eagerly watching for something to wriggle through the floodlights, will
forever change the way we think about life in our universe. If we find
not so much as a chemical trace of life, then we’ll know that life needs
more than just any watering hole to thrive. We’ll have the first concrete
hint that life on Earth is a more rare, more lucky phenomenon than we
had surmised. At the very least, we’ll know that our assumptions about
the requirements for a good home, extrapolated from our one example
of a living world, were wrong.
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Of course it will be impossible to prove definitively that Europa (or
any other planet for that matter) does not have life. What if the life
there is sufficiently alien that we just don’t recognize it? A negative
result on Europa would cause us to rethink how we go about defining
and looking for life, as did the slightly ambiguous negative results from
the Viking mission to Mars in 1976, the first mission ever sent to look
for life on another planet.
What if we do find something swimming under the Europan ice,
something that grows, breathes, reproduces, or dances the boogaloo
with an unmistakable signature of life? Just as Galileo’s original discov-
ery of Europa and company provided undeniable proof of a new world-
view, the discovery of life there would cause us to take a new look at
ourselves and how we fit in. The existence of another living world so
close to home would carry implications of a widely fertile, densely
inhabited universe. It would strongly suggest that life is no accident but
an inevitable step in Cosmic Evolution following from atoms, galaxies,
stars, and planets. Our pursuit would then be transformed from a nar-
row study of our own single biosphere to a comparative study of life in
the universe. When we have a second example we can begin to study
life as a general phenomenon.
All Earth creatures use the same chemical machinery: DNA to pass
life’s recipe into the future, proteins to build tissues and regulate chem-
istry, and so on. Does it have to be this way? Is this commonality pro-
found or accidental? Are cats, begonias, squid, slime and people all vir-
tually the same, chemically speaking, because proteins and DNA are
unique solutions to problems that nature cannot solve in any other
way? Or is this all the result of arbitrary early choices that have been
passed down to all life and become so enmeshed in the fiber of Earth
life that they seem essential?
The best way to answer such questions is to examine life that we’re
not related to. If there is a biosphere subsisting in the subsurface ocean of Europa, then it is almost surely isolated and independent of Earth.
(Such a claim cannot be made for Mars, since we know that Earth and
Mars frequently chuck rocks at each other.) The Europans, so far away
and buried under irradiated ice, have effectively been quarantined. If
we find that the Europans use DNA, then DNA is instantly elevated in
status. It is revealed as a cosmic standard, not just a terrestrial accident
or convenience.
Growing Up with Europa
203
If not DNA and proteins, then what do we share with our Europan
counterparts and what does Earth share with Europa? Does life trans-
form all of its home worlds in ways that transcend the local details of
evolution? What features do biospheres have in common? The answers
would bring some much needed sophistication to our efforts to define
and search for life elsewhere.
It is, of course, possible that we will discover that we are, after all,
related to Europan life, that we share common ancestors even with
these distant, iced-in creatures. Then we will have some thinking to do.
Such a finding would force us to completely reassess our agreed-upon
story for the origin of life on Earth. We might have to give another look
to some ideas that make scientists squirm, such as panspermia and even
intelligent design.*
Although our discussions of Europan life tend to focus on possible
bacteria, we might even find larger, more complex, advanced life there.
We know of no physical or biological law that rules out the Europan
equivalent of giant squid or sperm whales with songs of their own.
Even if we find only simple life there, such a discovery so close to home
would greatly brighten prospects for finding other intelligent life. If
Europa, too, is alive, then our universe must be bursting with life. And
in a universe like that, surely we will soon find someone else to talk to.
Our smashed preconceptions about the Jovian moons came from
applying local rules of planetary evolution, which we mistook for uni-
versal laws, beyond their jurisdiction. We missed something fundamen-
tal when we neglected to consider the tidal energy created within small
moons of giant planets. There may be an important lesson here for
astrobiology. Complex phenomena, such as planets or living organisms,
do follow physical laws, yet they cannot easily be predicted. The out-
come of natural “experiments” in planetary or biological evolution
depend on too many factors. Speculation and theorizing are fun, but we
learn the truth through exploration.
In some ways our current knowledge and beliefs about the oceans of
Europa are reminiscent of our pre-space-age ideas about Venus. The
bright Europan ice is playing a role that the bright clouds of Venus
played before 1962, deflecting deeper inquiry and providing cover for
*If we reject these theories out of hand just because we find them ideologically repulsive, then we are practicing pseudoscience.
