by Alex Boese
Short answer: no. Geologists feel quite sure that planets, left to their own devices, don’t blow up. Advocates of the exploded-planet hypothesis, however, beg to differ. They argue that planets do have a disconcerting tendency to abruptly and catastrophically detonate, and that there’s evidence this has already occurred multiple times within our own solar system.
The genesis of the exploded-planet hypothesis traces back to the mid-eighteenth century, when an astronomical puzzle emerged: the mystery of the missing planet. A German professor of mathematics, Johann Titius, noticed something unusual about the positions of the planets in our solar system. Their distances from the sun seemed to follow an orderly pattern. Each planet lay approximately twice the distance from the sun as its inner neighbour. Titius had no idea why the planets would be spaced out like that; he just noted that they were – except for one conspicuous exception. There was an extremely large, empty gap between Mars and Jupiter, and, according to the pattern, there should have been a planet in that space. He recorded this observation as a footnote in a book he was translating from French into German.
This wasn’t exactly an attention-grabbing way to announce a new discovery. The footnote could easily have gone unnoticed, but the German astronomer Johann Bode happened to see it and decided Titius might have stumbled upon a law that regulated planetary distances. So, Bode included the idea in an astronomy textbook he was writing.
That might have been the end of the story. Titius’s observation didn’t otherwise attract much attention, until 1781, when something happened that made it the focus of intense scrutiny. In that year, the British astronomer William Herschel discovered the planet Uranus, out beyond Saturn. This was the first time a new planet had ever been found – not counting the discovery by prehistoric sky-watchers of Mercury, Venus, Mars, Jupiter and Saturn – and, to everyone’s surprise, Uranus lay almost exactly where Titius’s pattern predicted it should, as a seventh planet. Astronomers at the time didn’t think this could be mere chance – they thought Titius and Bode had to be on to something and that the pattern really was a law. It came to be known as Bode’s Law, which was a little unfair, since Titius was the one who had actually figured it out. Bode had merely publicized it better!
The acceptance of Titius’s pattern as a fully fledged law immediately focused attention on that gap between Mars and Jupiter. If the pattern had been right about the position of Uranus, where was the planet in the Mars–Jupiter gap? An intensive search promptly began among astronomers throughout Europe, with the result that, in the early nineteenth century, something was found at exactly the right spot. But it wasn’t a planet. It was a bunch of asteroids. They were, in fact, the first asteroids ever found, and this region of the solar system became known as the asteroid belt.
If you go looking for a house, the position of which is indicated on a map, but when you get there all you find is a pile of rubble, it would be reasonable to conclude that the house must have been destroyed. Likewise, if you go looking for a planet, but all you find in its place is a cluster of large rocks, you might suspect that the planet had come to a bad end. That’s exactly what the astronomer Heinrich Olbers concluded. In 1812, he proposed that the asteroid belt between Mars and Jupiter must be the broken-up remains of a former planet that had either exploded or had been destroyed in a collision. In this way, the unsettling idea that planets may not be as stable as they seem, that sometimes they simply blow up of their own accord, entered the astronomical imagination.
Olbers’ disturbing vision didn’t remain scientific dogma for long. Astronomers soon abandoned it in favour of the more reassuring theory that the asteroids were mini proto-planets that had been prevented from combining together into larger bodies due to the interference of Jupiter’s massive gravity tugging on them.
Nor did Bode’s Law retain the status of a law. By tradition, it still bears that name, but the planet Neptune, discovered in 1846, beyond Uranus, was way out of sync with the pattern. So, astronomers concluded that there was no law of planetary distances, and that the pattern observed by Titius was mere coincidence.
But it was only a matter of time before the catastrophist view of nature reasserted itself. In 1972, the British astronomer Michael Ovenden published an article in Nature in which he revived Olbers’ spectre of a blown-up planet between Mars and Jupiter. Adding more drama to the story, he envisioned this vanished world to have been a massive gas giant, ninety times the size of Earth. He named it Krypton, after Superman’s exploded home world.
