Toward Distant Suns
by T. A. Heppenheimer
Copyright 1979, 2007 by T. A. Heppenheimer, reproduced with permission
Table of Contents
Chapter 1: The Planetary Hangup
In the beginning there were stars and the void, and together they formed the Galaxy. And the Galaxy was not without form, nor was it featureless, nor was there darkness upon its face. In this time of beginnings the Galaxy was already approaching an age of 10 billion years. It had a mature, well-developed form, with a central bright nucleus and a disk of stars, the brightest of which were bunched into long curving lanes.
Then as now, the Galaxy was unimaginably rich and varied, alive with the pulsing processes of astrophysics, continually stirred and changed by stellar upheavals. As in the sea with its ceaseless tides and waves, storms and winds, and profusion of living creatures that exist for their season, be it for mere days or for many years, so it is with that greater sea, the Milky Way. If we think of the heavens as unchanging, it is merely because our own season is not yet of galactic proportions. We measure our days by the turning of our planet, our lives by its motion around the sun, but the days of the Galaxy are measured by its own turning motion, which takes two hundred million years.
Nor was the Galaxy made up only of stars. A thin, rarefied dispersion of hydrogen pervaded throughout. To illustrate the awesome scale of the Galaxy, one need only compare the concentration of this gas on a mountaintop—where air grows thin and is hard to breathe but which still has ten million trillion molecules in each cubic centimeter—to that throughout most of the space that was then between the stars, which was at less than one atom per cubic centimeter. In places it gathered into patchy clouds, and there it was richer, denser, possibly reaching all of one hundred atoms per cubic centimeter. Yet even in such a rich cloud, a volume the size of Earth would have held only four hundred pounds of matter, less than can be packed into the trunk of a car. But so vast was the Galaxy that all its gas together may have amounted to the mass of thirty thousand trillion earths.
There were waves within the galactic sea of stars in those days like there are today, which are called spiral arms. They give form and beauty to photos of galaxies, but their significance is far greater than that. It was the spiral arms, the spiral structure of the Milky Way, that formed our sun. To say that spiral arms are like waves in the sea is indeed to describe a little of how they behave. Waves cross the ocean and wash up on shore, but the ocean itself does not flow to the beach. It remains in its basin, and the waves merely ripple its surface.
Similarly, spiral arms are produced by disturbances that pass across the starfields. These disturbances are known as density waves because they cause stars to cluster together more thickly, or with greater density, than in regions where these waves are absent. Density waves emanate from the core or nucleus of some galaxies, a bright and active region where millions of stars may be concentrated in small areas of space. These stellar concentrations tend not to be regular in form, but to gather in an elongated pattern known as a bar. Impelled by the rotation of the galaxy, the bar rotates, its gravity producing a spiral-shaped pattern of disturbances in the galactic disk. These disturbances influence the motion of stars, causing them to stay near the spiral pattern for fairly long periods of time before leaving. The resultant clusterings of many stars perpetuate these gravitational disturbances indefinitely, producing permanent spiral arms. These then are regions of gravitational attraction, which travel around a galaxy but do not actually transport the stars. Instead, the spiral arms form a pattern that attracts new stars as it advances, while other stars leave.
Spiral arms also attract masses of interstellar gas. Amid this gas are the rich, dense (by cosmic standards!) clouds, extending for tens or hundreds of light-years. These clouds contain the mass of ten thousand stars, yet they do not collapse under their own gravity. Their internal pressure supports them, just as a tire full of air under pressure will support the weight of a car. Ordinarily these clouds behave somewhat like clouds in the sky—they form, accumulate, break up, and are slowly pushed here and there upon the cosmic tides. Occasionally, though, such a cloud grows dense enough to approach the point where it would overcome this pressure, collapse of its own weight. When such gas enters a spiral arm, it may receive the extra compression which triggers this collapse. The cloud then compresses under its own gravity and breaks up into a myriad of smaller clouds. As these in turn collapse and compress, they form a shower of stars.
The largest of these are hot, bright, and blue, ranging up to thirty or fifty times the mass of the sun. They form so soon after the arm passes that they illuminate its position in space, marking its passage with their brilliance. But it is this very brilliance that is their doom. The hydrogen from which they form serves as fuel for their thermonuclear fires, and in the most luminous stars, it doesn’t last long—a few million years at most. When most of the hydrogen is gone, a core of helium is left. The star then contracts, increases its internal temperature, and calls on the helium as a source of energy. The helium responds and undergoes nuclear fusion, releasing energy while forming still heavier elements.
