Although there seems to me to be hope of making progress in exploring the potentially universal processes and structures of life, it is probably facile, but worth saying, that the most reliable way to investigate the scope of contingency in how physics limited life on Earth would be to examine another example of life. What are our prospects of ever finding another example of life outside our planet?
We do not know whether we will find outside our planet life that is truly an independent genesis, the evolutionary experiment run from scratch. In our own Solar System, we have found numerous watery environments, such as ancient Martian terrains that seem to have been habitable, potentially supportive of life. Perhaps there is life below the surface of that planet today. Substantial liquid water oceans exist in the icy moons of the outer Solar System, including Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan. Do they house independent experiments in evolution? Even if they do, a confounding problem could be that their biota might be related to life on Earth, existing as it would within our Solar System, where rocks, and potentially their hitchhiking life, have liberally been shared between the planets in the form of meteorites since the planets first coalesced from the early protoplanetary disc. Nevertheless, the search for life in our Solar System is a worthy scientific goal because if we did find life independently evolved from Earth, we could assess the universality of biology. If we find life related to Earth’s biota, or no life at all, we will learn less about biology, but we will have learned something about the distribution of life and its capacity or lack thereof to originate or be transferred within a solar system.
Besides the impressive strides in the robotic exploration of our own Solar System, extraordinary advances have been made in finding Earth-like worlds around other stars. Do these breakthroughs offer us any hope for answering our question of what features of life may be universal? It will be a long time, if ever, before we visit exoplanets orbiting distant stars and can sample their biospheres. Even planets a few to tens of light-years away would be multigeneration missions with the best conceivable propulsion systems we could build at the current time. It is premature to declare that the discovery of exoplanets opens up the real possibility of expanding the set of biospheres with which to explore life’s universality.
Nevertheless, retreating from the heady ambition of finding another biosphere to test some of the observations expressed in this book (although maintaining our hope that this might one day happen), there is something to be said for delving into a discussion of these quite remarkable exoplanetary discoveries. At the very least, in a more narrow-minded way, we might ask some slightly different questions. How alien are these other worlds? Even taking as our hypothetical starting point the life we know on Earth, would we expect the environments of these planets to channel evolution in alternative ways?
Although this may seem a little speculative compared with the more data-rich basis of our previous intellectual escapades, such thinking can sometimes illuminate the view of our home world, stimulating us to ask fresh questions about the forces that shape the products of evolution on Earth. Using other planets to see the Earth in a fresh light is often a fruitful way to open up the human mind. So in the interest of expanding our minds to the full scope of the question of whether the structure of life is universal, let us explore briefly this new horizon on alien worlds and indulge ourselves in some evolutionary thought.
When eager scientists turned their gaze to distant stars to find planets, they expected to find these worlds in planetary systems much like our own. In the outer reaches beyond a star, giant gaseous planets like Jupiter and Saturn would orbit, the frigid temperatures from the early dawn of those solar systems, like ours, favoring the condensation of hydrogen, helium, and other light gases that in the inner regions near the star tend to be vaporized into space. Closer to the star, where those gases disappear, are left rocky remnants, the collided and congealed fragments of material that built the so-called terrestrial planets, like Venus, Mercury, Mars and our own special home, Earth. Small bits of rock near a star, big balls of gas far away. That seems to work nicely.
It was something of a surprise then that when astronomers did detect another world orbiting a distant star for the first time, it turned out be a giant of a world, orbiting once around its sun in a preposterously short five days! So close was this planet to its star that astronomers had no choice but to call this new object a hot Jupiter. That discovery of this first so-called exoplanet in 1990, orbiting around the science-fiction-sounding star Pegasi 51 and 50.9 light-years away from us, threw astronomers into a certain confusion. How could it be orbiting so close? Such a large gaseous world must surely be vaporized? The discovery forced astronomers, early in the quest for these distant planets, to revise their models of how solar systems formed. Only one theory could properly account for these giant worlds so tightly hugging their parent stars: they had migrated from the outer reaches of their solar systems inward and taken up residence in neighborhoods thought to be exclusively owned by small rocky worlds. This discovery was the first inkling that our own Solar System may not be typical.
Pegasi 51b was the first in a flood of many peculiar types and orbits of planets that would litter the scientific literature as novel discoveries piled up. About twenty years after the first observation, the surprises keep coming, but several things have become apparent and will long endure. Our Solar System’s architecture is not typical. In other solar systems, planets are strung together in many configurations, with migratory planets throwing a wrench in the works and leading to a host of systems with small and large planets in different positions depending on how gravity arranged the alignment of worlds. Many planets do not occupy the nice near-circular orbits that are a characteristic of our Solar System’s major planets. Planets can take on wild elliptical orbits, tracing arcs that take them far away from their stars and hurtling inward in close shaves, a little like the comets that we are more familiar with. These extreme trajectories are a consequence of the gravitational perturbations and interactions that shaped these systems in their early years as new planets formed and others migrated. Some pathways are so extreme that planets can even be hurled out of their star systems entirely, exiled into the infinite abyss.
