Starting
at the beginning of our journey, I believe that the human child,
looking at the world beneath his feet and the sky overhead, and
wondering about it, is as good place to begin it as any. The child
is not merely curious about the color of the daytime sky (see
Appendix B) of course, but also the blackness of the night. But it
is not just the blackness which attracts him but the things that
shine within it and take that otherwise monotone of darkness away,
revealing the full glory of the universe through the tapestry of
night.
When the child gazes upwards at the night sky he sees not only the blackness but the many myriad of points of lights sprinkled against that inky backdrop. How many and how bright the points are depends upon from where he gazes, but they are spectacular nonetheless. Some of the lights stand out as special, or so he notices if he is keen enough an observer over many nights. They are generally brighter than the rest and appear to move slowly, thought stately, across the sky. Some appear only in the evening or the morning skies, while others traverse the entire arc of night, yet are not always there. They appear to ride, more or less, along a single line, which is known to astronomers as the ecliptic. The ecliptic, though our child does not know it, is simply the path the sun takes throughout the sky as a result of Earth orbiting her. The special lights seem to trail in her wake like fireflies.
These lights have special names. The ancient Greeks called them planetes, or wanderers, from which we derive the term planets. Of those visible to the naked eye, they include Mercury, Venus, Mars, Jupiter, and Saturn. One might add the moon and the sun to the list, and speak of the seven stars (although only one, the sun, is truly a star) as did the fool to the king in King Lear: (Fool: “The reason why the seven Starres are no mo then seuen, is a pretty reason”. King: “Because they are not eight.” Fool: “Yes indeed, thou wouldn’t make a good Foole.”).
What are these wandering “stars” that stand out so spectacularly in the night sky, and why do I begin my journey there? Certainly, they have piqued our curiosity as much as anything else in the natural world. The child eventually learns that the reason they wander against the seemingly fixed backdrop of stars is that they are members of our own solar system, planets in their own rights like our own, and so are relatively close by compared to the stars. He – by which, I mean I – also learns of two more planets, discovered over the last two hundred plus years, mighty in their own respects but too dim to be seen clearly from here on Earth: Uranus (which actually is just barely visible under the most optimum seeing conditions) and Neptune. And indeed, Earth we stand on is a planet as well, which we would easily see from the sky of any of the others should any of us be fortunate enough (and I believe some of us will have that fortune) to stand on one of them some day. But from whence does this knowledge come?
To the ancient Greeks and Romans the planets were the gods who inhabited Mount Olympus, but when people began in earnest to explore the natural world within the last few centuries, men like Galileo first pointed his primitive telescopes at them and discovered, lo and behold, that they were not the bright points of light the stars remained under even the highest magnifications, but showed clear discs which nobody, even the Catholic clergy of the day, could deny. Some of them even had small bright objects (which we call moons, or satellites) orbiting them, while others, such as Venus and Mercury, showed phases much like our own moon. The results of these simple observations was that the Earth-centered universe of Ptolemy was forever and at last obliterated and a new model of the heavens had to be found, one in which the Earth and the other planets circled the sun and not the other way around (though, as noted, some objects also orbited the planets themselves, such as our own moon or the four satellites orbiting Jupiter that Galileo discovered).
For me, the planets, and their moons, and the myriad other bodies of rock and metal and ice which form our solar system are such a marvelous beginning along our quest into curiosity, if only because so much has been learned about them in my lifetime. Also, as a boy it seemed my clear mandate to become an astronomer when I grew up (instead of the chemist and general scientific dilettante I actually became), so the night sky held a special fascination, perhaps because more than anything else it made me realize just how inconceivably vast the very concept of everything is.
In the early 1960s, when I was a small child just learning how to read and write, very little was known about the other worlds which inhabited our solar system. What was known was largely from the blurry images of ground-based telescopes and the simple spectroscopic and photographic equipment which was all that was available then. We also had some information from microwave and radio astronomy. So we knew some basic stuff; for example, that Jupiter and Saturn were huge gas giant worlds, Uranus and Neptune more modest gaseous worlds (still considerably larger than the Earth), that Mercury was almost certainly a sun-baked ball of rock as tidally locked to the sun as our moon is to Earth – indeed, it was probably a slightly larger version of our moon. Of Pluto, discovered only in 1930 by Clyde Tombaugh, virtually nothing was known for certain, even its mass and size. Finally, of all these worlds, only thirty two natural satellites were known, with essentially nothing known about any of them except that Titan, Saturn’s largest moon and probably the largest moon in the entire solar system, was the only one showing evidence of a substantial atmosphere, the nature of which was little more than speculation.
