The universe seems to be bursting with planets, and this is profoundly important — but not in the way we might expect
The eXtreme Deep Field, or XDF. The photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field and was our deepest view into the universe. Photo by NASA
Caleb Scharf is director of astrobiology at Columbia University in New York. He is the author of Extrasolar Planets and Astrobiology (2009), which won the Chambliss Astronomical Writing Award.
The rites of spring are many and varied. As a child in rural England, I was once given the chore of finding and rearranging the bulbs of a long-unattended flowerbed. I’m not sure if spring was a wise time to do this from a horticultural point of view. It seemed to me that, having survived the rigours of winter, these hardy little tusks of plant matter probably wanted to wait undisturbed for the Sun’s warmth to penetrate the blanket of earth above them. But such was the issued command, and so I began to brush away last year’s dead leaves and timidly poke about in the rich alluvial soil. To my small self, this patch of ground seemed huge and vacant. That is, until I happened on a tiny unyielding clump, barely distinguishable from the grainy clods around it. I’d discovered my first precious bulb.
It produced a momentary surge of optimism, this bulb. I had searched only a short time and already here was one of my quarry. A somewhat grim afternoon activity was transformed into a promising expedition. There might be a great population of these slippery living forms hiding in the soil. I imagined my grubby little fingers feeling for them, finding them. I imagined how my small trowel would lever up proud handfuls. Great riches and accolades awaited me.
Except that is not what happened. There were no other bulbs, no other signs of life. As I dug, poked and prodded with increasingly sore hands, the already intimidating spread of dark earth transformed itself into a barren expanse of cosmic proportions. How could this be? If I had found one juicy bulb merely minutes into my search, surely this hinted at a wealth of others?
The incident with the bulbs stung my pride. Its puzzling aftertaste didn’t fade until long into summer, when all manner of new things took over my life. But I have had many occasions to remember it in adulthood because it speaks to one of the most fascinating, challenging and frustrating questions that astrobiologists such as myself confront every day in our quest to find life elsewhere in the universe.
There is a commonality between the puzzle of a lonesome bulb in a mass of soil and the puzzle of whether or not we’re alone in the cosmos. Until quite recently we knew of only one life-harbouring planet in a single planetary system — adrift within a universe of more than a billion trillion stars. Our home was that single speck, the lone bulb in a great cosmic garden, and it raised essentially the same question: is this all? Or are there more?
We might imagine that our very existence in this vast universe makes the existence of other life a foregone conclusion, or at least very likely. But that conceit is profoundly misleading; it’s a victim of one of the most challenging aspects of statistical inference and probability. It’s an example of post-hoc analysis or a posteriori probability — that is, the evaluation of the significance of events that have already happened. This is a treacherous terrain, a place where statisticians know to tread carefully, because rare and common events are indistinguishable once they’ve occurred. And this caution is especially important for those phenomena for which we have few or no precedents. Just because life did arise on earth says nothing, in itself, about how likely it is to arise elsewhere.
As I dug in the garden that spring, I knew that other bulbs existed elsewhere in the world. But, until recently, we did not know for sure that other planetary systems existed at all, or what their abundance was, or what the potential for ‘habitable’ planets was. We certainly had our suspicions, some scientific, and some not, but no firm evidence one way or the other. This impasse has now been broken, in a most dramatic fashion, and the implications are extraordinary, for reasons you might not expect.
Since the ancient Greek atomists and the upending of a solipsistic worldview by Copernicus, we’ve toyed with the notion of a plurality of worlds, the idea that the universe is brimming with planets and the stuff that might be on them. But, apart from a few false starts, astronomy was hard-pressed to detect other worlds around even the nearest stars to our Sun. The surrounding cosmos has, when it comes to planets, been a great mound of barren soil as unprepossessing as my childhood garden.
This began to change in the early 1990s. First came the confirmation of planet-sized objects orbiting a distant pulsar — the fast-spinning, ultra-dense remains of a stellar core, left behind from an ancient supernova. Pulsars tend to beam out their pulses of radiation into the universe with astonishing regularity, but this one’s pulses exhibited small variations, tiny, time-stamped changes that revealed the gravitational tugs of planetary bodies. These are likely the zombie remnants of matter left after the star’s explosive death millennia ago, now re-coalesced into entire worlds. Unexpected and strange, these were the first hints of what was to come. Just a few years later, in 1995, a giant planet was detected orbiting a regular Sun-like star known as 51 Pegasi. This measurement was a feat of spectroscopic detective work, which registered the fine Doppler shifts induced on the star by the gravity of an unseen world.