Like Titius, Ovenden suspected there had to be an underlying order to the positions of the planets. He theorized that, over time, the planets will always settle into positions at which they have the minimum gravitational interaction with each other. He called this his principle of planetary claustrophobia. But what puzzled him was why the planets weren’t all in these locations. Specifically, Mars was relatively close to Earth, but far away from Jupiter. According to his calculations, it should have been closer to Jupiter, and this is what led him back to the idea of a missing planet.
He proposed that the current positions of the planets would make sense if a massive planet had once existed between Mars and Jupiter, but that it had abruptly ‘dissipated’ about sixteen million years ago. Following this disappearing act, Mars would have started slowly inching closer to Jupiter, though it would still take millions of years before it reached the position of minimum interaction.
What, however, could have made a planet dissipate so quickly? Ovenden saw only one possibility: it must have exploded. The bulk of the planet, he believed, must have been swallowed up by Jupiter, leaving what remained to become the asteroid belt.
Ovenden’s hypothesis inspired a show titled ‘Whatever Happened to Krypton?’ that toured North American planetariums in the late 1970s, but it didn’t particularly impress astronomers. Most of them ignored it, with the significant exception of one young researcher who was profoundly taken with the idea. This was Tom Van Flandern, who at the time was working at the US Naval Observatory. Up until then, Van Flandern had been an astronomer in good standing, who didn’t stray far from conventional wisdom, but the idea of spontaneously detonating planets deeply appealed to him. Under the influence of Ovenden’s hypothesis, he took a sudden, sharp turn towards scientific unorthodoxy. Over the following decades, to the bafflement of his colleagues, he transformed himself into a kind of weird astronomy celebrity, frequently expounding his against-the-mainstream theories (such as his claim that geological features on Mars show the handiwork of an extraterrestrial intelligence) on late-night radio shows.
As he pondered Ovenden’s catastrophist vision, Van Flandern concluded that there was more evidence, beyond the existence of the asteroid belt, of a long-ago planetary explosion. In fact, he came to believe that the solar system was a landscape scarred by the incendiary violence of its past, with a variety of solar-system oddities suddenly acquiring new significance in his eyes.
There was, for example, the curious pattern of cratering on Mars. The planet’s southern hemisphere is heavily cratered, whereas its northern hemisphere is relatively smooth. The conventional explanation for this ‘Martian dichotomy’ (as it’s called) is that it’s the result of a massive asteroid impact early in the planet’s history that created a magma ocean, resurfacing and smoothing out the northern half. Van Flandern instead argued that Mars must originally have been a moon of Ovenden’s exploded planet. So, when the planet blew up, the hemisphere of Mars facing towards it was hit, full force, by the blast, cratering it, while the opposite side remained unscathed.
Then there was the unusual colouration of Saturn’s moon, Iapetus. One half of it is dark, while the other half is bright white. Mainstream theory attributes this two-tone effect to differing temperatures on its surface, which produce an eccentric pattern of water-ice evaporation. The ice accumulates on the side that’s coldest, rather than where it’s relatively warm. (Although, the entire moon, by Earth standards, is freezing cold.) Van Flandern, however, argue
d that the dark half came to be that way when it was blackened by the blast wave generated by the exploding planet. Since Iapetus rotates extremely slowly, only one side of the moon ever faced the blast wave, which would explain, he said, why none of the other Saturnian moons show similar colouration – they all spun fast enough to be uniformly blackened.
As the years passed, Van Flandern’s vision of the violent past of our solar system grew ever more elaborate. He concluded that exploding planets hadn’t just been a one-time event in its history, but were a recurring feature. Initially, he upped the explosion count to two planets, which he designated by the letters V and K (K for Krypton, in a nod to Ovenden). His reasoning was that there were two distinct types of asteroids in the asteroid belt, so there must have been two planets. But, by the end of his life, in 2009, he had worked his way up to a full six exploded planets: two in the asteroid belt between Mars and Jupiter, another two to account for the Oort cloud of comets beyond Neptune, and a final two, just for good measure. Planets were apparently going off like fireworks in our solar system.
