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  The exploded-planet hypothesis also recalls a centuries-old dispute within Earth science between so-called catastrophists and uniformitarians. The latter argue that a good geological theory should be based on the identification of natural processes ongoing in the present, such as erosion and sedimentation. The theory will then extrapolate backwards, assuming that these processes have been occurring in a uniform, consistent manner over time. The catastrophists, on the other hand, maintain that sometimes weird stuff happens that doesn’t have an exact parallel in the present. On occasion, cataclysmic events shake up the whole system and the pieces fall back down in an entirely new order.

  Uniformitarianism traces back to the work of the late-eighteenth-century Scottish geologist James Hutton, regarded as the founder of modern geology. The cause of catastrophism, however, was first taken up by Biblical literalists, who insisted that Noah’s flood had produced the rock formations we find today. As a result, it gained a bad reputation within the scientific community. Uniformitarianism came to be regarded as the hallmark of proper Earth science.

  But, since the mid-twentieth century, catastrophism has been making a comeback. Researchers began to realize that rare events such as super-volcanoes and massive meteor impacts have had a profound effect on the history of life and the Earth. The recognition, in the 1980s, that an asteroid strike was the most likely cause of the extinction of the dinosaurs, marked a turning point in the rehabilitation of catastrophism. A strong tendency remains among scientists, however, to be sceptical of explanations that invoke catastrophes, and one can’t help but wonder if the exploded-planet hypothesis suffers on account of this old suspicion.

  As we grow increasingly aware of the many extravagant terrors that the universe is capable of unleashing upon us – rogue black holes that could materialize in the middle of the solar system and swallow us whole, or stray gamma bursts from distant galaxies that could abruptly incinerate us – perhaps the idea of exploding planets will come to seem less outrageous. If the phenomenon is possible, it would, after all, merely be one more item in nature’s awe-inspiring arsenal of destruction.

  Weird became true: the heliocentric theory

  In 1543, the Polish priest Nicolaus Copernicus published De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres), in which he offered what is arguably the greatest weird theory of all time. His outrageous claim was that the Earth revolved around the sun. This contradicted what almost every astronomer since the beginnings of astronomy had believed to be true, which was that the Earth lay at the centre of the universe and everything else – the sun, moon and planets – revolved around it.

  Nowadays, it may be difficult to think of Copernicus’s theory as being odd. After all, we know beyond a shadow of a doubt that the Earth does indeed revolve around the sun. From our perspective, his theory is the epitome of wisdom, which is why he’s popularly celebrated as a great thinker who managed to part the veils of superstition that had shrouded knowledge up until the sixteenth century. But, given what was known about the natural world in the 1540s, his theory was incredibly weird.

  Let’s consider why a geocentric, or Earth-centred, model made so much sense to ancient and medieval people. For a start, it aligned with the evidence of their own eyes. Anyone could look up and see that the sun, moon and planets travelled from one corner of the sky to the other. The heavenly objects were obviously what was moving, not the Earth.

  It also had the weight of tradition and authority behind it. Up until the sixteenth century, no scholar, with the minor exception of the ancient Greek astronomer Aristarchus, had ever seriously questioned the assumption that the Earth was at the centre of the cosmos, and Aristarchus had offered up the idea merely as a speculative hypothesis.

  Geocentrism also explained gravity in an intuitive way. Scholars since the time of Aristotle had taught that objects fell down because they had a built-in urge to move towards the centre of the cosmos, which was the Earth. It made sense that things would gravitate towards the middle, rather than towards a random point on the edge.

  Most importantly though, it worked. In the second century AD, the great Egyptian astronomer Claudius Ptolemy had devised a mathematical model of the solar system, based upon geocentrism. His system was quite complicated because he had been forced to introduce some creative geometry to explain why the planets occasionally appeared to move backwards in the sky. Nowadays, we know that this ‘retrograde’ motion occurs because we’re watching the planets orbit the sun as we ourselves simultaneously orbit it, but, to explain this motion from a geocentric perspective, Ptolemy had concluded that the planets circled the Earth while simultaneously circling around the path of their own orbit, in what is known as an epicycle. This meant that his system had all kinds of things spinning around each other, like gears turning around gears turning around other gears in a complex machine. But it did nevertheless accurately predict the movement of the planets, which seemed to indicate that it represented the way the cosmos really was.

  Then, almost 1,500 years later, along came Copernicus, who proposed tossing the geocentric cosmos overboard and replacing it with a heliocentric, or sun-centred, system. Why? Not because he had any new observational evidence about the movement of the planets. He didn’t. His entire argument rested upon the fact that he had devised a new mathematical system which, he claimed, could predict the movement of the planets as well as the Ptolemaic system.

  Copernicus had hoped his system would have no need for the complex epicycles and therefore would be simpler, but it didn’t turn out that way because he made one crucial mistake. He assumed the Earth revolved around the sun in a perfect circle, which it doesn’t. Its path actually takes the form of an ellipse – more like an egg shape. As a result of this error, he had to include epicycles anyway to make his predictions align with observed data. This made his system just as mathematically convoluted as the Ptolemaic one.