These new planets, and the ones to immediately follow, were utterly alien. The half Jupiter-sized companion to 51 Pegasi orbits once every 4.2 Earth-days, well within the orbit of Mercury in our own system. The next two planets discovered around other stars, 70 Virginis b and 16 Cygni Bb, further disturbed any tidy theory about the structure of solar systems. They have highly elliptical orbits that swing them to and from their parent stars with wild abandon. It was too soon to construct statistics about the abundance of these ‘exo’ planets, but the writing was on the wall: the cosmos makes worlds with an extraordinary diversity.
Since that time, the number of planets known to us has swelled beyond the dozens, the hundreds, and now teeters above a couple of thousand — maybe more, depending on how confident we are in our (largely indirect) methods of detection. These are the subtle Doppler changes in the spectral hues we see as a star tangos about its system’s fulcrum point, or the miniscule dip of light that occurs when a planet transits the unresolved face of its star, dimming the light we see by perhaps as little as 0.003 per cent. Or even the hour-by-hour changes of a distantly glowing stellar backdrop, as the gravitational distortion of a foreground star and its planets focuses and brightens the surrounding field of view. These ridiculously difficult measurements have become state-of-the-art components in the ultra-sophisticated technological machinery of planet hunting.
Indeed, the greatest barriers to expanding our catalogue of other worlds have less to do with our devices and more to do with the fact that stellar astrophysics is a messy business. The upwelling and down-welling of plasma on a star’s visible surface produces measurement errors and statistical noise that can obscure the delicate effects of surrounding planets. Especially those like Earth, whose own presence induces a mere 9cm per second motion on our Sun. Finding planets can be like looking for the gentle swaying of a wheatfield brought on by the beating of a bird’s wings, while all around a hurricane blows.
Despite these limitations, we now know that the diversity of planet sizes and orbital configurations is enormous. For example, the most numerous types of planet seem to be those somewhere between the size of Earth and a few times larger. Since we’ve yet to peer very far into the pool of much smaller objects, this statistic holds true for now. There is a kind of Copernican surprise lurking at the heart of this statistic: it means that the most numerous type of planet is not represented in our own solar system.
There could be about 23 billion stars in our Milky Way galaxy, each harbouring Earth-sized planets with life-friendly temperatures
Many exoplanets also follow orbits that are far more elliptical than any followed by the major planets around our Sun. More puzzling still, the most frequent type of configuration, the one that has earned the moniker of ‘the default mode of planetary formation’, is that of closely packed worlds, on orbits that take mere days or weeks to loop around their stars. These compressed versions of our own system seem, for now, to be far more normal than our own. But if we’re not normal, what are we? That’s a question we can’t answer yet, because our census of stars and planets is still woefully incomplete.
Nonetheless, there is something we can do with this wealth of data. We can make a statistical extrapolation from the worlds we’ve found to those we’ve yet to see. Although these different techniques of planet detection come with different biases and systematic effects, we’re sorting through the issues, and a consistent picture is starting to emerge, a picture of extraordinary cosmic wealth.
Just how many planets are there? This is such an active field of research that it can be hard to know which study to quote, so I’ll simply take one of the more recent and representative examples, whose results are pegged to NASA’s Kepler mission, a telescope that has patiently monitored some 140,000 stars in a distant patch of the Milky Way, looking for those tiny dips of light as mosquito-like planets nip across the face of their parent stars.
In this study, by the astronomers Courtney Dressing and David Charbonneau at Harvard, published in The Astrophysical Journal in February 2013, the focus is on stars smaller than our Sun. Not only is it easier to spot transiting planets around little stars, little stars are far more numerous than big stars. These ones, called M-dwarfs and K-dwarfs, are anywhere from half to one-10th the mass of the Sun, and there are 12 times more of them than of Sun-sized stars in our galactic neighbourhood. In other words, they represent 75 per cent of all stars in our galaxy, an excellent group upon which to work up population statistics. From the data we have now, it looks like planets ranging from half of Earth’s width to four times larger appear to be circulating around 90 per cent of these small suns, with orbits of 50 Earth-days or fewer. Bigger planets, and planets on larger orbits — well, there are going to be plenty of those, too. The estimated number of Earth-sized planets orbiting around these stars at a distance that allows for the possibility of a temperate surface (capable of holding that biological elixir — liquid water) varies from study to study, but it’s coming in at about one per seven systems. Given the concentration of these small stars locally, there is a 95 per cent probability that one of these potentially temperate worlds sits within a mere 16 light years of Earth. There could be about 23 billion stars in our Milky Way galaxy, each harbouring Earth-sized planets with life-friendly temperatures on their surfaces.
Twenty-three billion, give or take a few. This estimate tallies with others. Some studies produce numbers of Earth-sized planets closer to 17 billion, all in different orbital configurations. Others suggest a figure as low as 6 billion or so, but these are just the planets close to Earth in size. If we extend our reach to slightly larger worlds, the places now known as ‘super-Earths’, we’re back into the tens of billions. No matter how you slice the cosmic cake, you end up with a vast wedge of planets that we’d be happy to go and study, perhaps even land on and cautiously tiptoe about. These are mind-blowingly huge numbers. The cosmos makes temperate planets aplenty.