Of course, it doesn’t matter much how scarred and rubble-strewn our solar system might be. If planets can’t explode, they can’t have been the cause of the destruction. And, really, why would they explode? They’re giant lumps of rock and gas. There’s no obvious reason for them ever to detonate spontaneously.
Van Flandern realized this, so he and his fans set to work to identify the hidden explosive force that could transform planets into ticking time-bombs. Among the ideas they came up with were gravitational anomalies, antimatter beams from the centre of the galaxy, and even interplanetary war. It wasn’t until the 1990s that a more plausible possibility swam into view – conceivable because it didn’t involve any impossible physics, not because it in any way reflected orthodox thinking in science. It was the georeactor hypothesis.
The idea was that the cores of some planets might consist of massive balls of the highly radioactive element uranium, acting as natural nuclear fission reactors, or georeactors. Some isotopes of uranium are fissile, which means that when they absorb a neutron, they split in two, releasing energy and more free neutrons. If you collect enough uranium together, the fission of one atom will trigger the same in its neighbour, which will do likewise for its neighbour and so on. The process becomes self-sustaining, producing enormous amounts of heat and energy. This is how man-made nuclear reactors work.
So, imagine a lump of uranium five or ten miles wide, cooking away inside a planet. Under normal circumstances, uranium reactors just release energy; they don’t explode. Something has to compress the uranium into a tight ball, crowding the atoms together and causing them to reach a supercritical mass to produce a detonation. Nuclear bombs achieve this through the use of conventional explosives. A planetary georeactor, therefore, would remain stable as long as it was left alone. But there are things that could potentially set it off. The shockwave of a very large asteroid hitting the planet might do the trick, and, if something like this did happen, the resulting nuclear blast would absolutely have enough force to rip apart a planet, sending chunks of it flying clear out of the solar system.
The idea that natural nuclear reactors might exist at the cores of planets wasn’t Van Flandern’s idea. He simply recognized, as soon as he heard about it, that it could provide a mechanism for planetary destruction. The hypothesis was the invention of J. Marvin Herndon, a mining consultant with a doctorate in nuclear chemistry, who hit upon it in an attempt to answer an entirely different question: why Jupiter, Saturn and Neptune all radiate far more heat than they receive from the sun.
The standard answer is that these planets are still radiating heat left over from their formation, but Herndon thought they should already have cooled down – especially since, being mostly gas, they lack the insulation to trap in their primordial heat. He was mulling over this mystery while standing in line at the grocery store during the early 1990s when he had a sudden epiphany. He remembered that, back in 1972, French scientists in Gabon had discovered an underground pocket of uranium that, they realized upon analysis, had been acting as a natural nuclear reactor some two billion years ago, before exhausting its fuel. Similar naturally formed georeactors have been found subsequently. Their existence had actually been predicted in 1956 by the physicist Paul Kazuo Kuroda, but, at the time, the scientific community had scoffed at his idea. He had difficulty even getting it published – another example of a weird theory that became true!
So, Herndon reasoned, if it’s possible for a natural nuclear reactor to form in the crust of the Earth, as it evidently is, it might also be possible for one to form at the core of a planet. There’s plenty of uranium around, and it’s the heaviest metal commonly found in nature. Under the right conditions, during the process of planetary formation, it might sink directly into the core and concentrate there, triggering the fission process. If this had happened in the case of the outer planets, it could certainly explain the excess energy radiating from them.
Herndon originally applied this reasoning only to Jupiter, Saturn and Neptune, but he soon extended it to the Earth as well, making the case in several articles published in the Proceedings of the National Academy of Sciences. He pointed out that the Earth produces enough energy to power a strong magnetic field that shields us from the worst effects of the solar wind. Where, he asked, does all this energy come from? The conventional answer is that it’s a combination of residual heat, radioactive decay and gravitational potential energy, but Herndon doubted these sources would suffice. A georeactor, on the other hand, would easily power the magnetic field. Daniel Hollenbach, a nuclear engineer at Oak Ridge National Laboratory, used computer simulations to help Herndon confirm that this, in principle, was possible.