* * *
You could not fail to notice that I have overlooked two planets, the two of our most particular interest at that. Venus and Mars capture our imaginations and hopes precisely because they are the nearest worlds to our own, and thus, or so we thought / hoped, were also the nearest in their natures. Even without the benefit of interplanetary probes and the crude, atmosphere befogged instruments we possessed circa 1960, we could see how much promise they held. Rocky worlds like our own, with substantial atmospheres and possibly decent living conditions as good if not better than ours (albeit Mars probably on the cold side, and Venus a tad warm even for the hardiest frontiersmen), they invigorated our imaginations with tales of life and even intelligent beings which even the most skeptical could find believable. Percival Lowell could convincingly describe the “canals” he was certain he spied on Mars (a word which, in fact, is a mistranslation of the Italian word for channels, which their original, equally deluded, discoverer Schiaparelli called them), and the civilization which built them to keep their dying world alive was so believable that when Orson Welles broadcast H. G. Wells novel The War of the Worlds in 1938 thousands were panicked, convinced the invading Martians were all too real. Even well into the 1950s and 60s it was possible to populate the Red Planet with sentient beings with little in the way of scientific rebuke, as Ray Bradbury did in The Martian Chronicles.
Venus somehow inspired less creativity than Mars, perhaps because the dense foggy atmosphere that perpetually hid its surface from view made it seem less hospitable, at least to advanced, intelligent life such as our own. Still, and despite microwave measurements which suggested the planet too hot to be amenable to any life, legions of minds had no problem envisioning all sorts of exotic scenarios for our “sister” world (unlike the smaller Mars, Venus is almost exactly the Earth’s diameter and mass). From vast, swampy jungles, to an ocean-girdling world, to thick seas of hydrocarbons larger than anything on Earth, Venus was often envisioned as a planet as alive as our own.
* * *
All of these visions seemed plausible, even compelling, to imaginative minds right up until the 1960s, when the initial phase of the Great Age of Planetary Exploration blew them all into dust. We – or more precisely our robotic probes, launched into interplanetary space by Cold War ICBMs designed to drop nuclear bombs onto cities teeming with human life – learned that Venus was a searing carbon dioxide encased hell, hot enough to melt lead and with a surface atmospheric pressure equal to almost one kilometer beneath our oceans. Forget life: even our hardiest robots barely lasted an hour under such conditions. As for Mars, our smaller, brother world turned out to be positively welcoming in comparison to our sister, but Schiaparelli and Lowell were shown to be hopeless wishful thinkers; there were no civilizations, no canals, no sentient beings, no beings at all, not even simple plants.
Sometimes what curiosity discovers is that imagination has overreached itself. This is often considered to be curiosity’s downside; I suspect that much of the antagonism towards science comes from just this fact, that in collecting data about the universe we are to some degree destroying our creativity. Thinking about this complaint, I have come to the conclusion that it is not an entirely unfair one. Why do I say this? Because it is true, that in satisfying our curiosity we narrow the range of what “could have been” down into what is, and that is a real loss to real human beings in the real universe. There is no denying this.
At the same time, however, there is an opposite phenomenon which has to be added to the stew. In satisfying our curiosity, we just as often – indeed, perhaps more often – find that our imaginations have been in fact impoverished. It turns out that “there are more things in heaven and earth” than we ever came close to dreaming; that the ocean of actual realities extends far beyond the limiting horizons, out to lands and seas and possibilities we never suspected were out there. The reason I started this chapter with what the last forty years of planetary exploration has found is that nothing could be a better example of this discovery process in action.
Take Mars. The first Mariner photographs were crushing disappointments. Far from being a verdant world, the Red Planet looked more like our moon: crater-pocked, barren, lifeless. There were no signs either of life or any kind of intelligence. Even the atmosphere was less than what we’d hoped: a bare one percent of Earth’s surface pressure, and worse, composed almost entirely our of carbon dioxide, with no free oxygen or water vapor.