What exoplanet hunters around the world have found is nothing less than a feast of bizarre and wonderful places. Alongside the bounty of hot Jupiters, astronomers have found hot Neptunes, slightly smaller gas worlds the same size as Uranus and Neptune.
These were by no means the only oddities. Some gas giants orbit so close to their star that it is thought that the intense heat of the star causes the gases to expand into giant envelopes, inflating the atmosphere. These worlds, or puffy planets, have a very low density. One such object, HAT-P-1b, just larger than Jupiter, lies 453 light-years away and orbits its planet every 4.47 days. It has one-quarter the density of liquid water. This inflated ball epitomizes worlds that would have been unimaginable to astronomers just a few decades ago.
What interests us most are worlds where the conditions for life might exist. As exoplanet detection methods have improved, the size range of planets capable of being detected has been reduced to smaller and smaller spheres. Somewhere between the mass of the Earth and Neptune-like worlds, there is a gray region where planets make the transition between being giant gas worlds or small rocky worlds. Between these two extremes are the planets sometimes called super-Earths. The term is somewhat of a misnomer because these places are not necessarily Earth-like. Many of them are likely to be uninhabitable or might have originated in conditions very different from our own planet. Some are likely to be ocean worlds, where a good fraction of the mass is water.
Crucial to the quest to find truly Earth-like planets is to find planets in the habitable zone around a star, where the solar radiation is just high enough to allow liquid water to persist on their surfaces. Get too close, and you end up like Venus, the oceans boiled away, helped along by a runaway greenhouse effect where the dense carbon dio
xide in the atmosphere traps heat, causing all the water to evaporate. Form a planet too far away, and it generally remains a frigid, wintry world. The habitable zone, sometimes called the Goldilocks zone, is the annulus where temperatures are just right, the zone in which our own evolutionary experiment has explored its capacities.
It was a planet orbiting Kepler 452, about fourteen hundred light-years away, which took the first title of being a genuinely Earth-like world. Although it is slightly larger with a diameter about 60 percent greater than Earth’s, it was the first Earth-like world to be found orbiting a Sun-like star and within the habitable zone. It is slightly more aged than our own world—it is about six billion years old.
The sheer quantity of data gathered by instruments such as the Kepler space telescope has been used to suggest that about 5 to 7 percent of Sun-like stars in our galaxy might host Earth-sized planets in the habitable zone. Take these numbers and scale them up to the whole galaxy, and you are confronted by outrageous numbers, guesses that the Milky Way contains something like eight billion Earth-sized, potentially habitable worlds! Well, we can argue about whether that should be five billion or ten billion, but this is nitpicking. The discoveries of exoplanets have revealed just how common small rocky worlds are.
How did astronomers find these planets? The story of exoplanet discovery has filled the pages of entire books, and although this story is somewhat tangential to our focus on life and physical principles, it is well worth a brief detour. The discovery shows the fascinating connections now being made between researchers in the physical sciences and those in the biological sciences; both groups are motivated by an interest in understanding the physical conditions on other planets, conditions that we assume would influence any life.
Two decades ago, astronomers and biologists had very little to say to one another (apart from general coffee-room chatter). However, the methods that astronomers are using to look for exoplanets are revealing Earth-mass planets that we might eventually examine for signs of life. In this endeavor, physical and biological sciences come together, and the forging of this alliance may well yield a rich harvest of ideas about how astrophysical and physical principles shape the conditions for planetary formation, the surface characteristics of planets, and, therefore, the potential physical environments in which life might emerge. This is one of the most exciting emerging areas of astrobiology.
It is not immediately obvious how to detect a planet around the blazing inferno of a star. The light reflected from even the grandest Jupiter-like objects is many billions of times less than that generated by the immense burning fusion reactor of a star. To overcome these limitations requires some ingenuity. And astronomers have brilliance in no short supply.
Picture yourself with a telescope staring at a distant solar system edgeways. Caught in the lens of your telescope is the bright star that lies at the center of the solar system, but then the star dims. Gracefully passing in front of it is a planet in orbit, its opaque form blocking out some of that starlight for as long as it passes in front. That dimming is not much, maybe 1 percent of the light or less, but with a good telescope and some equipment to measure the light, you can see the temporary drop in intensity as the planet passes by. This method, the transit method, has its limits—the solar system must be observable edgeways. It is no good observing such a system from the top down since as the planet orbits the star, it will never intersect with the star’s light. Nevertheless, the transit method has been extraordinarily successful. The Kepler space telescope, named for seventeenth century astronomer Johannes Kepler, who first elaborated the laws that define the orbits of planets, used this method to find over a thousand planets. The limitation of needing solar systems that are oriented edgeways to us has not hindered scientists from finding a bounty of planets.
