opment where each unit is identically built as far as the eye can see.
Like houses in some urban neighborhood with a storied past, planetary
systems are a hodgepodge of style and size.
Is there a silent majority, or even a noisy minority, of well-behaved,
“normal” planetary systems obeying the rules and living quiet lives? Is
there a vast population of systems like our own that haven’t yet called
sufficient attention to themselves to be picked up with our crude tech-
niques? Statistically, even if our kind of solar system is quite rare, say one in one hundred thousand, then there must still be many millions of
them, given the 10,000,000,000,000,000,000,000 stars the universe
has to work with. Even if “good” solar systems are a tiny fraction of
the whole, they might still be quite numerous.
And then, of course, we have to ask, what makes us so sure that the
good ones are like ours?
O N T H E E D G E O F K N O W I N G
For four hundred years we’ve believed in extrasolar planets. Now, the
universe has shown us a sign and our faith has been rewarded. This is
the beginning of the end of our long isolation in the realm of the Sun.
The galactic distribution of planetary systems is being revealed to us,
but not all at once. It’s more of a striptease than a grand unveiling.
With every new detection (or nondetection) our view improves slightly.
For astrobiologists, the most interesting extrasolar planet discoveries
won’t be made until we know about the harder-to-find planets, until we
can overcome our strong observational limitations and uncover the true
range and prevalence of planetary environments in the universe. The
picture will come gradually, and the most mind-blowing revelations
may never make the news.
What does it all mean? The most secure answer right now is “Come
back and ask us in a few years.” But we’re not like that. Because we are
nerds. Even when the data is piling in so fast that we know that it will
all become more clear in months, we can’t resist speculating, interpret-
ing, pontificating even.*
For the next few years, the new discoveries will continue to be giant
*It’s a common but little discussed problem: Do you suffer from premature speculation?
Enter the Exoplanets
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planets and the excitement will be over finding “other Jupiters” and
Saturns. But what about finding “other Earths,” small, rocky worlds
that might harbor biospheres like our own? They are out there, too,
and we’ll start finding them before long.
A satellite called Kepler is going to find them. It will not have to
travel far, only into Earth orbit, for a clear view of the stars. Kepler will find planets by watching for the faint flickering of far-flung suns. The
tiny gravitational wobbles induced in other stars by alter-Earths are
beyond the limits of our current searches. But we don’t need to see the
wobble if we can catch the shadow of the planet itself, as it passes our
line of sight to its star.
Which reminds me of an effect I’ve sometimes noticed looking out of
airplane windows. When I fly, I always try to get a window seat. I just
love to be that high and watch the world go by, and I especially enjoy
pondering and analyzing and explaining anything that appears mysteri-
ous.* On some of my first few trips back East after moving out to
Arizona, I noticed something odd. As I watched the ground on a night-
time approach to an East Coast airport, I saw the lights madly flicker,
in a way that they don’t on approaches to cities in Arizona.
After noticing this on several flights, I decided that I, being Joe
Scientist and all, should be able to explain it. Then it hit me like a
branch in the face. It’s the trees. The lush Northeastern forests versus
the sparse Western deserts. Massachusetts, New York, and greater D.C.
are much more heavily treed than Arizona—at least in the flat parts
where they build major airports. Approaching Dulles or Logan at
night, the flickering lights you see betray the branches of ubiquitous
trees hiding in the darkness. Each little twig is briefly revealed in a
twinkle as it occults the lights behind it. And that, my friends, is just
how Kepler will find the little planets that our other searches cannot detect. If the galactic forest grows thick with small planets, Kepler will catch the occasional twinkling when one passes in front of a star.
It amounts to eclipse watching, from light-years away. Orbiting just
beyond the murky, turbulent distractions of Earth’s atmosphere, Kepler
will continuously monitor the brightness of one hundred thousand stars
for four years, watching for the brief flickering of distant partial
eclipses. If such a dimming is due to an intervening planet, it will occur
*For this reason, if you ever fly with me, I recommend you bring headphones, so you can pretend you’re listening to music.
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at regular intervals as the planet orbits its star. The amount of darken-
ing will tell us how large the planet is, and the orbital period will tell us
its distance from the star. The biggest drawback is that a planet must be
orbiting in just the right orientation so that it crosses the line of sight
between us and its star. Inevitably, we’ll miss many more planets than
we’ll catch with this technique. Still, we’re quite sure to catch a small
but reliable fraction of them. About half a percent of planets should, at
random, be oriented in the right way to be caught by Kepler. This is not the way to take a complete census, but it’s a great way to get solid statistical data on how common planets of various sizes and orbits are.
