“What?” cries the marquise. “Don’t you remember that you assured
me there were seas on the Moon that one could distinguish from here?”
“I’m sorry to say that’s only a conjecture,” answers the philosopher.
“Those dark places that are taken for seas are perhaps only great cavities.
At the distance we are, it’s understandable not to guess quite accurately.”
Frustrated by the philosopher’s flip-flopping, the marquise quips,
“That’s a great deal of ignorance based on a very little science.” I won-
der what she would think of a modern astrobiology conference.
Later that evening the philosopher tries to convince the marquise
that all the other planets are inhabited, once again using reasoning that
still has a modern ring. Fontenelle was well aware that the telescope
was not the only seventeenth-century Dutch optical invention that had
opened up new realms of the cosmos to humanity. In the 1670s Antonie
van Leeuwenhoek, the unsung Galileo of the microscope, reported the
discovery of a multitude of invisible, microscopic organisms squirming
in every drop of ordinary water. If there is abundant and ubiquitous life
hidden from our senses, Fontenelle reasoned, why shouldn’t this be true
on cosmic as well as microscopic scales?
“Take all of these species of animals newly discovered, and perhaps
those that we easily imagine which are yet to be discovered, along with
those that we’ve always seen, and you’ll surely find that the Earth is well
populated. Nature has distributed the animals so liberally. . . . Can you
believe that after she had pushed her fecundity here to excess, she’d been
so sterile toward all other planets as not to produce anything living?”
Plurality of Worlds
25
Today we’ve identified a great many more of those species “yet to be
discovered.” We’ve found strange critters cavorting around undersea
volcanoes and microbes slumbering miles beneath the ground. In an
echo of Fontenelle, these modern discoveries are trumpeted in newspa-
per headlines as hopeful signs for alien life.
Fontenelle even discusses the significance of exotic life-forms, today
dubbed extremophiles. He proposes a hidden biosphere of rock-eating
organisms living deep underground on other planets: “Even in very hard
kinds of rock we’ve found innumerable small worms, living in imper-
ceptible gaps and feeding themselves by gnawing on the substance of the
stone. Imagine how many of these little worms there may be, and how
many years they’ve subsisted on the mass of a grain of sand. Following
this example, even if the Moon were only a mass of rocks, I’d sooner
have her gnawed by her inhabitants than not put any there at all.”
Fontenelle argued for infinite variety among intelligent life-forms on
other planets, mirroring the bountiful diversity of life on Earth. He
imagined that life elsewhere would take turns that we cannot predict,
as it adapted to local environmental conditions. He endowed his aliens
with new senses and other capabilities unknown to us.
In response, his marquise declares, “My imagination’s overwhelmed
by the infinite multitude of inhabitants on all these planets, and per-
plexed by the diversity one must establish among them; for I can see
that Nature, since she’s an enemy of repetition, will have made them all
different.”
On the fifth evening, the philosopher makes the final leap, placing
inhabited worlds around the distant stars. He wonders about the diver-
sity of solar systems, something that we are today just starting to figure
out. The marquise asks, “May these systems not, despite this similarity,
differ in a thousand ways? After all, a basic resemblance doesn’t exclude
infinite differences.”
The philosopher responds, “Definitely. But the difficulty is to figure
it out. What do I know? One vortex has more planets revolving around
its sun, another has fewer. In one there are subordinate planets which
revolve around the larger planets; in another there are none. Here
they’re gathered around their sun like a little platoon, beyond which a
great void extends . . . elsewhere they travel around the edges of the
vortex and leave the middle empty. I don’t doubt that there may be
some vortices deserted and without planets. . . . What do you want
from me? That’s enough for a man who’s never left his own vortex.”
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L o n e l y P l a n e t s
The final conversation ends with a discussion of the birth and death
of stars. When the marquise expresses sorrow that some stars expire,
the philosopher offers the comforting thought that new stars might also
be born. Foretelling one of the most important insights of twentieth-
century astronomy, he asserts, “I also believe that the universe could
have been made in such a way that it will form new suns from time to
time. Why couldn’t the proper matter to make a sun, after having been
dispersed in many different places, reassemble at length in one certain
place, and there lay the foundation of a new system?”
Here Fontenelle rejects the ancient notion of the immutability of the
heavens, instead seeing continual change among the stars and hinting at
the modern notion of galactic ecology, the great cycle in which new
generations of stars are continually reincarnated from the ashes of their
ancestors.
Throughout his long life (February 1657 to January 1757—one
month short of a century) Fontenelle published numerous revisions of
Conversations, each time bringing it up-to-date with the latest astro-
nomical observations—adjusting the diameter of Venus or the distance
to Saturn in accordance with the most recent measurements.
