Jupiter's complicated relationship with life on Earth

Astronomy is a science that fascinates and captivates people of all ages, all across the globe. Documentaries about space are sure-fire rating winners, and people have an endless appetite to learn more about our cosmos. But not every story that is told about the universe holds true - and over the years, many myths have entered our collective consciousness, taking route in the fertile soil of our collective imagination.

If you’ve ever watched a documentary about the Solar system’s most massive planet, Jupiter, you will doubtless have heard the story of how the giant is our apparent gallant protector. Without Jupiter, the Earth would be pummeled by impacts from asteroids and comets, rendering our planet utterly uninhabitable.

It’s a great story, with one minor drawback - it simply isn’t true! Or, to be more accurate, the story is far more complicated than that old but widely held, myth would have you believe.

So let me take you on a story of a planet with a split personality, one that has more in common with Dr Jekyll and Mr Hyde than it does with Sir Lancelot and the Knights of the Round Table.

To start our story, let us journey back, to the year 1770. Just a few months after Captain Cook first landed in Australia, an astronomer called Charles Messier discovered a new comet. Now, it might be that you’ve heard of Charles Messier before. He was the author of the ‘Messier Catalogue’, which has become something of a bible to astronomers - a guide to the best clusters, galaxies, and nebulae in the night sky. As a child fascinated with astronomy, the Messier Catalogue was my go-to guide to the sky’s most spectacular sights, and I know for a fact that I’m far from alone in that. 

What is remarkable about Messier’s catalogue is that it was never intended to be the basis for a scenic tour of the cosmos. Instead, it was Messier’s personal list of fuzzy blobs in the sky that aren’t comets. Messier, an avid comet hunter, spent his nights scouring the sky with his telescope, searching for fuzzy blobs that moved from night to night, trying to identify these ghostly transient visitors. 

And as he scoured the sky, he kept coming across other fuzzy blobs, all over the place. So he kept a list of all the fuzzy things he saw that didn’t move - that were always in the same place in the night sky. The things that, at a quick glance, could be mistaken for comets - but that were instead something totally different. Thus was born the famous Messier Catalogue.

In addition to finding and cataloguing more than a hundred of the sky’s prettiest fuzzy blobs, Messier did manage to discover a number of real comets through the course of his career. Of those comets, the one he found in 1770 was perhaps the most spectacular and was certainly the most interesting. 

Messier discovered this new comet on 14th June 1770. Over the next few nights, the comet grew rapidly larger and brighter in the sky - quickly becoming visible to the naked eye. But it soon became obvious that the comet looked strange. Rather than having a small, well-defined head, and a spectacular tail, the head of the comet was particularly large - several times greater than the diameter of the full Moon.

On 1st July of that year, the comet had its closest approach to the Earth - it tore across the sky at an unprecedented rate - vast, fuzzy, and bright enough to be seen even from the centres of the largest cities. After that, it faded but remained visible for several months - with Messier himself making the last recorded observation of the comet, in October. 

What had happened? The comet was unusual in several ways - it looked weird (with a big fuzzy head far larger than normal for a comet). It moved strangely - tearing across the sky far faster than comets normally moved (up to 42 degrees in 24 hours, at its fastest). And it brightened unusually rapidly. 

It didn’t take astronomers long to work out what was going on. The comet (which became known as Comet Lexell, after the mathematician who first calculated its orbit) had come remarkably close to our planet. Indeed, it was (and remains) the closest observed cometary flyby of our planet in recorded history. 

At its closest, the comet was just 2.2 million kilometres from the Earth (about six times farther from Earth than the distance to the Moon). It was that proximity that led to the comet’s rapid motion across the sky, that gave it such an unusual appearance, and that caused it to brighten so quickly.

But the same studies that helped to explain those oddities threw up another peculiarity. They revealed that the comet was orbiting the Sun with a period of just 5.58 years. It was what we now call a ‘short period’ or ‘Jupiter family’ comet. 

Now, that’s all well and good - there are several hundred such comets now known in the inner Solar system. But Lexell’s comet was particularly bright. Why had it never been seen before? If it was going around the Sun every five and a half years, why wasn’t it seen in 1764, or 1759? It was almost as though it had come towards the Earth totally out of the blue. 