But those were just the initial impressions. More Mariner missions, two Viking orbiters and landers, and a slew of other robots hurled at Mars over the last twenty years, not to forget images from the space-based Hubble telescope, have shown it to be a world even more remarkable than we had thought. For one thing, there are amazing geological structures, some of the largest in the solar system: Olympus Mons, the giant shield volcano, is larger than any mountain on Earth several times over, and Cannis Marineris, a Grand Canyon like our own but which would stretch the entire breadth of North America. As for life, Mars now is probably (but not certainly) dead, but it once clearly once had all the elements for life, if several billions of years ago: a thicker atmosphere, warmer temperatures, flowing surface water, a likely abundance of organic or pre-organic molecules. The photographic and chemical evidence, returned from our probes and telescopes, have shown us this past and opened the door to our understanding of it. With some hard work and a little luck, in the coming decade or two we will finally have the answer to the question of whether life on Earth is unique or not, and, by implication, is common in the universe or not. Or if not, why not. Either way, at the very least the ramifications for our own existence are staggering.
This alone could justify the time and energy, and money, spent to satisfy our curiosity about other worlds. But this turns out to be just the beginning. The solar system’s biggest surprises have come in the exploration of the outer planets. It turns out that we knew pathetically little about these worlds and their moons, or the forces that have shaped their evolution. We had a few hints, but we mostly dwelled in ignorance and speculation. Starting in the late 1970s with the Pioneer 10 and 11 missions, then the Voyager and other probes, that ignorance was stripped away in the most spectacular fashion. Pioneer and Voyager returned pictures of worlds far more dynamic than what we had expected, in ways we had not foreseen.
Consider tides. Here on Earth the tidal effects of the moon and, to a lesser degree, the sun, make our oceans rise and fall in gentle cycles. The reasons for tides is a straightforward application of the inverse square law of gravity: the closer two objects are to each other, the more strongly they are pulled together, and so the faster they have to move in their respective orbits to avoid falling into each other. The net result of this dynamic is that the near sides of such objects are moving too slowly and try to fall together, while the far sides are moving too fast and thus want to pull away. On Earth, that means that the oceans on the side facing the moon fall toward it ever so slightly, while the oceans on the opposite side try to drift away. It is a very humble effect, just a few feet, or tens or feet, either way. Nothing to write home about.
Tides can do much than rock the seas of a world, however. The rock comprising Jupiter’s innermost large moon, Io, is largely molten, thanks to the heat generated by tidal forces by both the parent planet Jupiter and the other Galilean satellites. The result is the most volcanically active world in the solar system by far, not excluding Earth. Io’s surface is liberally pocketed by volcanic calderas of all different sizes, which spout sulfur and other molten minerals tens to hundreds of kilometers above and across its surface in a steady rain of debris; a surface so new that it contains not a single impact crater. If the tidal stresses in Io’s guts were just a smidgeon stronger than they are, the world would be literally torn apart by them. That indeed might be Io’s ultimate fate, to be fractured and rendered into a new ring for the giant planet.
The tides are cruelest to Io because it is closest to Jupiter, but they do not leave the other large moons at peace either. The next Galilean satellite, Europa, may prove to be the most intriguing place in the entire solar system outside of our own planet. I must make a brief digression to explain why. Most of the solar system’s matter does not consist of rock and metal but of light elements, such as hydrogen, helium, carbon, nitrogen, and oxygen, and their various chemical combinations – water, ammonia, methane and a variety of small hydrocarbons – chemicals composed of carbon and hydrogen. In the inner solar system these substances are largely in gaseous or liquid form, making it a challenge for the small worlds (including ours) inhabiting this region to even maintain a hold on them in the teeth of the sun’s fierce radiations and her perhaps fiercer solar wind (a steady stream of electrons, protons, and other particles constantly being blown out by the sun, which can easily blow away weakly held atmospheres) , but starting at the distance of Jupiter the sun’s output is diluted enough to let these substances condense into their solid phases: ices. Starting with Jupiter, ice is not merely a thin coating over rocky worlds and moons but comprises the bulk of these bodies. The most predominant of them is water ice, which at the temperatures prevalent in the outer solar system essentially is rock, albeit a low density kind.
The cores of three of the Galilean satellites, Europa, Ganymede, and Callisto, are normal rock like the inner, “terrestrial” planets’, but they are covered with mantles of liquid and solid water many tens to hundreds of kilometers deep. Europa in particular consists of a relatively thin skin of cue ball smooth water ice over an abyssal ocean far, far deeper than any sea on Earth. Again, it is the tidal kneading of Jupiter and its other moons which generate the internal warmth which keeps this ocean in a liquid phase.