Astronomers have found some other clever ways to find planets. You can detect these worlds by observing the wobble they cause in the star around which they orbit. Imagine or recall yourself at a wedding, watching couples dancing at a reception. A couple grasp each other’s hands and spin wildly around. They spin around their common center of mass, around an imaginary point that lies somewhere between them, and both become dizzy at the end. As they leave the floor, the unfortunate young woman is intercepted by her large uncle, who also demands a dance. The petite woman holds his hands, but now this time, his mass is so large she flies around him, his position almost fixed in the dancefloor. However, he is not entirely fixed. The common center of mass between them is an imaginary point that lies around the edge of the gargantuan uncle or even within him. He too wobbles and spins around the common center of mass, relatively less affected than she is, with her small and light frame that spins around his bulk. When they have finished, she is positively giddy, and he too lurches and stumbles back to his table.
So too in the world of astrophysics. A planet does not strictly orbit its star. Both objects orbit around their common center of mass. However, a star is so massive that the common center of gravity is essentially buried in the star; it hardly notices the tiny planet tugging on it as the smaller body dances around the star. Although the effect is small, it is still there and the star subtly wobbles around like our slightly unstable uncle, its spin influenced by the mass of the planet. If there are many planets in orbit, the movements will be more complicated, the speed and pattern of the lurching revealing many worlds.
How can we, many hundreds or thousands of light-years away, see this almost imperceptible change? The good news is that this is where ice cream takes center stage. As I’m sure happens to you every summer, the mere sound of the tune from an ice cream truck causes you to rush out into the street in happy exultation. Now as the truck approaches, the pitch of its melody is high, but as the vehicle passes you (your disgust at its failure to stop overwhelmed only by your fascination with physics), the pitch gets lower. You can observe the same effect with the more sobering sound of an ambulance.
The Doppler effect, named for Christian Doppler, an Austrian physicist who proposed how it worked in 1842, causes this alteration in pitch. The effect is easy to understand. As the ice cream truck approaches you, it is emitting sound, but since the vehicle is moving, each successive soundwave is produced closer to you than the previous one, hence the time between the waves is reduced (they become higher frequency, which is perceived by you as the higher pitch of the sound). When the rascal driver disappears into the distance, each successive wave of sound emitted is further away, since the truck is traveling into the distance, making the frequency of sound lower; the sound waves are essentially stretched.
What does ice cream have to do with exoplanets? It transpires that the Doppler effect also influences light, because, like sound, light moves in waves. If a glowing object like a star is moving toward you, its light will be ever-so-slightly bluer than if the star were stationary, since as it comes at you, those wavelengths of light are slightly squashed to shorter wavelengths, in the blue region of the spectrum. Similarly, as the star travels away, then the light is slightly redder, because by stretching light, the wavelengths become longer.
As the giant star wobbles around the common center of mass with the planet that orbits it, seen sideways from Earth, the star very slightly seems to approach and recede. This wobble introduces into its spectrum a subtle shifting of the light, bluer as the star comes toward you, red as is recedes. By accurately observing the changing spectrum of light, this Doppler shift method of detecting exoplanets, sometimes also called the radial velocity method, can be used to ascertain the mass of orbiting planets. Combine these data with information from the transit method, which also gives you the size of the planet, and we can work out the density of a planet, which is important if we are to establish its composition.
With all these bizarre new worlds, it would be easy to return to H. G. Wells’s speculations about silicon beings walking along the shores of liquid iron oceans. We could declare that now that we have found an apparently endless variation in forms and sizes of world
s, we must also be open to the same possibilities in biology. However, despite the plethora of physical features we might find on these planets—features that might suggest strange environments for alien life—we can bring to bear our discussions in this book on our assessment of how physical principles might shape any life on them.
From observing the spectra of light, we know that all exoplanets are made of the same elements in the periodic table. The knowledge that exoplanets are made from the same periodic table, an extraordinarily trivial point of fact (no one was expecting anything else), leads to many straightforward and immediate points. The same restrictions that make carbon better for all those complex molecules we observe in the interstellar medium, on Earth and in life, characteristics with their roots in the quantum world, apply on exoplanets as well. On any rocky world, we would expect silicon to be primarily in minerals and would expect carbon to be the preferred choice for building self-replicating, evolving entities. Thus, at the atomic scale, we are in a position to make some predictions about the potential structure of life on other planets, if it exists.
The fact that water is a common molecule in the universe means that it is likely to be the most common liquid to slosh around on the surface of rocky worlds around distant stars. It may be mixed in with other compounds such as ammonia and, under enough pressure, liquid carbon dioxide, but it is an abundant solvent likely to stare any would-be biological experiment in the face from the moment of its inception.
Even the energy that other living beings might gather from the free energy in electrons must be the same sort of energy sources we observe on Earth. The electron donors and acceptors in the periodic table or in the compounds made from universal elements are the same anywhere across the universe. They are not an idiosyncrasy of Earth; they are mandated by the thermodynamics of the elements and compounds made from them. Temperature and pressure conditions will alter the thermodynamic plausibility of given reactions, and the abundance of different compounds will change what sources of energy are available to life, just as these variations occur across Earth. We cannot necessarily predict which types of energy sources will be more favorable or prevalent on and within a given exoplanet without some substantial knowledge of its geochemistry and geophysics. However, the reactions available from the tool kit of the periodic table will be the same.
The Equations of Life Page 26