For that matter, why has the comet never been seen since? The ‘D’ in its modern designation (D/1770 L1) designates a lost or destroyed comet - and, indeed, after its 1770 apparition, the comet was lost to us - never being seen again. So what was going on? What could be the answer to the mystery of the amazing vanishing comet? 

The solution to the mystery became clear when astronomers extrapolated comet Lexell’s path backwards and forwards through time - finding that the Solar system’s most massive planet, Jupiter, was responsible for both the comet’s sudden appearance and equally abrupt vanishing act.

It turned out that, in 1767, the comet had approached Jupiter whilst moving on an orbit that would have brought it nowhere near the Earth. Most likely, it was a member of a population of objects we now call the Centaurs - icy cometary nuclei held in cold storage in the outer Solar system. 

But as it swung past the Solar system’s largest planet, Jupiter’s gravity grabbed hold of it and flung it inwards, moving it to its new 5.58 year-long orbit, and sending it perilously close to the Earth. In other words, it took an object that would otherwise have come nowhere near our planet and threw it towards us! So that explained the comet’s sudden appearance - it was a new visitor to the inner Solar system, flung our way by the vagaries of Jupiter’s immense gravitational pull. 

But why was the comet subsequently lost? Well, just twelve years after its initial encounter with Jupiter, the planet and the comet had a reunion, of sorts. As comet Lexell swung out to aphelion (the point in its orbit farthest from the Sun), Jupiter was waiting for it. The comet swung close to the giant planet once again and was flung outwards, away from the inner Solar system. 

Exactly where it ended up is a mystery - our observations from its 1770 apparition aren’t good enough to tie down exactly how that encounter with Jupiter would have gone. But one thing is for sure - comet Lexell was ejected from the inner Solar system - taken from harm’s way, and flung onto an orbit that could no longer threaten the Earth. For comet Lexell, at least, Jupiter played both villain and hero in just a short,12 year period. 

This all brings us back to that great astronomical myth: Jupiter, the Earth’s great protector. Does our giant neighbour really act to shield us from impacts, giving us the space we need for life on Earth to survive and thrive? Or does it instead play a more sinister role, threatening Earth with debris, flung our way from orbits that would otherwise come nowhere near our planet?

The story of comet Lexell hints at the answer - rather than simply being our protector, or a celestial villain, Jupiter’s role is much more complicated. If anything, the planet has a two-faced nature - with those two faces perfectly illustrated in the case of comet Lexell. 

One side of Jupiter’s dual nature is that of an ally - after all, it does throw some potentially threatening objects far from Earth, preventing them from colliding with our planet. But at the same time, Jupiter has a dark side. It can take objects that would otherwise come nowhere near our planet and shift them onto orbits that penetrate the inner Solar system, placing them on orbits that can imperil life on our planet. 

In order to determine whether Jupiter can truly, in sum, be considered our friend and protector, these two roles must be weighed against one another. If it turns out that Jupiter is better at protecting Earth than threatening us, then maybe it would be fair to uphold the myth. On the other hand, if Jupiter poses more of a threat than any benefit it offers, then we should consider it more of a foe.

To get a feel for Jupiter’s role in the current Solar system, I carried out a series of detailed computer simulations, a little more than a decade ago, working with my dear friend and mentor, Prof Barrie Jones (who, sadly, is no longer with us). 

We asked a simple question - for the modern Solar system, how often would the Earth be hit by celestial projectiles, were Jupiter more (or less) massive than the giant planet we know and love? 

For our first test, we considered projectiles flung inwards from the Asteroid Belt - a population of Solar system small bodies called the ‘Near-Earth Asteroids’, which are currently thought to contribute roughly three-quarters of all impacts on our planet. 

The near-Earth asteroids are chunks of rock and metal, moving on orbits that cross those of the terrestrial planets. They are short-lived objects - at least, on astronomical timescales. Typically, any given near-Earth object will be removed from the inner Solar system on a timescale of a million years, or so. 

That removal comes in a variety of forms. They can be torn asunder by a close encounter with one of the inner planets or the Sun. They can collide with one another, or the inner planets, or even our star, meeting a fiery and explosive end. They can be ejected from the Solar system entirely, after a close encounter with one of the planets (usually Jupiter). They can even be destroyed by sunlight - with radiation from our star gradually making them spin faster and faster until they eventually tear themselves apart.