By that time, though, the star is in deep trouble. It no longer possesses its once vast reserves of nuclear fuels; instead it faces an energy crisis. The heavy elements are not only poor sources of nuclear energy, they are difficult to ignite. This is particularly true of carbon. A massive star may fuse its helium to develop a core of carbon, which then heats and compresses as the star further contracts. But instead of igniting and fusing to release more energy, the carbon may detonate. The result is a stellar cataclysm, a supernova explosion. For a few weeks the exploding star will shine with the brilliance of the galaxy itself, hurling its matter outward, back into the primordial gas from which the star originally formed. Yet in this stellar death there is the promise of new life. Amid the supernova gases are the heavy elements formed prior to and during the explosion, from which planets may form.
About 5 billion years ago there was a cloud of gas within the Milky Way, differing little from the many that have existed before or since, with one exception: It contained atoms that in time would create you and I. This was the protosolar cloud. We cannot say anything of its form, its extent, or its age, but it is believed that some 4.7 billion years ago, with formation of the Solar System still 150 million years in the future, a spiral arm passed through the cloud and triggered the collapse of a portion of it. This portion formed a massive star, which shone brilliantly for a brief season—10 million years—and then exploded. This explosion enriched other portions of the cloud with heavy elements formed within that star, but did not otherwise influence the cloud. In its massive and quiescent grandeur, the protosolar cloud continued to exist.
Then 100 million years after that explosion, perhaps 50 million years after its debris reached our vicinity, another spiral arm passed through our cloud. Again it formed a new generation of massive stars, one of which was only sixty light-years from what would become the Solar System. This star too evolved and exploded, producing still more heavy elements to enrich the protosolar cloud; but it did something more. Like the immense hydrogen bomb that in fact it was, the supernova detonation sent a shock wave coursing through the cloud. It was weaker than the spiral arm and would have had little effect, except that the explosion was so close—a mere 350 trillion miles or so. Just as the shock from a hydrogen bomb can cause a building to collapse, so this shock caused parts of the protosolar cloud to collapse.
The supernova sent tongues of gas penetrating deep into the protosolar cloud, seeding it with yet more of the elements of life, and in due time the gestation of Sun and Solar System was under way. The astrophysicist refers to these intrusive gaseous tongues as the result of a “Rayleigh-Taylor instability” arising from the interactions of masses of gas, but we may think of it as the cosmic kiss, the interstellar sexual act that conceived our world.
How can we know of such things? In the mid-sixties, scientists at Berkeley found traces of the gas xenon in some meteorites. There was an excess of the isotope xenon 129, which proved to have formed from radioactive decay of another isotope, iodine 129. Four heavier xenon isotopes were also in evidence: Xe-131, -132, -134, and -136. The proportions present showed they had formed from fission of a different radioactive element, plutonium 244. Since these radioactive isotopes decay at known rates (17 million years for the half-life of iodine 129 and 82 million years for plutonium 244), it was possible to determine that the plutonium and iodine were formed no more than 100 million years before the meteorites. These elements are found only in the cores of supernovae, so they thus represented telltale evidence that there had been just such a stellar explosion.
In 1969 a meteorite fell near the Mexican town of Pueblito de Allende. Known as the Allende meteorite, its fragments were rushed to laboratories at Caltech and the University of Chicago, which had sensitive equipment for the study of moon rocks. The key findings came out of Caltech, from a laboratory with the formal name of The Lunatic Asylum. Only a very good scientist could get away with such a name, and indeed its two most senior inmates, Gerald Wasserburg and Dimitri Papanastassiou, are so well regarded that their colleagues count it quite an honor if they can get an invitation to work there. One who got such an invitation was Typhoon Lee. It was Lee who detected excess quantities of the isotope magnesium 26 in small grains within Allende. This most likely was produced by decay of another radioactive isotope, aluminum 26, which also must have come from a supernova. In contrast to the longer-lived iodine and plutonium, aluminum 26 decays very rapidly; its half-life is 720,000 years. This means it could have formed no more than a few million years before the Allende meteorite. The most appealing explanation is that the same supernova that formed the aluminum also was a second supernova, which triggered the formation of the Solar System. The conclusion of science is that from delicate and subtle studies of tiny grains in meteorites we can see evidence for important events leading to the origin of worlds.