Currently, Kepler is scheduled for launch in 2007. So if all goes well,
later in this decade we should be getting some good information on the
demographics of Earth-size planets in our home galaxy.
When we do find these “other Earths,” they’ll be too far away to see
directly. We’ll know their sizes and orbits and what kind of star they
haunt. Can we learn anything else about them? Perhaps. If some of
them are found around nearby stars, then we should be able to identify
gases in their atmospheres with spectroscopy. This is no easy trick—
analyzing reflected light from dark little objects so far away and so near
their bright stars. You need an extremely precise way to separate two
sources that are so close together and vastly different in brightness.*
We don’t have the ability to do this yet, but we’ve got plans like you
wouldn’t believe. An ambitious mission called Terrestrial Planet Finder
is scheduled for launch in 2014 (don’t be shocked if this schedule slips).
This amazing machine will be able to see details one hundred times
smaller than the Hubble Space Telescope. It should be able to measure
the atmospheres on any Earth-size planets up to fifty light-years away.
We don’t even know exactly what gases to look for in these distant
atmospheres. Remember, we still don’t have a Galactic Life Detection
Manual. Until someone beams one down to us, we’ll have to wing it.
We certainly want to look for oxygen, water, methane, ozon
e, and CO2.
We’ll look for things that seem anomalous, such as gas mixtures that
are out of equilibrium, begging for reaction and explanation, or any
other characteristics that seem surprising or unlikely to be caused by
geophysical processes acting alone. For example, if we find a planet
with both oxygen and methane, then something must be actively sup-
*For example, the Sun is 1 billion times brighter than the Earth.
Enter the Exoplanets
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plying one of these, or they would rapidly consume each other. That
something could be life, or even alien technology.
With a range of techniques, we are zooming in on the demographics
and lives of solar systems in our galaxy. Soon we will have some solid
knowledge of the numbers and types of planets inhabiting local space.
We’ll even know a little bit about what some of these worlds are made of.
Yet it will be a long time before we have a comprehensive understanding
of the lives of entire planetary systems. This fully developed theory of
comparative planetology is way off in the future—we’ll only be able to do
it once we’ve had an opportunity to explore dozens of systems close up.
That’s right, I’m talking about interstellar exploration—not remote obser-
vation, but actually going there. This kind of exploration seems quite far
off now—probably as distant as voyages to the planets seemed at the
beginning of the twentieth century. Yet by century’s end planetary explo-
ration was being taught in history class. Interstellar exploration could
begin by mid-twenty-first century if things go well.
Given this timescale, studying planetary evolution for a living
involves a certain intellectual self-flagellation. Why devote your career
to a problem you know will only be solved after you’re gone? I don’t
know. Maybe it’s that cathedral thing. Whenever I see a giant cathedral
that took generations to make, I think of the people building it who
knew that they would not live to see it finished. You spend your life
working on the flying buttresses, knowing that the roof won’t go up for
a hundred years. Why bother? Some worked under duress. But some
did it because they believed that they were serving a higher purpose and
contributing to something larger than themselves. You feel you are a
part of something built to last. Perhaps that’s why we do what we do—
wrestling with the unsteady foundation stones of a theoretical edifice
that will be completed by some later generation.
T H E G O O D N E W S F O R M O D E R N M A N
The recent discoveries of extrasolar planets are quite encouraging for
our ultimate chances of finding extraterrestrial life. They confirm a
mode of thinking that all our theories for life elsewhere depend upon.
As seers, natural philosophers, and astronomers have long surmised,
for reasons ranging from mystical to mathematical, there are indeed
many worlds out there. Score one for arguments by analogy. You do get
the impression that we really can figure out some of the details of the
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rest of the universe even while confined, as we are, to our tiny little cos-
mic corner. Even when we had no way to find other planets, most of us
believed, because it seemed so unlikely that our star alone should have
orbiting companions. Like our belief in extraterrestrial life today, this
was based more on informed intuition than direct evidence. Can our
strong faith in the existence of other living worlds make the same extra-
ordinary passage, from deeply felt belief to confirmed knowledge?
Whether or not there are a lot of places like Earth in the details, we’ve
already discovered something important for life out there. We know
there are a huge number and variety of environments on various kinds
of planets that we have barely begun to detect, catalog, or explore.