To cut a long story short, the near-Earth asteroids are short-lived. But their numbers are continually replenished by new debris, ejected from the Asteroid Belt under the combined influence of the Sun’s radiation (which can push kilometer-sized objects all over the place, given enough time) and the gravitational pull of the Solar system’s giant planets. 

So, to Jupiter’s role. If Jupiter were solely Earth’s ally, from the point of view of the near-Earth asteroids, one would expect that, the more massive our giant neighbour becomes, the fewer near-Earth objects would impact the Earth. A bigger ally would equate to a lower impact rate on Earth. Equally, if Jupiter were more our enemy, the reverse would be true - a bigger enemy would result in a higher impact rate on Earth.

To test this, we set up our simulations, featuring the Asteroid Belt, the Earth and Mars, and Jupiter, Saturn, Uranus and Neptune. From one simulation to the next, we varied the mass of Jupiter, and then counted the number of impacts experienced by the Earth in a ten million year period. 

The results were, to put it mildly, unexpected.

We expected one of two simple scenarios to occur. Scenario 1 was ‘Jupiter - the ally’, with the number of impacts on Earth falling away as the giant planet became ever more massive. Scenario 2 was ‘Jupiter - Earth’s foe’, and would feature an increasing frequency of impacts on Earth, as Jupiter’s mass grew ever larger.

Instead, the number of impacts on Earth started really small, grew to a peak value (when Jupiter was about one quarter its ‘true’ mass), then fell away again. The impact rate in our Solar system (with the Jupiter we know and love) was higher than when Jupiter was absent, or really tiny, but was dwarfed by the worst-case scenario - when Jupiter was a little bit less massive than Saturn. 

So, as far as the near-Earth asteroids go, Jupiter is neither friend nor foe, but something in between. But a Jupiter a little bit smaller than Saturn would be the ultimate enemy. To swap analogies for a moment, imagine the tale of Goldilocks and the Three Bears. Our modern Solar system is the equivalent of a porridge that is neither too salty nor too sweet but not really any better tasting than the plain, unaltered porridge. The case where Jupiter was a bit smaller than Saturn, however, is like Goldilocks coming across a bowl of porridge laced with strychnine. 

The near-Earth asteroids are but one of three populations of Solar system small objects that pose an impact threat to the Earth. The others are objects like comet Lexell - ice-rich objects that move on highly elongated orbits, whose origins lie in the icy depths of the Solar system’s outer regions. 

Like the near-Earth asteroids, comets are short-lived, on astronomical timescales - they can fall apart, collide with the planets (and the Sun), or be flung out from the Solar system, never to return. But, just like their asteroidal cousins, their numbers are continually replenished, with new comets being continually flung into the inner Solar system, replacing those lost as the millennia fly by.

The short-period comets are the grand-children of a vast population of objects that lurk beyond the orbit of Neptune - the trans-Neptunian objects. The great majority of those objects are stable on timescales far longer than the age of the Solar system - but a steady trickle gradually evolve onto orbits that cross that of Neptune. 

At that point, the object’s fate is sealed - and it will almost certainly be destroyed or flung out of the Solar system within just a few tens of millions of years. On crossing the orbit of Neptune, it has become a ‘Centaur’ - a member of a transient population of objects moving on chaotic orbits between those of Jupiter and Neptune. As time goes by, the new Centaur will be the ball in a game of planetary pinball - being flung from orbit to another as a result of close encounters with one or other of the giant planets. 

The most likely outcome of that game of pinball is that the Centaur will be ejected from the Solar system, never to return. But fully one-third of all Centaurs will eventually be flung into the inner Solar system - usually by Jupiter - becoming a short-period comet. This actually takes us back to the start of our story - comet Lexell, which dazzled observers back in 1770, was one such short-period comet, flung inward from the Centaur population by Jupiter to imperil the Earth. 

To test whether Jupiter acts as more of a friend or foe for the short-period comets, we set up a suite of simulations that followed the evolution of objects in the Centaur region. From one simulation to the next, we varied the mass of Jupiter, to see how the impact rate on Earth varied as a function of Jupiter’s mass.

Once again, the results were startling. Just as was the case for the near-Earth asteroids, the Earth saw relatively few impacts when Jupiter was tiny. At large Jupiter masses, too, the impact rate on Earth was relatively low. However, when Jupiter was just a little bit less massive than Saturn, the impact rate at Earth was greatly enhanced. 