Yet perhaps there is more. Supernovae are rare; perhaps one star in a thousand will detonate in that fashion, turning the nearby skies to fire and incinerating its planets in titanic seas of flame. Even though supernovae can serve as agents of planets’ birth, at the moments of explosion they are the fulfillment of the prophecy of the Bhagavad Gita: “I am become Death, the destroyer of worlds. ” It is not the usual thing that star formation is triggered by a supernova. Is there in this then a uniqueness of our Solar System? Did this supernova, in triggering its formation, endow it even before its birth with additional quantities of heavy elements, and perhaps with other features as well, rendering it more suitable as an abode for life? When that supernova lit up the Galaxy, was it a primordial Star of Bethlehem, which announced not the salvation of mankind but rather its ultimate emergence?
Yet this cannot truly be so. If it took much longer for stars to evolve to supernovahood than for a collapsing gas cloud to form its complete retinue of stars, it would be a rare matter of chance for a supernova to trigger star formation. By the time the largest of the new-forming stars had evolved and exploded, there would be no more left of the placental cloud from which more stars might form. But in fact the collapse of a cloud is far from a quick or uniform event; large parts of it may resist collapse, or else compress only slowly over millions of years. There is ample opportunity for part of a cloud to collapse, form stars, give rise to a supernova, and thus trigger the collapse of still other regions of the cloud. However, stars do not necessarily stay in the environs where they were born, but often move long distances. By the time a bright star is ready to explode, it may be far away from the remnants of its cloud.
We do not know. We do not yet understand how important supernovae are in triggering the formation of stars like the Sun. Most solar-type stars form almost certainly without this intervention. Our own case has probably been repeated again and again to form worlds unknown. There may be some degree of uniqueness in our Solar System being fathered by a supernova, but how much so we cannot say. Yet how piquant it will be if it is found that virtually all sunlike stars arise quietly, gently, following passage of a galactic density wave, but that we have the special distinction of a system rendered particularly favorable to life from having been conceived in violence.
These earliest phases of the Solar System’s history are shrouded in questions unanswered, but the next stages in its formation are much better understood. At an early date, the matter of the Solar System gathered itself together under its gravity and separated itself from the rest of the protosolar cloud. It was somewhat as though this cloud were a glassful of water tossed out a window, which separated into drops; one of these drops was the solar nebula, the direct antecedent of Sun and planets. The solar nebula also proceeded to contract, to compress itself due to its gravity. As it did so, its rotation, which it had inherited from the Galaxy, speeded up, forcing its matter to spread out to form a disk, perhaps ten billion miles across. In form it resembled the Milky Way, but was a hundred million times smaller.
By no means was this disk a steady, stable structure like Saturn’s rings. It was much more like the atmosphere of Jupiter—wildly turbulent, driven by raging storms such as no mariner ever faced. Strong currents resulting from variations in pressure and temperature stirred it vigorously. New nebular material, itself turbulent, continually fell upon it, agitating it even more. Parts of the disk contracted or condensed further under gravity, producing large gaseous blobs, which moved through the primitive solar nebula and produced still more turmoil.
This tumultuous activity produced movement resembling the cracking of whips. A whip is long, flexible, tapering; when we crack it, we start with a rapid motion at its thick end. This motion produces a wave, which moves along its length. As the wave advances, the decreasing thickness of the whip makes the wave speed up. By the time the wave approaches the end of the whip, the thinness of its tip has brought the wave to and beyond the speed of sound. The crack of a whip then is actually a sonic boom produced by the supersonic speed of its tip, the first man-made object to exceed the speed of sound.
In the solar nebula the turbulent motions produced waves that traveled toward the surfaces of the disk, where the gas was less dense. As these waves advanced into thinner gas, they sped up; and when they passed the speed of sound, they became shock waves. As these shocks penetrated toward the disk surfaces, they became so powerful that they blasted nebular material back into space. In this fashion the disk lost mass. It evolved from a state containing as much matter as the Sun to a disk that had only perhaps one-thirtieth that amount, or thirty times the mass of Jupiter.
