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.
* * *
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.