And as before, it turned out that the story was far more complicated. Rather than solely being an ally, or solely an enemy, it turns out that Jupiter can play both roles.

The final group of objects that pose a threat to life on Earth are the long-period comets. Like their siblings the short-period comets, these dirty snowballs move around the Sun on highly elongated orbits. But where the short-period comets return every few years or decades, the long-period comets take far longer to orbit the Sun, with periods measured in millennia, or even millions of years.

Just like the other populations of threatening objects, the long-period comets are short-lived objects, on astronomical timescales. They have a tendency to fall apart, to collide with things, or to be flung out from the Solar system by one or other of the Solar system’s planets, never to return. 

But where do these objects come from? Just like the asteroids and short-period comets, their numbers must be continually replenished, for so many to still grace the inner Solar system. And whilst the parent populations of the other two populations can be directly observed, that of the long-period comets is somewhat more mysterious - a vast reservoir known as the Öpik-Oort cloud (or ‘Oort cloud’, for short). 

More than 70 years ago, two great astronomers independently realised that there must be a source reservoir for the long-period comets, replenishing their numbers against the continual attrition of disintegration, collision, and ejection. Both Ernst Öpik and Jan Oort hypothesised that the Sun was surrounded by a vast, distant cloud of comets - stretching out to more than halfway to the nearest neighbouring star. 

That vast cloud contains a truly astonishing number of dirty snowballs - with estimates of that population suggesting some ten trillion cometary nuclei greater than 1km in diameter lurk out in the Solar system’s icy depths. The mind-boggling thing is, though, that, because the volume of the cloud is so immeasurably vast, comets in the Oort cloud are rarely closer to their nearest neighbours than the Earth is to the planet Uranus. In a way, the Oort cloud turns a vast volume of space around the Solar system into a giant snow globe - a vast region of mostly empty space, littered with trillions of tiny, kilometre-sized snowflakes.

These Oort cloud objects float around the Solar system in cold storage - taking hundreds of thousands or million years to orbit the Sun, and at such great distance from our star that, to them, it wouldn’t even be the brightest star in the sky. 

Only tenuously bound to the Solar system by the Sun’s gravity, they are continually nudged and tweaked by passing stars. In much the same way that the Earth’s oceans move in response to the tides raised by the Moon and the Sun, the motion of objects in the Oort cloud is perturbed by the ‘Galactic Tide’ - the combined gravitational pull of all the objects in our galaxy, as well as feeling more concerted nudges and bumps from denser concentrations of mass closer to the Solar system (such as giant molecular clouds).

These many and varied influences cause the orbits of the objects in the Oort cloud to continually shift, tilting and stretching, on timescales of millions of years. The cloud continually leaks members, with objects being stripped from the Sun’s grasp entirely, to roam space alone forevermore. 

But some Oort cloud objects have a different fate - instead of being torn away, they are instead nudged inwards, onto orbits that cross those of the Solar system’s planets, falling into the inner Solar system to become new long-period comets. 

Almost all of history’s most spectacular comets are such long period comets - comets such as C/2006 P1 McNaught, which was an incredible sight in early 2007, or C/1995 O1 Hale-Bopp, which was visible to the naked eye for an astonishing 18 months through 1995 and 1996. Unlike their short-period siblings, the long-period comets always catch us somewhat by surprise, falling into the inner Solar system from the cold icy depths of the Solar system. The next great comet is undoubtedly already falling inwards, to grace our skies in years or decades to come - but as of yet, it remains undetected.

Of the three populations of potentially threatening objects we’ve discussed, the long-period comets are thought to pose the lowest ongoing risk - contributing somewhere between five and ten percent of the total impact threat to the Earth. Nonetheless, it is interesting to consider what role Jupiter plays in determining the true level of risk they pose. Could this be one case where Jupiter truly is our friend?

In this case, the auspices were good - the main way that long-period comets are removed from the Solar system is as a result of Jupiter’s gravitational influence. Most long-period comets end up being ejected from the Solar system - given a kick by Jupiter’s gravity that increases their speed enough that they leave, never to return. 

Interestingly, the comets are so tenuously held by the Sun’s gravity that Jupiter can fulfil this role even if the comet comes nowhere near the giant planet - even a distant tweak from the Solar system’s largest planet can be enough to ensure a long-period comet is flung out to travel amongst the stars!