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rational fantasies of alien life. As far as we know, anything may lurk
beneath the ice.*
We are the only generation who will see the planets transformed
from objects into places, and who will ever experience the first rush of
knowing, for sure, that other worlds are out there among the stars. We
might even get to be the ones who learn whether Earth has living com-
pany in the solar system. We are tasting the fruit promised four hun-
dred years ago by Galileo’s revolution. If we keep exploring and stay
healthy, with a little luck you and I may live to go ice fishing on Europa
and, just possibly, by finding the first real aliens, plunge into a bracing
new sea of cosmic awareness.
*Another interesting parallel is that sulfuric acid, the stuff of the Venusian clouds, has been identified by Galileo in the surface ice of Europa, causing us to wonder just how acidic the ocean there may be. The recent discovery of acid-resistant extremophiles on Earth opens the question of whether highly acidic conditions would preclude life on Europa, or on Venus.
Enter the Exoplanets
13
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Are you ready now? Then close your eyes, and tap
electronic edition
your heels together three times. And think to your-
self—There’s no place like home; there’s no place
like home; there’s no place like home.
—G LINDA , THE G OOD W ITCH OF THE N ORTH
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What do you do when you know that you know
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that you know that you’re wrong?
electronic edition
You’ve got to face the music.
You’ve got to listen to the cosmo song.
—SUN RA
M E D I O C R I T Y
Hey, have you heard the news? We’ve finally found other planets out
there, orbiting distant stars. Lots of them. This discovery is recent and
surely profound. But what does it mean for our place in the grand
scheme?
Is Earth the best of all possible worlds, just an average world, or the
only world there is? In the last four hundred years science has nar-
rowed down the options, dispatching the third one to the dustbin of
history. We know that our world is one among many. But is it only one
of many, or is it something more as well? Does Earth belong in the spe-
cial and gifted class of planets? As befits a species stumbling through
adolescence, we are wondering where we fit in, seeking our peers, look-
ing for answers.
Expectations about more distant planets have been, of necessity,
based on the local crew. If our planetary system were somehow typical,
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it would be a gift to science.* It would make our job so much easier,
because if we can safely generalize from our own solar system, then we
already know what goes on out there in the rest of the galaxy. If our
planetary family is ordinary, then it may not be all that unusual for one
or more terrestrial planets to end up in the habitable zone where liquid
water sloshes onto ripe shores. Some of these watery worlds will
become biospheres, perhaps following the same biochemical pathways
that life has found here, creating oxygen-rich atmospheres, ozone lay-
ers, forested continents, animal crackers, and Internets. If it happened
once, it can happen again, right?
Today, we all know that we are nothing. We learned in grade school
that we inhabit an ordinary planet circling an average star floating
inconspicuously and, until recently, innocently in a backwater arm of a
galaxy containing hundreds of billions of other stars, a humdrum
galaxy that is itself undistinguished among more galaxies than you
could shake a stick at in a zillion and a half lifetimes.
Jeez. Maybe there is no reason to think that our planet is special in
any way at all. This is an extreme application of a general principle in
cosmology, known as the principle of mediocrity, which says that the
universe is basically the same everywhere. Wherever you go, there you
are. There is nothing unusual about our position, and the view from
anywhere else should look more or less the same as ours. This thought
is comforting to scientists because we like to believe that our conclu-
sions do not depend on where in the cosmos we happen to be born. We
are not looking for regional truth. What we seek transcends location,
and that’s a much easier trick if location isn’t important.
The principle of mediocrity is a close relative of the arguments by
analogy that have made people believe in a plurality of inhabited worlds
for hundreds of years, at least. Can we use it to deduce the existence of
habitable planets and life out there? From the remarkable uniformity in
all the qualities we can observe in distant realms, should we deduce a
sameness in the features we cannot yet observe? It has worked before.
We assumed, by analogy, that planets were around other stars. Now we
know we were right. Do you suppose that life might be another of those
features found more or less everywhere? As Fontenelle, in 1686, said
about the view of nearby Saint-Denis from the towers of Notre Dame,
*Not to look a gift universe in the mouth, but if we were too typical, it would be highly suspicious.
Enter the Exoplanets
207
everything one can see there resembles Paris: steeples, houses, and so
forth. So we conclude that Saint-Denis, like Paris, is inhabited. If the uni-
verse tends to repeat itself, in gross architecture and building materials,
does that mean that it is fitted with the same fine trim and landscaping
elsewhere, achieving an economy of detail that we just can’t quite make
out yet?