In this condition there was, at last, a reasonable degree of calm. The nebular disk was now too rarefied to sustain its former storms and turbulence. Now, for the first time, solid grains of dust, droplets of ice, and particles of rock could form. These condensed within the nebula, just as if they were hailstones condensing within a cloud. And like hailstones, they rained downward—not into the growing protosun, but to the midplane of the disk. These particles formed a thin disk within a disk, concentrated midway between the upper and lower surfaces of the gaseous nebula. The solar nebula thus resembled a hamburger sandwiched between two buns, or a phonograph record midway between two Frisbees.
This dust disk was the direct antecedent of Earth and the planets. Once it had formed, it behaved somewhat like a thin film of water, which beads up into many small droplets due to surface tension. The dust disk had no surface tension, but it did have gravity. It broke up into numerous small regions, and in each of these regions gravity caused the dusty material to coalesce. In only a few thousand years, the rocky or icy material of the Solar System was gathered into small bodies and planetesimals, which in the vicinity of what would be Earth were a few miles in diameter.
The planetesimals in turn bumped and jostled together as they went along, each to its separate orbit. Where they collided at high speed—a hundred meters per second or more—they tended to fragment, to shatter one another back to the primordial dust from which they formed. But at lower collision speeds, they tended to stick together. In this fashion many of the planetesimals gathered together, forming larger bodies. Eventually, in each of several regions of the Solar System, one such body grew larger than the rest. The transition from planetesimals to planets now was unstoppable.
At some time during these events, Jupiter and Saturn formed. They may have begun as rocky or icy cores, larger than others, which attracted much gas from the surrounding nebula because of their gravity, or as condensations or accumulations of gas within the nebula, which attracted many planetesimals from surrounding space. We do know that they formed quickly, during a time of no more than a few million years, and before the Sun itself was well formed. For in all of this, there was as yet no true sun.
In the inner regions of the disk, gases collected to form a massive concentration. As this concentration developed, it lit up with the first solar fires. These were not the true thermonuclear fires of stars, but rather the heat produced as the protosun compressed itself under gravity. As it compressed, it generated very strong turbulence in the inner regions. As in the cracking of a whip, this turbulence again sent shock waves coursing through the outermost layers of the protosun, blasting great quantities of material outward. In this fashion there flowed outward a steady stream of gases, at the rate of several million trillion tons per year. This stream scoured the Solar System, cleansing it of dust and gas, and removing what was left of the solar nebula. Jupiter and Saturn now would grow no more.
After some tens of millions of years of this so-called T Tauri phase, the Sun’s interior finally reached the temperatures and pressures at which thermonuclear reactions could begin. The Sun now atoned for its turbulent youth by beginning a ten-billion-year career as a steady, reliable source of energy. In the newly cleared Solar System, warmed now by this new star, the final accumulations of planetesimals and protoplanets were swept up by growing planets or scattered by Jupiter to yield the last stages of planet growth. This was a final flurry of violent action, as the impacting bodies tore great craters and basins in the young planet surfaces. Then even this subsided. The violent energies were spent; the clashing upheavals had run their course. There was peace among the planets.
Astronomers disagree endlessly over the details, but many would agree that the origin of the Solar System took place more or less as described. Then, is there anything special or unique in this? Did our Earth sidestep some pitfall that must have befallen many other forming planets, so that our Sun gained a retinue of planets where it might well have had only a scattering of dust? The answer is yes. There is good reason to believe that rocky planets like Earth are far from usual, that there is a “planetary hang-up, ” which causes most stars to form without such companions.
This hang-up begins in the stage when the solar nebula first forms a disk. Such disks are known to be unstable and tend to form the shape of a bar. As the nebula further evolves, the bar can divide in two, forming a binary star. Such binaries are indeed quite common, and can take various forms. If one of the companions has more than one-sixteenth the mass of the Sun, it will shine like any star.
If its mass is between one-sixteenth and one-hundredth the Sun’s, though, it will not succeed in lighting internal nuclear fires but will glow with the feeble heat of gravitational compression. This glow continues for a billion years, then ceases; and the star thereafter persists as a “black dwarf, ” a massive body which emits no light [Author’s footnote: not to be confused with a black hole, which is hundreds of times more massive]. If it has less than one-hundredth of solar mass, it already is no more than ten times the mass of Jupiter. Such a body glows only briefly, if at all, and we would call it a planet.