Each of these, in our current ignorance, should be regarded as a poten-
tial niche for some kind of life.
Extrasolar planet detections have contributed to a new air of excite-
ment and optimism about alien life, and a new respectability in scien-
tific circles for the serious study of exobiological questions. Following
our nonlinear childlike curiosity, we approach the truth of the universe
through successive approximations. Every answer leaves us unsatisfied
and provokes in us new questions. Planets elsewhere? The answer is
yes. Yet they are not what we thought they’d be. Are we allowed to
change the question? Can we get one more wish? What we really want
to know is if other worlds are as friendly, fecund, and funky as our gur-
gling, green Earth. In searching, we’ll repeatedly be surprised by avant-
garde solar systems that we would never have thought up, and nontra-
ditional orbital arrangements that we didn’t even know were legal.
Life is a capacity the universe has to make small pockets of intense
order and beauty. It thrives on certain physical conditions found on Earth.
How specific and rare the required conditions are, we don’t know. All of
our ideas about life elsewhere are still educated guesses based on an
extrapolation from this single example. Yet, any way you look at it, no
matter what the planetary census ultimately shows, these exoplanets pop-
ping up around numerous stars in our galaxy are comforting on many lev-
els. The question “Are we alone?” is really many questions. In answer to
one of them we can now definitively say, “No. Ours is not the only fam-
ily of planets.” Phew. Didn’t think so, but it’s sure nice to know. It’s reas-
suring when our widely accepted conjectures become empirically sup-
ported truths. Sometimes we need a sign to show us that we’re not just
lost in the wilderness following our own footprints, chasing our shadows.
The universe is telling us that we are on the right track.
Exobiology: Life
14 on the Fringe
Surely one of the most marvelous feats of the twenti-
eth century would be the firm proof that life exists Image unavailable for
on another planet. All the projected space flights
electronic edition
and the high costs of such developments would be
fully justified if they were able to establish the exis-
tence of life on either Mars or Venus.
—STANLEY MILLER AND HAROLD UREY,
W R I T I N G I N Science, 1959
The most important scientific discovery that could be
Image unavailable for
made in this century is the discovery of life else-
electronic edition
where in the universe.
—ED WEILER, NASA ASSOCIATE
ADMINISTRATOR FOR SPACE SCIENCE,
Q U O T E D I N Science, 2002
D E T E C T I N G A N D P R O T E C T I N G L I F E
From the beginning, scientists advocating space exploration have been
motivated by the search for extraterrestrial life and have used this
evocative topic as a means of rousing public and official support. Yet
they have had to be cautious. Few scientists have been willing to predict
short-term success, especially after the “disappoi
nting” results of our
first few planetary probes. Do we want to stake the future viability of
planetary exploration on a long shot of uncertain odds?
Born at the fringes of the space program in the late 1950s, exobiol-
ogy focused on two questions: detection and protection. How do we
detect life on another planet? We can only search using a framework of
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preconceived ideas about what we are looking for, yet the ultimate
object of our search is to test these same ideas. This paradox has forced
us to accept fluid definitions and provisional search strategies.
The “planetary protection” question makes visible some ethical and
practical conundrums raised by our new ability to travel across space:
How do we protect other planets from contamination by organisms
hitchhiking on our spacecraft? How do we bring back samples of other
planets while protecting Earth from even a remote possibility of dan-
gerous contamination? Is new knowledge worth a small risk of inter-
planetary ecological disaster?
The first respectable modern scientist to publicly voice questions
about such cross-contamination was Nobel Prize–winning biologist
Joshua Lederberg. In 1958, he published an article in Science express-
ing his fear that our impending Moon probes might inadvertently con-
taminate our natural satellite. That year, spurred by Lederberg’s early
activism, the National Academy of Sciences adopted a resolution call-
ing on scientists to “plan lunar and planetary studies with great care”
and encouraging “the evaluation of such contamination and the devel-
opment of means for its prevention.”
At that time the issue was couched in language that was more practi-
cal than ethical. The fear was not so much that we might commit an
unforgivable and irreversible moral blunder by sacrificing one planet’s
biosphere to another’s clumsy curiosity. Rather, the danger was of ruin-
ing future opportunities for scientific research presented by the study of
uncontaminated worlds. The worry was about a crime not against
nature, but against science.
Lederberg convinced the powers that be that we should take appro-
priate precautions as we began sending our contraptions beyond
Earth’s gravitational embrace. The problem of protection was given
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