Liquid water is one of the most important ingredients to life on Earth, so wherever else in the universe we encounter it we are also encountering the possibility of life. On Mars the presence of flowing water billions of years ago raises that possibility. What Pioneer and Voyager and later missions have done is show how parochial our thinking on this subject has been. The kilometers-thick water ocean beneath Europa’s and other satellites’ icy surfaces no doubt contain their share of organic and other pre-biotic chemicals, as well as free oxygen, and over the eons of being warmed and mixed in this lightless abode who can say what might have assembled itself? We know little enough about life’s origins here to make all kinds of speculation plausible, speculation that will be answered only by sending more and better probes to that world. By, in short, satisfying our curiosity.
Which leads me again to the most important lesson once again, which is in how in satisfying our curiosity we often broaden our perspectives, not narrow them as critics claim. In reaching out, we find more than we ever thought we would, and our lives become immeasurably richer. This is what our science, our passion to know, has given us.
* * *
The fundamental premise, and primary lesson, of science is that there are no magic fountains of truth. There are no books with all the answers, no machines to solve every problem, no authorities with all the answers, no voices in our heads, no golden compasses or other devices waiting to be opened to spoken to in just the right way. All we have are our own limited senses, our own seemingly unlimited minds, our own hard work and perseverance. And this we find true whatever our questions or whatever mysteries the universe puts before us. Actually, there are no mysteries either: there is only what we have not yet understood, because we have not yet figured out how to explain it.
So we press on resolutely, our feet on the ground and heads down but our eyes always facing forward. And we take the pleasure of learning what we learn, in the steps and pieces that we learn it. It is a process that is, at times, grim. But what it yields is pure treasure.
As amazing as the moons of Jupiter have turned out to be, you have to go out still further to find the most amazing moon of them all. The somewhat smaller planet Saturn and its entourage of satellites orbits the sun at a distance twice that of Jupiter’s and ten times further out than Earth’s from the sun. Still a glare too fierce to be gazed at directly, the sun only provides one percent of the warmth and light here that it shines down on us. Furthermore, the effects of tidal interaction between Saturn and its moons is not as potent a force as it is in the Jovian system: there are no raging volcanoes or vast underground oceans of liquid water (with one possible exception). If anything, compared to Jupiter, the Saturnian system would seem to be a quiet backwater where little of interest might be found. Yet something of the most enormous interest is found right here: Titan.
Titan was known to be unique long before we sent any robots to explore it. Unlike all other moons in the solar system, a star passing behind Titan (an “occultation” in astronomer language) will fade and twinkle briefly before disappearing completely, similar to the way the stars twinkle when seen from Earth’s surface. The reason for both phenomena is the same. Atmospheres will refract and scatter the light that passes through them. Titan is the only satellite in our solar system with a substantial atmosphere; one that is, in fact, considerably more substantial than our own.
This in itself would have made it an object worthy of our curiosity. Atmospheres are living things. They continuously grow and regenerate themselves lest they escape away into space, courtesy of the lightness of their molecules, the temperature, the strength of the solar wind, and other factors. They eventually dissipate when left on their own, though this may take billions of years. On Earth, for example, the nitrogen and oxygen which comprise ninety-nine percent of our atmosphere go through chemical and biological cycles which keep them ever fresh throughout geologic time.
Titan’s atmosphere is not only substantial, it is several times as dense as our own. Also, like Earth’s, it is largely nitrogen: ninety-eight point four percent of it is this gas, as compared to seventy-eight percent here. Even more interesting is the other one point six percent, which is largely hydrocarbons – simple, organic molecules – like methane and ethane. Thanks to the sun’s ultraviolet rays, which are still potent at this far reach in the solar system, these hydrocarbons have given rise to even more complicated molecules which comprise the orange smog which permanently hides Titan’s surface from all outside eyes. They also form the basis for clouds and various kinds of precipitation which rain down on this moon’s icy surface, forming the terrestrial equivalent of lakes and rivers.
As a possible womb for life, however, Titan has a problem. Its distance from the sun and shielding cover of hydrocarbon smog mean that the surface temperature here is almost three hundred degrees below zero Fahrenheit. This is so cold that even the nitrogen comprising the bulk of its atmosphere is on the edge of liquefying. Not only is the water so crucial to life on Earth completely frozen into a thick mantle as on other outer moons, but other molecules important to the life’s beginnings here, such as ammonia and carbon dioxide, would be rock-hard solids at these temperatures as well. Moreover, any chemistry which could happen would occur at a pace that would make a snail look like a jack-rabbit on caffeine. Looking over all these factors, biology would seem to be a non-existing subject on Titan.