During our first decades of exploring our solar system, we learned
that planets evolve in ways that largely depend on accidents of birth, and
chance occurrences. As a result, planetary fates cannot be predicted
from their initial states. If planets were like electrons or atoms, it would
be different, because if you’ve seen one electron, you’ve seen them all.
Atoms are somewhat more complex than electrons. They are not all the
same, but they can be grouped into a hundred or so types (the elements)
that are easily classified (in the periodic table) and understood. If plan-
ets were like atoms, by studying a small sample we could know them all.
Will we ever be able to assemble all the planets we know into some-
thing like a periodic table of planetary types? No. Because planets, and
apparently entire solar systems (by which I mean stars with retinues of
orbiting planets), are more like people. Each is a complex individual,
shaped by the unique experiences of a long life in a chaotic, changing
environment. In the details, no two will ever be alike.
Imagine if you were trying to understand human beings as a general
phenomenon but you only had intimate knowledge of one person.* If
from studying your subject you deduced that everyone needs to sleep,
everyone experiences puberty, and that people respond to music, you’d
be right. But, depending on the peculiarities of your randomly chosen
subject, you might also conclude that everyone is way into heavy metal,
that Falun Gong exercises at lunchtime are a daily ritual for all human-
ity, or that everybody must get stoned. In generalizing from your ran-
dom sample of one, you would draw some correct inferences and be led
astray with others.
Let’s face it. We need more planets.
M O R E P L A N E T S , P L E A S E
In truth, it would have been shocking and disturbing if they weren’t out
there. Nevertheless, their number and nature were entirely unknown
*I suppose you could say that this is the situation we are all stuck in.
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until October 1995, when the efforts of patient and diligent observers
paid off, producing one of the most marvelous discoveries of the twen-
tieth century.
The discovery was made by Swiss astronomer Michael Mayor and
graduate student Didier Queloz. Using Haute-Provence Observatory in
southern France, they were attempting, in late 1994, to find planets by
taking detailed spectra of Sun-like stars. They knew they could not see
the planets themselves with any telescopes yet built, but they knew also
that large planets, as they orbit, should cause their stars to wobble
slightly, first approaching us and then receding. By watching for the
subtle spectral shifts induced by a star’s rhythmic motion toward and
away from us, t
hey hoped to turn up clear signs of a planetary compan-
ion. Others had tried this and failed, but Mayor and Queloz had devel-
oped new spectroscopic techniques that could detect velocity changes,
in a distant star, of just thirteen meters per second. This would be
barely sufficient to detect Jupiter if observing our Sun from a nearby
star, so they thought they had a shot at it.
In late September they began observing a star called 51 Pegasi.* This
star is nearly a dead ringer for the Sun. It is located about fifty light-
years from here: star number 51 in the constellation Pegasus, the
Winged Horse. By December of that year, they had noticed irregulari-
ties in the star’s spectrum. It seemed to shift slightly every time they
observed it. Yet the shifts were large, amounting to velocity changes of
up to sixty meters per second. They first suspected a problem with their
equipment. By January 1995, they saw a pattern to these shifts, yet
something was strange.
The star’s velocity was varying periodically, as would be caused by
an unseen planetary companion. Yet the variation was far too fast to
be any kind of planet we knew or expected to find. The period of oscil-
lation was just four days. By contrast, in our solar system, the fastest
planetary orbit is that of Mercury (the winged messenger), which takes
a spin around the Sun every fifty-nine days. Following universal laws
worked out in the sixteenth century by our old friend, the freaky
genius Kepler, close-in planets orbit more rapidly than distant ones.
Thus in our solar system Mercury’s year is the shortest and Pluto’s the
longest. Kepler’s third law of planetary motion tells us exactly how far
*Or just 51 Peg to its friends.
Enter the Exoplanets
209
away a planet is from its star if we know the orbital period. If the four-
day period they saw was really the signature of a planet, then it was
orbiting a mere 5 million miles from its star. By comparison, Earth
orbits at a safe distance of 93 million miles, and Mercury, the inner-
most planet in our system, is 36 million miles from the Sun. The data
seemed to indicate something very strange indeed: a large, Jupiter-class
planet following a tiny orbit, only one-seventh the radius of Mercury’s
orbit.
They kept quiet and kept gathering data. By March, Mayor and
Lonely Planets Page 31