It’s quite striking to see how past thinkers seldom separated the existence of a planet from the existence of life on it
So what does this mean for us, and our puzzlement over our place in the universe? It certainly tells us that Earth is likely one of many other planets in the cosmos with at least some attributes in common. But it feels like it should mean more. Doesn’t it mean something for our questions about whether there’s other life out there somewhere? It does, but not in the way we might first assume.
From the time of the atomists of ancient Greece, the idea of a plentitude, even infinitude, of other places in nature, other worlds, has had considerable allure. More than 2,000 years ago, the philosopher Democritus is said to have written:
There are countless worlds of different sizes. In some of them, neither the sun nor the moon are present; in others, they are larger than ours, still others have more than one of them... some prosper, others are in decline... Some worlds have no animal or vegetable life, nor any water at all.
Here, the ‘worlds’ are really universes — conceptual, metaphorical constructs of other natural places. But ideas like these inspired the later atomists such as Epicurus and his followers to embrace more tangible models of plurality: notions of countless places more akin to real planets and Earth-like environments, all existing within an infinite space. Not everyone was on board, naturally. The idea that new Earths existed elsewhere was fiercely opposed by Aristotle, the great champion of geo-centrism.
But when Copernicus issued his bold cosmological vision of a de-centralised Earth in the mid-16th century, a new generation began to wonder if the universe might be filled with planets like ours. In Rome the priest, philosopher and scientist Giordano Bruno was burned at the stake for heresy in 1600. His heretical actions included his vigorous promotion of the idea that the stars were suns with their own planets. A bit later, in the influential book Entretiens sur la pluralité des mondes, or Conversations on the Plurality of Worlds (1686), the French polymath Bernard Le Bovier de Fontenelle mused on the possibility that all stars harboured worlds like those of our solar system. Piece by piece, these ideas made the universe bigger and bigger, and our place in it smaller and smaller.
Voltaire got in on the act, too. In his satirical work Micromégas (1752), giant beings from Sirius and Saturn puzzle over microscopic Earth and its tiny denizens, discovering to their surprise that humans actually have intelligence. But when these superior beings leave humanity a book that purports to explain the meaning of existence, it is found to contain only blank pages.
The idea that the universe is filled with planets drew supporters from a variety of intellectual disciplines. The natural philosopher Christiaan Huygens was an enthusiast, the astronomer William Herschel took to it, and so did the poet Alexander Pope, who wrote, in 1734:
He, who through vast immensity can pierce,
See worlds on worlds compose one universe,
Observe how system into system runs,
What other planets circle other suns,
What varied Being peoples every star,
May tell why Heaven has made us as we are.
There is a remarkable commonality among all these dreams of plurality. All of them share the unquestioned assumption that planets, or plantlike environments, equate to life, and vice versa. In fact, it’s quite striking to see how past thinkers seldom separated the existence of a planet from the existence of life on it.
Today though, astronomers constantly fret over whether or not a world is the right size, the right composition, or in the ‘habitable zone’ — the region around a star that allows for liquid water. We don’t assume that planets are necessarily occupied. Instead, astronomers and astrobiologists like myself spend our days trying to figure out if there are any planets with the right characteristics to harbour life.
Whether we’re one in a billion, or the product of a predictable life-generating apparatus, the outcome looks the same to the observer after the fact
Yet, for all of this scientific caution, we’re still wishful thinkers. We still want to be able to find life. Like many of my colleagues, I have, on several occasions, succumbed to that alluring view that planets equal life. But the crucial difference between us and past advocates of plurality is that we have information they never had. We actually know that planets are abundant, that they are an atomist’s dream no longer. And yet, the abundance of planets itself actually changes very little about the probability that life exists elsewhere in the universe.
Instead, what it does is fundamentally alter the nature of the question itself, and that in turn leads to an even deeper and rather surprising truth. To understand it, let’s return to my story of the lonesome bulb in a spring garden and bring it into the realm of the cosmos.
Imagine, for a moment, an alternative reality. Imagine we discovered that the Sun was the sole planet-harbouring star in the galaxy, or for that matter the universe. What would this actually tell us about the probability of abiogenesis, the spontaneous generation of life? It’s tempting to think that our existence would be proof positive that, so long as you have a planet, life is extremely probable. If it weren’t, the likelihood that the one planetary system in all existence would carry life would seem fantastical.