The incredibly long orbital periods of the long-period comets posed a real problem for us, however - we simply didn’t have the computing power to simulate enough comets and count their collisions with the Earth, as we’d done for the other two populations. Instead, we had to follow a more simple path - we simulated the evolution of a vast number of long-period comets under the influence of the gravity of the Sun and the giant planets, and kept track of how many were ejected over time. The faster comets are ejected, the fewer passes they make through the inner Solar system, and so the less chance they have to hit the Earth.

In this scenario, then, if Jupiter truly is a friend to Earth, scenarios with more massive Jupiters would see comets ejected more rapidly from the Solar system, never to return - and hence a lower impact rate at the Earth. And, at least for the long-period comets, that is what we found. The scenarios where Jupiter was most massive yielded the fastest removal of long-period comets from the system - proving ever more effective at purging the Solar system of these threatening objects as its mass increased.

All of this brings us back to that age-old question - is Jupiter Earth’s friend or our foe? Our simulations revealed that, rather than there being a simple answer to this question, the story is far more complicated. For the short-period comets and near-Earth asteroids, Jupiter actually acts to increase the impact risk to the Earth over that we’d experience were Jupiter absent - but the situation would be far worse were Jupiter significantly less massive than the behemoth we know and love. If Jupiter were comparable to, or somewhat less massive than, Saturn, then the Earth would be pummeled by impacts from asteroids and comets far more frequently than is the case in the real Solar system.

But there’s yet another added complication here. We imagine that an Earth with no impacts would be best for life - and that might suggest that the Earth would have been even more idyllic were our giant neighbour not present. But if impacts were less frequent, would that really be a good thing? If the reign of the dinosaurs had not been brought to an unfortunate end by a rock from space, would we be here, right now, to learn more about Jupiter’s influence? 

Looking further back, to the formation of the Earth, impacts on our planet caused by objects flung our way by Jupiter might just have played a key role in ensuring that the Earth became the wonderful habitable world we know today. Without the icy objects flung our way by Jupiter from the outer asteroid belt and outer Solar system, the Earth could well have been bone dry - a desiccated husk of a world. 

Without Jupiter, then, the Solar system would be a very different place. Yes, the Earth would be hit less often - likely far less often - but that might not be a good thing. Our planet could well be an arid place, more like Arrakis than Kamino.

One of the natural biases we fall prey to when considering life beyond the Earth is the idea that a planet must be exactly like the Earth for life to develop and thrive. This led, many years ago, to the birth of the ‘Rare Earth hypothesis’ - the idea that the Earth is so special, and the various factors that contribute to our planet’s habitability are so unlikely to recur, that we are unlikely to ever find life elsewhere in the cosmos. 

The idea that Jupiter is a shield to the Earth, protecting us from frequent impacts that would sterilise our planet, is one of the centrepieces of the Rare Earth hypothesis. It suggests that planetary systems that don’t contain Jupiter-like planets on Jupiter-like orbits would be worse places to look for alien life - since any Earth-like planets in those systems would be sterile, battered husks. 

Our research tells a different story - in fact, we find that the influence of giant planets, like Jupiter, on the impact rates on Earth-like planets is far more complicated than previously believed. But beyond that, our work shows how we could actually get a feel for the real impact flux on any potentially Earth-like planets we discover. When it gets to the point that we are seriously beginning to search for evidence of life beyond the Solar system, using the world’s largest telescopes to gaze, unblinking, at Earth-sized planets around other stars, looking for hints of life upon them, we will want to pick the most promising targets for that search.

The same kind of simulations we carried out to investigate Jupiter’s dual nature - friend and foe - could easily be used to examine the impact risk on distant, alien worlds. We now have the technology to find and measure belts of debris in distant planetary systems - their analogues to the Asteroid belt and Edgeworth-Kuiper belt. We can now find the planets in those distant systems. Put the two together, and we can model how those planets perturb the debris in their system - and we can track that debris as it threatens the other planets therein.

It’s an exciting thought that our work might not just have helped answer an ancient question (or, at least, demonstrate how muddy the waters actually are), but that it might also help us move forwards in our efforts to answer an even larger question - are we alone? 

But that’s a story for another day!

Video credit - Asteroid simulation: Scott Manley/YouTube
Video credit - Jupiter Perijove 9 flyover: Sean Doran/NASA/SwRI/MSSS/Juno Mission/YouTube