This brought the exploded-planet hypothesis right to our doorstep. Herndon himself didn’t make the connection that a georeactor might detonate, but others did. If he was right that there’s a five-mile-wide ball of burning-hot uranium at the core of the Earth, then it might only be a matter of time before our home world becomes the next Planet Krypton.
But, forget about the possibility that the Earth might explode – what if it already has? This idea forms the most recent and arguably most sensational development in the ongoing saga of the exploded-planet hypothesis. Obviously, the explosion couldn’t have been big enough to completely demolish the Earth – we’re still here. But it could have been enough to leave some dramatic evidence behind, which now floats above our heads most nights: the moon.
The origin of the moon is a genuine mystery to scientists, which may seem paradoxical given what a familiar object it is. You’d think they would have figured out where it came from by now. And yet, explaining how the moon formed has proven to be extremely difficult.
The problem is that the moon is very large – far too large for the Earth’s gravity to have captured it if it happened to have been speeding by. Moon rocks are also chemically almost identical to the rocks on Earth. It’s as if a giant ice-cream scoop took a chunk out of the Earth’s mantle and placed it in orbit, up in the sky. The challenge for scientists has been to explain how that scoop got up there.
The leading theory, proposed in the mid-1970s by the planetary scientists William Hartmann and Donald Davis, imagines that a Mars-sized object, nicknamed Theia by astronomers, struck the Earth at an angle, causing a huge chunk of the mantle to splash up into space, where it initially formed a ring around the Earth before eventually congealing into the moon. The problem with this theory, as planetary scientists acknowledge, is that some chemical signatures of Theia should have remained in the lunar rocks, but they don’t seem to be there. Which brings us straight back to the exploded-planet hypothesis.
In 2010, the Dutch scientists Rob de Meijer and Wim van Westrenen proposed that the moon could have formed by being blasted out of the side of the Earth. They hypothesized that, while the Earth was still very young, a large concentration of uranium formed near its core. An asteroid impact then might have detonated th
is uranium, launching a massive chunk of the Earth’s mantle into space, where it became the moon. This would explain the chemical similarity between the Earth and the moon, because they were once one and the same.
Van Flandern didn’t live long enough to learn of this newest wrinkle to the exploded-planet hypothesis, but he doubtless would have appreciated it.
Asteroid rubble where a planet should be, a scarred solar system, georeactors, the moon overhead … These are the clues that lead like a trail of breadcrumbs to the conclusion that our planet has the potential to suddenly go boom. But, before you rush out to buy exploding-planet insurance, rest assured that orthodox science sees no reason to worry. As the saying goes, keep calm and carry on!
For a start, astronomers are pretty sure that no planet has ever exploded in the solar system. They point out there’s actually not that much stuff in the asteroid belt between Mars and Jupiter. If you lumped it all together, its total mass would only be a few one-hundredths of the mass of our moon, which hardly seems enough to be the remains of a planet, even if much of it did disappear into Jupiter.
Geologists, likewise, seriously doubt that georeactors could ever form at the core of planets. It’s true that uranium is very heavy and there’s plenty of it around, but uranium likes to bond with lighter elements, particularly oxygen, which should prevent it from ever sinking to a planet’s core. We’re probably safe.
However, if one were inclined to indulge paranoia, such assurances might not completely satisfy. After all, saying that uranium should bond with oxygen is not the same as saying that there’s absolutely no way for a planetary georeactor to ever occur. Nothing in physics or geology explicitly forbids it. What if a planet formed from materials low in oxygen? Then perhaps it could happen. At least, it might be vaguely within the realm of possibility – and, with it, the chance of explosive apocalypse.