In 1976 the astronomers Helmut Abt and Saul Levy used the 84-inch telescope at Kitt Peak, Arizona, in a careful study of stars similar to the Sun. They examined 123 such stars, all within 85 light-years of the Sun, and all visible to the unaided eye. They were not the first to examine these stars, but they had better equipment than earlier astronomers, so their results were more complete. They concluded that of these 123 stars, 83 have binary companions that are true stars. They were unable to get good data on the existence of black dwarfs and planets, but they suggested there would be 20 black-dwarf companions, and 25 companions that would be planets resembling Jupiter. In other words, it appeared likely that all 123 sunlike stars have companions, ranging from Jupiter-size to sizes similar to the Sun itself.
What difference does that make? The answer is that such companions tend to prevent the growth of planets such as Earth. If a star has a binary companion (and most do), then it will be most doubtful that small rocky planets can form. This is the planetary hang-up, and we ourselves nearly fell victim to it. Astrologers speak of the influence of Jupiter over the lives of mortals, but their speculations are mere trifles. The truth of the matter is far more chilling: Had Jupiter been only slightly larger or nearer, or on a slightly less regular orbit, Earth as we know it would not exist.
Recall those early days when the matter of Earth was bound up in billions of small planetesimals, which had to collide and stick together if proto-earth was to grow. To do this, it was essential that conditions be steady and even, that the planetesimals be kept on orbits that were very nearly circles. If their orbits departed from this, and became even slightly elliptical, the planetesimals would collide at speeds too high to stick together and instead would tend to shatter. As long as there was no external disturbance on the planetesimal orbits, they could maintain their circular orbits, and the growth of planets would proceed apace. However, only a few hundred million miles away was Jupiter.
Jupiter’s orbit was elliptical, and Jupiter, pulling with its gravity on the planetesimals, tended to shift these small bodies onto orbits that were also elliptical. This effect was so strong that had there been no countervailing effects tending to reduce Jupiter’s influence, Jupiter would have prevented planetesimals from combining anywhere in the Solar System.
Fortunately, the presence of the solar nebula tended to soften or attenuate Jupiter’s influence. Each planetesimal responded to the gravity not only of Jupiter but also of the nebula itself, and to the extent that Jupiter had only a small fraction of the nebula’s mass, Jupiter’s effect was reduced to a small fraction of what it would otherwise have been. Since the nebula took time to dissipate, the planetesimals were granted a reprieve. As long as the nebula existed, Jupiter’s disruptions were held at bay. Still, the nebula’s presence was far from a foolproof security. When the nebula mass fell below about ten times Jupiter’s mass, its attenuations became insufficient to keep Jupiter from disrupting growth in a region outward from about three times Earth ‘s distance from the Sun. Some time later the nebula lost more mass and had no more mass than Jupiter; then the disruption of growth crept inward as far as Mars. Not long after, it was the turn of Earth to feel the disruptions; but by then, fortunately for us, there had been enough growth to permit Earth to form in spite of Jupiter. The nebular reprieve had lasted long enough.
Still, Jupiter was not through yet. In its very act of dissipating or reducing its mass, the nebula, which heretofore had acted to weaken Jupiter’s influence, now introduced a new effect. It cooperated with Jupiter, and in so doing, produced a new threat to the growth of planets.
To understand this effect, we must think of an orbit as an ellipse, a flattened shape similar to an oval, which points in some definite direction. We may recall the game of Spin the Bottle, and imagine the bottle is ellipse-shaped. The direction to which an orbit points is not fixed, but slowly changes as the orbit shifts; we call this shift a precession. All orbits precess, usually at different rates, due to the various gravitational tugs they experience from different planets. Ordinarily precession causes no problems.
If a planet should find its orbit precessing at the same rate as Jupiter, however, something new will happen. The planet’s orbit, even if originally a perfect circle, will become elliptical. It will become more and more noticeably so for as long as its precession is matched to Jupiter. If this lock is not broken, the orbit may become so strongly elliptical that the planet will cross the orbits of other planets, so that eventually it would be destroyed in a collision of worlds.
What controls this precession? For Jupiter and Saturn, it is mostly the gravity of one another and, in early days, that of the nebula. For a planet, or for the planetesimals from which it might form, the precession is controlled mostly by Jupiter, Saturn, and the nebula. The importance of the nebula depended on its mass. As this mass diminished, at any distance from the Sun there would be some special value for the nebula mass that would make the precession rates of local orbits match that of Jupiter. What would happen next would depend on how fast the nebular mass was changing. If the change was slow, the precession rate would stay close to Jupiter’s for a long time, and an orbit would become elliptical indeed. Only if the change was fast could orbits stay nearly circular.