We shouldn’t think so narrowly, however. Life does not require water so much as it needs some liquid medium, and as noted, compounds like methane and ethane, gasses on Earth, do exist in liquid form both on Titan’s surface and in its atmosphere. True, any biochemistry would proceed with agonizing slowness, but the solar system has been around for almost five billion years, and that might be just enough time for something to happen. We won’t find anything resembling a … well, even a bacterium is probably pushing it … on Titan, but some kinds of self-replicating entities – the most basic definition of life – might exist there. Or whatever could lead up to such entities under more favorable conditions. Either way, when we do find out, we will certainly learn some lessons applicable to how life came to exist on Earth, what that requires and what must be forbidden for that grand event to occur. All of which makes the time and energy and resources necessary to do the finding out worth it.
* * *
Our robotic exploration of the solar system has rewarded us with much more than volcanoes and canyons and possible new possible niches for life. For one thing, knowing about a place is often the first step to going there; it is certainly a necessary step. I call the last forty plus years the initial phase of the Great Age of Planetary Exploration, and there should be little doubt anymore that that is what it is. The twenty-first century will assuredly see us plant our footsteps on our neighboring worlds, the moon and Mars for certain, and the centuries to come will see their thorough colonization and exploitation.
What about beyond? We have come a long way in our travels in my lifetime, but at the same time, we have hardly begun to crack the door open. I loved astronomy as a child, but what excited me the most were not the planets but the stars. In reading about them, I learned that the stars were other suns like our own, possibly with their own worlds and God-knew-what on them, perhaps, one dared hope, some of them even people like ourselves: either way, it was and is an overwhelmingly staggering thought, especially when you contemplate how many stars there are.
Curiosity will eventually take us to the stars, but this is a journey that will take far longer and require much more resources than exploring our own solar system, because the distances involved are so much vaster, by a factor of a million and more. So much greater that it will change what it means to be human in some ways – though our passion to know will hopefully remain intact. We cannot travel to the stars yet, but their light comes to us, rains down on us in fact from every direction we look. And light is a code which, when unlocked, reveals a universe more amazing than dreams.
The six inch Newtonian reflector telescope I received for my eleventh birthday was a wondrous, magical device. With it, I could easily make out mountains and craters on the moon, view the planets as multi-colored discs along with their larger moons, resolve multiple star systems into their components, and in general enjoy many things of the nighttime sky which the naked eye alone can never see. And yet still the stars are so distant that they remained points of light in the blackness, brighter and more variously colored yes, but points nevertheless. Yet even had that telescope been more powerful a device, the miles of air and dust and water vapor I would still have had to peer through would have smeared my vision with unending twinkling and wavering, rendering it of maddeningly limited use. Even the simple question of whether other stars besides our own possessed planetary systems – and so, possibly, life and intelligence – would have been forever beyond its capacities.
The most powerful telescopes humans have ever built can collect a thousand times and more as much light as my childhood toy. They are perhaps the ultimate monuments to our lust for knowledge and understanding, sitting on their mountaintops above much of our world’s blurring atmosphere and now, in the form of the Hubble Space Telescope, even floating in space entirely beyond it. The ones on Earth wield corrective optics and sophisticated computer software to compensate for atmospheric disturbances. Not only do they gather much more light, but that light can be gathered it over hours, even days, of viewing times and stored it on sensitive electronics to be analyzed and manipulated using other ingenious software packages running on other powerful computers.
Light. It is a substance far more valuable than the most precious of metals (it is also far more mysterious, as Appendix A explains). It’s greatest value is not merely allowing us to see the universe around us, however. If you know how to decipher and decode it, and understand what comes out of doing so, light can tell you almost anything you could ever want to know about whatever you are gazing upon. I’m serious: it is that amazing a substance. For example, the science of spectroscopy, the analysis of light by wavelength, allows us to deduce the chemical composition of an object or substance simply by the light it creates, reflects, or transmits. This feature of light, discovered in the nineteenth century, has given us the elemental compositions of the stars and other astronomical objects, a gift we once thought we would never be granted. Light can also tell us the temperature of things and the ways its constituent atoms are chemically bonded together. Not a bad day’s work for something we take so much for granted.
