And yet conventional theory required them, so they gained the status of geological orthodoxy.
Not all geologists joined the anti-Wegener pile-on. There were some bright spots of resistance to the mainstream, including the South African scientist Alexander du Toit and the esteemed British geologist Arthur Holmes. But the majority firmly rejected continental drift. By the 1950s, the conventional wisdom was that the theory was not only wrong, but that it had been definitively disproven.
Left to their own devices, geologists probably would have continued on forever denying that the continents could move, but advances in other disciplines forced change on them. Sonar technology developed during World War II had led to a revolution in ocean-floor mapping. By the 1960s, this had made it possible to see in detail what lay beneath the water. With this new knowledge, the evidence for continental drift became overwhelming. These maps allowed researchers to see that the fit of the continents significantly improved if one connected them at the continental shelf, this being the actual, geological edge of the continents, as opposed to the coastline, which can change with rising or falling sea levels.
Even more dramatic evidence came from a topographical map of the North Atlantic Ocean floor, produced in 1957 by Bruce Heezen and Marie Tharp. It revealed a massive ridge system running down the centre, like a spine, exactly paralleling the two opposite coasts. All the way down the centre of this ridge system was a rift valley where molten rock was welling up from the mantle. You could almost see in real time the two sides of the ocean floor spreading away from each other, pushing the continents apart. In the 1960s and ’70s, Heezen and Tharp completed maps of the remaining ocean floors, showing similar ridge systems running throughout them as well.
As geologists became aware of this new information, it only took a few years for the majority of them to abandon their belief in unmoving continents. Researchers then developed the theory of plate tectonics, which represented an overall vindication of Wegener’s concept, though with an important revision. He had imagined only the continents moving, somehow bulldozing through the ocean floors, but plate tectonics envisioned the entire crust of the Earth, including the ocean floors, separated into massive plates that the continents rode on top of. These plates constantly jostled around, spreading apart, sliding past each other, or colliding (in which case, one slowly disappeared, or subducted, beneath the other). All this motion was driven by convection currents deep within the mantle. By 1970, this was firmly established as the new geological orthodoxy. Land bridges and fixed continents were quietly scrubbed from textbooks.
Unfortunately, Wegener didn’t live to witness his vindication. He had died long before, in 1930, while leading a research expedition to Greenland. His body remains there to this day, entombed in the ice where his colleagues buried him, now covered by hundreds of feet of snow and drifting slowly westwards with the North American Plate.
What if ten million comets hit the Earth every year?
A lot of stuff falls to Earth from space every year. It’s hard to know how much exactly, but scientists estimate that it’s as much as 80,000 tons of material. The bulk of this is space dust, but there are also numerous pea-sized meteorites that burn up in the atmosphere. However, in 1986, the University of Iowa physicist Louis Frank suggested that this estimate was too low by many orders of magnitude. To the grand total, he argued, needed to be added ten million ‘small comets’ that struck the atmosphere every year. Each of these contained, on average, one hundred tons of water. So that added up to one billion tons of water that was falling to Earth annually from space.
To most scientists, this was crazy talk. For a start, they had never heard of such a thing as a ‘small comet’. To them, comets were icy objects of fairly significant size, measuring anywhere from one to ten miles across. If one were to hit the Earth, we would know it. If we were lucky, it would detonate with a force equivalent to a nuclear bomb. If we were unlucky, the impact would wipe out our species. And, even if small comets did exist, it seemed wildly implausible that ten million of these things could be striking us annually and somehow no one had noticed until now.
The problem was, Frank hadn’t conjured these small comets out of thin air. He had pictures of them: tens of thousands of satellite images. If these images weren’t documenting small comets hitting the atmosphere, what were they showing?
For the first part of his career, until he was in his mid-forties,
Frank was a well-respected scientist who adhered to conventional views. He specialized in plasma physics. His accomplishments included making the first measurements of the plasma ring around Saturn and discovering the theta aurora, a polar aurora that looks like the Greek letter theta when viewed from space.
Even after he developed his small-comet hypothesis, he continued his work in plasma physics. As a result, colleagues who knew him from this research often didn’t realize he was the same Louis Frank notorious for the small comets. He joked that it was as if there were two of him, like Jekyll and Hyde: ‘One appears to be the most conservative of scientists, the other a maverick hell-bent on destroying the very foundations of science.’
The events that led to the small-comet hypothesis began in late 1981. One of his students, John Sigwarth, was analysing ultraviolet images of the Earth’s atmosphere taken by Dynamics Explorer, a pair of polar-orbiting NASA satellites. He was trying to find evidence of atmospheric ripples that might have been caused by gravity waves, but he kept noticing dark specks in the pictures. There were over 10,000 images, and almost all of them featured at least one or two of these annoying specks.
Sigwarth’s first assumption was that something was wrong with the camera’s electronics, and he worried that all the images were ruined. He notified Frank, and together they began trying to figure out the cause of the specks. They first tried systematically eliminating any possible technical problems, such as computer glitches, radio transmission noise or failing sensors. At one point, they even suspected it might be paint flecks on the camera. But, one by one, they ruled them all out.
Eventually, they looked at successive picture frames, which led them to discover that the specks could be followed from one frame to another, gradually fading in intensity. The specks also moved mostly in the same direction. This suggested that the specks were an actual phenomenon in the atmosphere, not instrumental error. But what could it be?