Now let us venture inward from Jupiter’s orbit, which is 483 million miles from the Sun, and examine the damage done by these effects. For the first 150 million miles or so we find virtually nothing more than empty space. There may once have been bodies orbiting here, but they apparently were swept up or ejected by Jupiter. This fact is ominous, for only if their orbits were markedly elliptical could they have suffered such a fate.
From about 320 to 200 million miles from the Sun, we find a collection of orbiting bodies, the asteroids. Here are no large planets or regular, nearly circular orbits. Instead, here are thousands of small objects, none of which are more than a few hundred miles across; most are much smaller. Their orbits are very elliptical, several times more so than Jupiter’s. Even more telling, the total mass of asteroids is small; some one ten-thousandth that of Earth.
Here amid the asteroids we see the wreckage, the devastation wrought by Jupiter. For a brief halcyon time, the nebula was massive enough to keep Jupiter at bay so that planetesimals could begin to combine and grow. The early dissipation of the nebula allowed Jupiter to interrupt this growth. Then as the nebula slowly continued to dissipate, the precession effect moved with savage force amid these small, weak bodies, wrenching their orbits into ellipses. With this, the asteroids collided full tilt, at speeds of several miles per second, returning many of them to dust. This grinding-down or shattering continued till there were simply too few asteroids left to collide very often. Of the rest of them, there was nothing left but dust blowing in the solar wind.
We leave these scenes of carnage and destruction and proceed farther sunward. At 140 million miles from the Sun is a planet, Mars. Yet is it really a planet, or merely the largest of the asteroids? It is so small and shrunken, only a tenth the mass of Earth, and its orbit still is much more elliptical than Jupiter’s. Evidently the planet-forming processes went further here, but in the end again it was Jupiter that carried the day.
Finally, 93 million miles out, we do find a true planet—large, fully formed, on a nearly circular orbit. Evidently, in Earth we have at last reached beyond the effects of Jupiter, have entered a region where the king of the planets ceases to reign. But. . . .
We are already four-fifths of the way from Jupiter to the Sun!
We look back over our shoulder at Jupiter and shudder. It was so close, such a near thing. These disruptive effects, which penetrated with full force so far sunward, depended only on Jupiter’s mass and orbit and on the history of the nebula. When compared to other binary-star companions, Jupiter is so small, its orbit so much more nearly circular than most others. Had Jupiter been slightly larger, nearer to the Sun, or more elliptical in its orbit, then Mars today might be merely a scattering of asteroids and Earth a small, shrunken, wizened planet like Mars. We may think of the mythology of the Greeks, and be thankful that mighty Jupiter did not hurl its gravitational thunderbolts far enough to harm our planet. It was a very near thing.
Yet while we might search star after star and find nothing but binary companions and scatterings of asteroids, still there is hope. It is no accident that the orbits of Jupiter and Saturn are as nearly circular as they are, for these planets grew by accumulating gas from the nebula. As they did so, their orbits became more circular.
Moreover, many stars form not as binaries but as members of triple or multiple systems. These stars then follow strange, unusual orbits, which may result in the ejection of the smallest of them. They are literally thrown out due to close encounters with the gravity of their fellows. In just this fashion were the spacecraft Pioneer 10 and II ejected, following close approaches to Jupiter that gave them enough energy to escape. If a star is ejected early enough, it will still have the form of a solar nebula, and its evolution may proceed free of disruption from a binary companion or even perhaps of a large planet or black dwarf.
There is a third possibility. A king may wreak havoc with his army, but in his own castle there is calm. Similarly, Jupiter, disrupter of worlds, has in its immediate vicinity a most regular and stable collection of large satellites, which could well be regarded as planets in their own right. There may be many such systems of worlds, dancing attendance upon the binary companions of sunlike stars. It may be that what the binary companions take away via the planetary hang-up, they then return by forming planetlike bodies in their immediate neighborhoods.
Even so, the existence of the planetary hang-up gives us reason to appreciate Earth’s uniqueness. We thus may continue onward in meditating upon our significance in the Galaxy; we may look farther, probe more deeply, and in particular we may consider the origins of life. And as we do so, we may remember the words of Paul of Tarsus: “We know in part, and we prophesy in part. . . . Now we see through a glass, darkly.”
Toward Distant Suns Chapter 2 Table of Contents