Human ingenuity and the laws of physics are a dynamic combination which seems to have no limits. The question of whether life and intelligence exist elsewhere in the universe hinges partly on whether planetary systems are common or a unique aberration of our own star. Unfortunately, merely looking through our telescopes, or even recording what comes from them with our most powerful technology, can’t answer this most critical of questions: the light from even the dimmest star is so overpowering that it completely masks the feeble reflected glow of any planets it might own. It’s like trying to pick out a the tiny twinkle of a lit match sitting astride a lighthouse beacon’s full fury.
Until the 1990s, that would have been the beginning and end of the quest. But light holds other secrets for the mind clever enough and determined enough to pry them out and exploit them. One of those secrets, which Edwin Hubble used in the 1920s to show that the universe is indeed expanding as Einstein’s General Relativity (but not Einstein himself) predicted, is the ability to tell how fast an object is moving either toward or away from us. The so-called Doppler effect (see Appendix C for a fuller explanation) is easier described using sound rather than light, but the principle is the same: when a sound-emitting object is approaching us, the distance between sound wave peaks and troughs is shortened because the object has moved part of that distance toward us in the meantime; when moving away from us, the distance is increased for the same reason. Thus, in a standard example, a train whistle’s pitch drops suddenly as the train swoops by us.
The same modification of wavelength happens with light, although it is much smaller (because light travels so much faster). It is also trickier to use in an astronomical setting because, after all, we don’t know what the wavelength of the light is when the object is at rest! This is not a problem in planet-hunting, however, as we shall see. The other piece of cleverness in our scheme lies in the fact that, according to Newtonian physics, two gravitationally bound objects revolve around their common center of mass, a point not precisely at the center of either object; the common notion that the moon revolves about Earth, or Earth about the sun, arises because in these cases the larger object is so much more massive than the smaller that the center of gravity of the system is very close to the center of the larger object.
The basic picture starts to emerge: if a star has planets, then the star itself is revolving around the system’s center of mass. This causes the star to wobble about ever so slightly as its planet(s) revolves about it. We may or may not be able to detect this wobble; it depends on how large it is and, more importantly, the angle of the wobble with respect to us. If the angle causes the star to alternately approach and recede from us, this will give rise to a, albeit very small, Doppler shift of its light from our vantage point. It is this regular, cyclic change in the shift we are interested in, which is why the rest wavelength is not important; from its size and other details, we can infer not only the existence of planets, but their masses and orbits. This, needless to say, is where the main difficulty of the technique comes into play, in the “ever so slightly” aspect of the wobble. Only the most resourceful analysis of a sufficiently large enough set of observational data has a prayer of picking this wobble out from all the other motions of a star and everything else in its vicinity.
* * *
Resourcefulness is something Homo sapiens sapiens has never been in short supply of, and thanks to modern technology data can be almost as astronomical as the stars themselves. Assuming you can get enough time on the instruments, that is. The most powerful telescopes in the world are difficult to get that time with; curiosity combined plus the size of the universe makes for far more research proposals than time will ever permit conducting. As a result, the powers that control access to them must be convinced that it will be spent on something that is both worthwhile and possible to do, and convincing them is itself a challenge for the resourceful.
Whether our solar system, and by implication life and intelligence, is unique in the universe or not is a question that, at the end of the 1980s, appeared to be unanswerable in my lifetime.
Besides, ours was the only solar system we knew of. Straightforward physics suggests that the inner planets of a system should be terrestrial – composed of rock and metal, like Earth – and that the larger, gas and ice worlds will be found further out. Gas / ice worlds such as Jupiter, Saturn, Uranus, and Neptune are largely made from small molecules like hydrogen, helium, water, ammonia, and methane; these substances are volatile and are boiled off a newly forming world if it is too close to its sun, while further out they can condense in enormous quantities as they are by far the most common materials in the solar nebula.
So you expect Jovian worlds to be found only in stately orbits far from a star, if it has any. Nature, happily, has a way of not cooperating with our expectations – and of rewarding our willingness to test them. When the first extra-solar planet was discovered orbiting a sun-like star, 51 Pegasi, only some fifty light-years from our own solar system, it stunned the astronomical community only by showing a mass approximately half of our Jupiter’s, while at the same time being in an orbit which was only some five million miles from its sun (as opposed to Earth’s 93 million miles), with an orbital period of only some four and a quarter days. Similar systems were discovered in the ensuing years, also of gas giants in very close proximity to their stars.