But this is post-hoc analysis, which can be misleading. The fact that life exists on this imaginary solitary Earth tells us next to nothing about whether life is probable or highly improbable because if we hadn’t already emerged on this planet then we wouldn’t be able to ask the question in the first place. Whether we’re one in a billion, or the product of a predictable life-generating apparatus, the outcome looks the same to the observer after the fact. It would be just like my childhood self getting his hopes up because he’d quickly come across a bulb in one spot of a great and untilled patch of soil.
Of course, this is not entirely fair. In the real universe, there is a teeny bit of information in the fact that microbial life appears to have emerged very early on Earth, just a few hundred million years after our planet assembled. This does seem to tell us a little about how life arises in the universe, but as a single data point it places few meaningful constraints on the range of rates for abiogenesis. Life could still crop up often, or very infrequently — we don’t know which, because in either case we’d be here to ask the question.
But what if we found a second planet in this imaginary universe, around another star that could potentially harbour life? Suppose too that we were able to test whether or not there was life on this other world. Whether the answer was positive or negative, we’d be able to significantly improve our knowledge about the frequency of abiogenesis. The more habitable planets we found in our pretend cosmos, the more we could test them, in order to build and refine a mathematical model for the likelihood of life.
We live in a universe that allows us to get some measure of our own significance
Now let’s bring ourselves back to this universe, and our galaxy, the one full of temperate planets — tens of billions of them. What do all these planets tell us? They don’t tell us how common life is, but they do give us a shot at finding out. If we lived in a cosmos with only a few planets, we could never deduce the true probability of abiogenesis with any precision, even if they all harboured life — as imagined by earlier advocates of plurality. We might never be lucky enough to find one of these worlds within examining range, and all would be lost among the stellar fields of the Milky Way. The existence of billions of planets gives us a chance to write the equation, a chance to pin down the relationship between habitability and actual habitation. And it could have larger implications still.
Over the past century, scientists have noticed that many of the root physical characteristics of our universe, from the strength of gravity to the values of fundamental constants that determine atomic structure, all seem precisely aligned with the conditions that appear necessary for life to exist. Change a few of these constants and there’d be no stars like the Sun, there’d be no heavy elements like the carbon that builds our complex molecules, and so on.
Of course, it’s pretty obvious that if things weren’t the way they are then we wouldn’t be here to notice it in the first place. In that sense, this apparent ‘fine-tuning’ of the universe for life is just a selection effect, an unavoidable bias of our existing in the first place. This ‘fine-tuning’ of nature is especially mysterious if you suppose that ours is the only universe, the only version of reality that has ever existed. Why should a one-off universe be tuned for the grubby little molecular pieces of our life? It’s a thorny problem that can lead to all kinds of interesting, and sometimes wild, interpretations. But fundamental physics and cosmology might hold the answer to this mystery.
Ever since the American philosopher and psychologist William James first coined the term ‘multiverse’ in 1895, there has been increasingly good reason to think that our universe is actually only one of many, part of a multiverse of other regions of space and time separated by distance and time, or dimension. Some theoretical frameworks suggest that there could be upwards of 10 to the power of 10 to the power of 16 distinct universes. Indeed, our investigations of the cosmos suggest that the same quantum physics of the vacuum that spawned our universe, and the awesome energy of inflating space that blows it up into the kind of place we occupy, is also well-suited to generating all of these other universes. We even have a shot at testing this through astrophysical measurements. If the theory holds, our fine-tuned universe will cease to be mysterious; it will simply be a part of a larger multiverse, a part that turned out suitable for life.
The special thing about our planet-rich universe is that it’s tuned both for life and for finding out about life. If we were back in our imaginary universe with only one planetary system, we would have no way of learning how frequently life arises. We live in a universe that allows us to get some measure of our own significance. There is nothing in our present understanding of the nature of life or the universe that says this was absolutely necessary, yet here it is.
It’s not clear that we’ll ever be able to solve this puzzle. To start with, we need to build that equation for abiogenesis, we have to dig deeper in the astrophysical dirt to find places in the universe that might harbour life, to follow nature’s breadcrumbs, as it were. The strategy is straightforward: seek out more worlds that might share some of Earth’s characteristics, and search their surfaces for the chemical signatures of life. It won’t be easy but, unlike 20 years ago, we now know we have a galaxy’s worth of planets to chase, and we know that if we persevere, the equation will eventually come into focus.
That this work is even possible has, in a very real sense, already changed our universe. Not because it’s told us anything quantitatively new about life elsewhere, but rather because it’s raised the stakes for evaluating our significance, our cosmic loneliness, to a whole new level. Not only are the fundamental properties of our universe aligned, and tuned to, the needs of life, they also promise success in our quest to discover life’s frequency, origins, and perhaps the very causes of this tuning itself. And it didn’t have to be like this at all. If you turn it over in your mind enough times, you realise that Albert Einstein was right when he said: ‘The most incomprehensible thing about the universe is that it is comprehensible.’
25 June 2013