  In addition, it actually had some stark disadvantages when compared to the geocentric model. By moving the Earth away from the centre of the cosmos, he lost the explanation for gravity. In fact, if the sun was actually at the centre, it wasn’t clear why the Earth didn’t plunge downwards into it. Copernicus had no answer to this puzzle.

  Even more disturbingly for his contemporaries, the heliocentric system put the Earth in motion. If Copernicus was to be believed, we were all living on a giant spinning rock, hurtling through space. It didn’t feel like the Earth was moving. If it was, his critics asked, why wasn’t there a constant headwind blowing from the direction of its travel? (At the time, no one knew that space was a vacuum.) Why wasn’t everything on its surface flung off into space? Again, Copernicus had no answer to these troubling questions.

  With all these problems, most scholars at the time naturally rejected his heliocentric model. Any reasonable person should have rejected it, based upon the slim evidence Copernicus provided. It was only in the seventeenth century that other scholars fixed the shortcomings of his theory. This began when the Dutch spectacle-maker Hans Lippershey invented the first telescope in 1608, which promptly inspired the Italian scholar Galileo Galilei to build his own. With it, he saw moons around Jupiter, which proved that not everything was orbiting the Earth. Soon after, the German mathematician Johannes Kepler showed that the dreaded epicycles could be eliminated from the Copernican system if the planets moved in elliptical orbits rather than perfect circles. And finally, in the 1680s, the Englishman Isaac Newton tied everything together by developing the theory of gravity. This explained why apples fall to the ground and also why the planets move in ellipses – they’re constantly falling towards the sun and missing it. The accumulation of all this new evidence convinced scholars of the validity of the heliocentric theory.

  But, given the weirdness of what Copernicus had suggested (in the context of sixteenth-century knowledge) and the weak arguments he presented for it, one has to wonder what exactly inspired him to come up with his theory in the first place. What was he thinking? Th
is question has long puzzled historians.

  One idea is that he may have been trying to enforce a belief that heavenly objects move in perfect circles. Both ancient and medieval scholars took it for granted that celestial objects naturally moved in circles rather than straight lines because, they felt, the circle was a perfect shape and therefore was appropriate for the heavenly sphere. The Ptolemaic model took a few minor liberties with this principle because the sheer complexity of the system made having to tweak the rules in various ways inevitable. Copernicus may have decided that this made it irredeemably corrupt. It’s certainly true he rigidly enforced the principle of circular heavenly motion in his own system, which, as we’ve seen, is why he ended up having to fall back on epicycles.

  A more controversial idea, put forward recently by the University of California historian Robert Westman, is that Copernicus was trying to defend and improve astrology, the study of how the planets supposedly influence the health and fortunes of people on Earth. The validity of astrology was something that, again, both ancient and medieval scholars took for granted. But, in the late fifteenth century, the Italian philosopher Pico della Mirandola had attacked it as an inexact science. One issue he singled out was the difficulty of knowing the exact positions of Mercury and Venus with respect to the sun in the Ptolemaic system. If astronomers didn’t know this, he asked, how could astrologers possibly make accurate forecasts?

  Copernicus solved this problem in his heliocentric system. If everything was orbiting the sun, the positions of the planets became unambiguous. The faster a planet orbited the sun, the closer it was to it. Perhaps Copernicus had hoped that this new clarity would lead to more accurate astrological predictions. Westman points out that Copernicus did both live and study with an astrologer while attending university.

  Whatever may have been Copernicus’s underlying motive for developing the heliocentric theory, it’s clear that he wasn’t driven by what we could consider today to be good reasons. His beliefs were deeply rooted in medieval assumptions about the nature of the world. In essence, he managed to come up with the correct model of the solar system for entirely incorrect reasons. He lucked out.

  Nevertheless, he left behind a powerful legacy of iconoclasm. He showed that it was possible to come up with a weird theory that directly challenged the most basic assumptions about the world and to end up being right. Weird theorists have been trying to emulate his example ever since.

  What if our solar system has two suns?

  Something in outer space is killing Earthlings. Every twenty-six million years, it slaughters a whole bunch of us. It causes many species to go entirely extinct.

  The existence of this extraterrestrial assassin was first detected in the early 1980s by the University of Chicago palaeontologists John Sepkoski and David Raup, who had compiled a huge database of marine fossils found in sedimentary rocks. It was the most comprehensive database of its kind and it allowed them to start examining various large-scale patterns of evolution, such as when families of marine life had gone extinct and how often this had happened.

  As they graphed their data, what they found shocked them. There was a distinct periodicity in the rate of mass extinctions. The spikes in their graph were unmistakable. Approximately every twenty-six million years, for the past 250 million years, a whole group of species had abruptly disappeared. They checked and double-checked their data, but the periodicity seemed to be a real phenomenon.

  What, they wondered, could have caused such regularly repeating mass extinctions? They couldn’t think of any natural phenomenon on Earth that would recur on a twenty-six-million-year cycle. So, when they published their findings in 1983, they suggested that the mass die-offs must have been triggered by something non-terrestrial. There was a cosmic serial killer on the loose.