In one sense, this should not have surprised us at all. Such planetary systems ought to be the first discovered as they are the easiest to detect: a large planet orbiting close to its sun will produce the largest Doppler shift effect, and hence be easiest to detect. It was just that no one had suspected such systems to exist at all, or at most, to be exceedingly rare. Gas giants, after all, could only form far from their parent stars, otherwise as mentioned the intense stellar radiation and stellar wind will blow the light elements away. Clearly, that was what had happened with Earth’s solar system. So what had gone awry in systems such as 51 Pegasi?
The basic physics of planetary formation are likely to be correct. Therefore, 51 Pegasi b (the official designation of the planet) must have formed at more Jovian-like distances: a good one hundred or so times further out from the present position. Various interactions with other bodies in the system, or even with other stars, have since gradually spiraled 51 Pegasi b in to its current orbit, very close to its sun. This hypothesis is not unreasonable; it was known that planetary orbits could be highly unstable over time spans of billions of years. No doubt, catastrophic interactions with other bodies in the 51 Pegasi system had occurred in this time: smaller, closer, possibly terrestrial (even Earthlike) planets had been bulldozed out of the system permanently, into cold interstellar space.
This just leads to the next question, however. Why has our own solar system been apparently so stable during its four and a half billion years of existence? If anything, the gas giants such as Jupiter and Saturn have done us a good turn by sweeping smaller bodies out of the system which otherwise might have collided with us, or herded them into relatively stable asteroid belts. Have we been just incredibly fortunate in this regard? Why didn’t Jupiter eject our own world, not to mention Mercury, Venus, Mars, and the moon into the interstellar abyss?
The number of additionally discovered systems similar to 51 Pegasi have made this question more than a trifling compelling. It suggests that systems harboring life-bearing worlds are rarer than we had supposed, relying on a mixture of luck and physical laws which we still have but an inkling as to their workings. It seems that once again, in our attempts to gratify our curiosity, we have only given it more fodder to feed on. One thing is for certain: repeatedly, we find our attempts to uncover the secret orderings of things to humble us again and again as to how little we still understand. We think we are taking the Russian dolls apart one by one, into ever deeper levels of understanding, only to find ourselves as baffled as when we had begun.
* * *
I am not trying to sound defeated. I do not believe that we are, or will be defeated. Progress in knowledge, in science, does proceed. Little by little, our curiosity is satisfied. It is merely that it never proceeds in the nice, round, little steps we always expect it to. No, there are fits and starts, backtrackings where we seem worse off than when we had begun, strategic retreats here and there before we make the next jump forward. If anything, this makes the whole journey that much more exciting, and fulfilling. At the end of each day, we can sit and watch the sunset, happy in what we have achieved and that much more edgy and restless for what tomorrow might bring. For we know that, like today, it will bring something, just not the nice, neat packages of knowledge that, actually, would have been quite boring to receive, but a mixture of new questions and mysteries with which we can set out for further explorations – with just enough genuine new understanding to leave us feeling satisfied. That is the way of knowledge, the path that curiosity invariably takes us down. Isn’t it one filled with restless throbbing and hope? I believe that it is.
Furthermore, since the discovery of the 51 Pegasi planet, almost fifteen years ago, astronomers have been aiming their instruments at the sky with the hopes finding more planetary systems, and not only that, planetary systems more like our own solar system. And they have been successful well beyond anyone’s expectations. Over the last few years systems have been found with planets more similar to our own; these includes “super-Earths”, which are rocky terrestrial worlds akin to our own, only much larger, and other large planets, similar to our own gas giants but smaller. Some of these worlds have even revealed the tantalizing tastes of substances such as oxygen and water, absolutely essential to life as we know it. It seems quite likely now that over the next ten-twenty years we will discover Earth-like planets circling other stars in our galactic neighborhood. And where there is life, there is certainly the possibility of intelligence.
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Well, I certainly hope I have whetted your appetite for what is to come. At this point, I myself must admit that it is uncertain just what ground I will cover, what areas will be explored, what mysteries will be unveiled. Perhaps that is as it should be. Curiosity is a passion which you never know for certain where it may lead you. You only know it will go somewhere; that there will be a resting spot somewhere in the future you can perch upon and gaze at the territory covered, while the campfire dims and the last of the evening meal lingers on your palette.