A S T R O T H E O L O G Y
The seeds of pluralism, spread in the seventeenth century by Fontenelle
and his contemporaries, took root and grew in the next. General recep-
tivity to the concept of alien life was fostered by the growth of natural
theology—the quest to learn about God through the study of nature.
Natural theologians were not afraid, like Galileo’s inquisitors, to look
through the telescope. What could reflect more fully the glory of the
creator than a universe filled with inhabited planets?
Between 1700 and 1750 at least a dozen books were published advo-
cating pluralism, mostly on the basis of natural theology. The most
successful popularizer of this view was the Reverend William Derham,
who published Astro-Theology, or A Demonstration of the Being and
Attributes of God from a Survey of the Heavens in 1714. Derham asked,
“What is the use of so many Planets as we see about the Sun, and so
many as are imagined to be about the Fixt Stars? . . . The answer is, that
they are Worlds, or places of habitation.”
Derham’s natural theology removed the potential conflict between
science and religion for those with pluralist inclinations. Encouraged by
Plurality of Worlds
27
the freedom to speculate that this reconciliation allowed, scientists and
mathematicians began to dedu
ce the characteristics of life on other
worlds, sometimes with questionable results.
In 1735, German mathematician Christian Wolff published a for-
mula for calculating the size of creatures on other planets. Assuming
that (1) height is proportional to the size of the eye, and (2) that the
square of the diameter of the pupil is proportional to available light
intensity, he calculated that the average height of those living on Jupiter
was precisely 13.57 feet! This calculation was cited for more than a
century by both sides in the pluralist debate, and Wolff’s method was
used to compute sizes of organisms on several other planets.
Today, of course, this seems arbitrary if not downright loopy. We can
regard it as a good example of how scientists sometimes use math inap-
propriately, sticking equations where they don’t belong in an effort to
make our answers seem more “scientific,” more exact, and thus more
legitimate. If the assumptions behind our calculations are absurd (as
Wolff’s now seem with the hindsight of three centuries), then our
results, beneath their shiny mantle of quantitative credibility, are noth-
ing but garbage. Which of our current efforts will someday be placed in
this same pile?
W O R L D S W I T H O U T E N D
The daring and liberating pluralist vision captivated Enlightenment poets
and philosophers. Immanuel Kant, one of the last Enlightenment philoso-
phers, is widely known as a giant of philosophy who forcefully questioned
the sources of all knowledge. Kant defined the Enlightenment as the
“emergence of man from his self-imposed infancy.” As a young man,
Kant had turned his attention to astronomy with equally impressive
results. In his Universal Natural History and Theory of the Heavens, written in 1755 when he was thirty-one years old, he made several important
and lasting contributions to cosmology. Like the first tadpole bravely gaz-
ing above the surface of the pond and daring to contemplate frogdom,
Kant had a vision of a universe many orders more vast than anyone had
previously conceived.
Kant, standing five foot two and weighing less than a hundred
pounds, was a shrimp of a man with a Galaxie 500 mind. Though he
didn’t see much of planet Earth during his stay here, never in his life
traveling more than fifty miles from his hometown of Königsberg, Kant
28
L o n e l y P l a n e t s
gave humanity an unbelievably enlarged universe. Pondering the diffuse
glowing band we see at night and call the Milky Way, he made a
tremendous conceptual leap, deducing that this view is an optical illu-
sion born of our position within the Milky Way. Kant realized that if
our Sun is embedded within a flattened disk of innumerable suns, then
the far shores of this disk, too distant for individual stars to be seen,
would appear to us as the milky band we see stretched across the night.
Kant had found us the Milky Way galaxy. Indeed, he found the other
galaxies as well.
Contemporary thought held that the numerous “spiral nebulae,” dif-
fuse smudges of light spread randomly through the sky, were enlarged
puffs of gas or dust as distant from us as the stars. Kant, noting their
elliptical shapes, proposed that they were other entire “Milky Ways,”
much farther away than the stars we can see, each a far-flung disk com-
posed of numberless stars. He even speculated that these other Milky
Ways were themselves bound together in larger groups held together by
gravity—what today we call clusters of galaxies.
Kant and his fellow Enlightenment philosophers saw themselves as
leading the human race in a courageous jailbreak from a prison with
walls built of superstition and prejudice; they were freedom fighters for
reason. In his galaxies, Kant found the perfect physical manifestation
of the Enlightenment quest to cast aside comforting illusions and
unquestioned assumptions in search of deeper, wider truths. “The cos-
mic space,” he wrote, “will be enlivened with worlds without number
and without end.”