  An astronomical murder mystery immediately caught the attention of scientists. As sleuths took up the case, they quickly decided that, if something coming from space was regularly killing Earth creatures, it was almost certainly one of two things: asteroids or comets. Asteroids are essentially big rocks, whereas comets are lumps of ice, dust and rock. A big enough one of either, if it impacts the Earth, can cause serious death and destruction.

  But these would merely be the murder weapon. The more puzzling question was what force might be wielding that weapon. There had to be some astronomical phenomenon that was periodically flinging those objects in our direction. But what was there out in space that exhibited such a regularly repeating pattern over such a vast time scale?

  One idea, suggested by Raup and Sepkoski, was that the spiral arms of the Milky Way Galaxy might be the culprit. Our solar system orbits the centre of the Milky Way once every 230 million years, but we’re moving slightly faster than the arms of the galaxy rotate. As a result, as we travel along, we move in and out of the arms. It was possible that, whenever we moved into an arm, the slightly higher density of matter there was gravitationally disturbing the orbits of asteroids and comets, causing a bunch of them to fall into the inner solar system, where some of them hit the Earth.

  It was an interesting idea, but analysis revealed that the periodicity was all wrong. We only cross into an arm about once every hundred million years. This gave the spiral arms a pretty good alibi; they couldn’t be the killer.

  NASA scientists Michael Rampino and Richard Stothers offered another idea. They suggested that the perp could be the flat plane of the galaxy. The Milky Way is a gigantic flat disc of matter that spins around. Our solar system moves around with it, but as it does so it simultaneously bobs up and down with a wave-like motion, rising slightly above the surface of the disc, then sinking below it, over and over again. The two researchers argued that, each time our solar system passed through the plane of the galaxy, this might cause a gravitational disturbance that disrupted the orbit of comets, sending them on a collision course with our planet.

  The length of the periodicity was approximately right. We pass through the plane of the galaxy every thirty-three million years. But there turned out to be other problems. The matter in the plane of the galaxy is very diffuse. Astronomers found it hard to believe that passing through it would produce much of a disturbance. Also, we’re currently in the middle of the galactic plane. If the hypothesis of Rampino and Stothers was correct, we should be experiencing a mass extinction just about now, but, according to the timetable drawn up by Raup and Sepkoski, the next one isn’t due for another thirteen million years – luckily for us! The two periodicities, therefore, didn’t sync up. Once again, the suspect had an alibi.

  With spiral arms and the plane of the galaxy ruled out, University of California physicist Richard Muller stepped forward with an altogether more radical hypothesis. He proposed that our solar system has two suns. There was the familiar one, which we all know and love, but there was also a companion star, an evil twin that was periodically flinging comets at us.

  Even people who think they know nothing about astronomy are sure of at least one basic fact: our solar system has one sun. Look up in the sky and there it is, shining away. There are not two of them, as there are on Luke Skywalker’s home world of Tatooine. So, to claim that our solar system actually has two suns might seem perverse. And yet, Muller had a reasonable argument to back up his claim, and astronomers were willing to hear him out.

  The hypothesis was primarily Muller’s brainchild, but he had help working out the details from Marc Davis of Lawrence Berkeley Laboratory and Piet Hut of Princeton University. All three appeared as co-authors on the article detailing the hypothesis that appeared in Nature in 1984. They explained that our sun might have a distant companion that circled it in an extremely elongated, elliptical orbit, taking a full twenty-six million years to complete one orbit. At its furthest distance, this companion would be a vast fourteen trillion miles from our sun, but over time it swept in closer, until it was a mere three trillion miles away.

  At this distance, it would pass through the Oort cloud, a massive cloud of trillions of comets that su
rrounds our solar system at its furthest edge, and each time it did so it would dislodge billions of comets from their orbits, sending them falling into the solar system. A few of those billions would inevitably end up hitting the Earth. After having wreaked this havoc, the death star would then retreat back into the depths of space, on its long arcing journey, to return again in another twenty-six million years. This cycle, the three authors suggested, had been repeating for hundreds of millions of years.

  As an explanation of the recurring mass extinctions, this hypothesis worked. There were no problems of non-syncing periodicities that could possibly give the death star an alibi. More significantly, what other explanation could there be? Astronomers were running out of things in the universe that could plausibly be hurling comets at us every twenty-six million years.

  The authors also noted that the majority of known stars, over two thirds of them, are thought to have companions. So, statistically, it was more probable than not that our sun was part of a binary system. It was true that our sun’s companion would need to have a highly eccentric orbit in order to allow it to pass through the Oort cloud only once every twenty-six million years, but that didn’t make it impossible – just slightly odd.

  The three authors suggested naming our sun’s hypothetical companion Nemesis, after the Greek goddess of vengeance. And, if that name didn’t work out, they wrote, George might work as an alternative. This was apparently an attempt at scientific humour. However, George didn’t make it through the editorial process. The editor at Nature made an executive decision and chose Nemesis.

  There was just one problem with their hypothesis, which they noted with a touch of understatement: ‘The major difficulty with our model is the apparent absence of an obvious companion to the Sun.’