It would be nearly two hundred years before astronomers could con-
firm Kant’s intuition, and in some ways we have still not caught up
with him. The distant galaxies are so far away that, unlike the planets
of our solar system or even the “fixed stars” in our own galaxy, it is lit-
erally difficult to imagine traveling to them or communicating across
the interminable, empty intergalactic stretches. Even science fiction
writers, who are encouraged to bend the rules to help the rest of us
probe the edges of the possible, rarely imagine intergalactic travel or
cross-talk. When Star Trek writers need an incredibly far-off land as a
plot device, they choose a remote quadrant of our own Milky Way
galaxy. Whether they’re accidentally marooning Voyager in the distant
“gamma quadrant” or twisting space into a wormhole reach to Deep
Space Nine’s “delta quadrant,” these visionaries actually restrict them-
Plurality of Worlds
29
selves to a minuscule patch of universe. They never venture even as far
as nearby Andromeda, which at only 12,000,000,000,000,000,000
miles is a stone’s throw away compared to any other galaxy.*
It is as if, even in a fictional universe where faster-than-light “warp
drive” is accepted as a quotidian transportation option, it would strain
credulity to postulate extragalactic visitation or communication. So, in
a sense, the discovery of external galaxies throws us back conceptually
to the other “worlds” of the ancient Greek Epicureans—a universe
composed almost entirely of places that are real, but forever out of
reach. The difference, of course, is that we can see the galaxies, even if we still don’t quite dare dream of traveling to them. And now we know
we live in one.
Kant also came up with one of the first physical theories that
explained the origin of planetary systems. In Kant’s scheme, stars begin
life as rotating clouds of gas that spin faster as they collapse under the
weight of their own gravity. These flattened, spinning clouds create
rings of material around a young sun that eventually coalesce into
orbiting planets. Kant had conceived a crude picture of what would
later be called the nebular hypothesis, understood today as the way
solar systems form.
Kant ended his book with detailed speculations on the nature of
extraterrestrial creatures. Among these were the seeds of an idea that
persisted, in various forms, for centuries: that there is a progression in
stages of evolution with distance from the Sun, so that planets closer to
the Sun are younger and more primitive than Earth and those farther
out are more highly evolved. This framework led Kant to believe that
the creatures on Mercury and Venus are dimwits, and that the
Jupiterians and Saturnians are geniuses, with earthlings occupying
“exactly the middle rung . . . on the ladder of beings.” Later in life,
embarrassed by these youthful conjectures, he insisted that the more
speculative final chapters be censored from
reprints of Universal
Natural History and Theory of the Heavens. But he never lost his plu-
ralist convictions. In his 1781 masterpiece, Critique of Pure Reason,
written when he was fifty-seven years old, he wrote, “I should not hesi-
tate to stake my all on the truth of the proposition—if there were any
*With your unaided eyes from a dark site, you can easily see Andromeda, a faint, soft oval smudge floating off the knees of Cassiopeia.
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L o n e l y P l a n e t s
possibility of bringing it to the test of experience—that at least some of
the planets which we see are inhabited. Hence I say that I have not
merely the opinion, but the strong belief, on the correctness of which I
would stake even many of the advantages of life, that there are inhabi-
tants in other worlds.”
Encouraged by Enlightenment philosophers, astronomers in the eigh-
teenth century increasingly adopted pluralism. Among them was
Edmund Halley, the British Astronomer Royal who is most famed for
Halley’s Comet and who argued that all planets “are with Reason sup-
pos’d habitable.”
A L U X U R I A N T G A R D E N : P L U R A L I S M G O E S
M A I N S T R E A M
At the age of nineteen, astronomer William Herschel was forced to flee
his native Germany when the French invaded his hometown in 1757.
He settled in England and became one of the most famous scientists of
the Age of Reason. Systematically mapping the heavens with a tele-
scope, he was in many ways the first modern astronomer. It is difficult
to determine how much of his reputation was really due to the work of
his brilliant, diligent, and unsung sister Caroline, who helped build his
telescopes, performed many of his observations, and did most of his
calculations.
William and Caroline were both professional musicians who pursued
astronomy on the side, but together, in their spare time, they observed
and classified numerous stars, and they were the first to see that the
diverse colors and types of stars belonged to an evolutionary sequence.
They realized that stars have life cycles with visibly different stages.
William likened our brief glimpse of stellar evolution to a glance at a
garden with plants at all stages of growth. In The Construction of the
Heavens, he wrote, “The heavens . . . are seen to resemble a luxuriant
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