Chapter 20
Life in the Universe
Introduction
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We have discussed the nine planets
and some of the moons in the Solar
System, and have found most of
them to be places that seem hostile
to terrestrial life forms.
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Yet a few locations besides the
Earth—most notably Mars, with its
signs of ancient running water, and
Europa, with liquid water below its icy
crust—have characteristics that
suggest life may have existed there in
the past, or might even be present
now or develop in the future.
Exobiology is the study of life
elsewhere than Earth.
Introduction
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In our first real attempt to search for life
on another planet, in the 1970s, NASA’s
Viking landers carried out biological and
chemical experiments with martian soil.
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The results seemed to show that there is
probably no life on Mars (see figure).
A small, inadequately funded British
probe, Beagle 2 (the first Beagle having
been Darwin’s ship), failed as it
approached Mars in 2003. (One reason
it became so popular among the public
may have been that it was supposed to
send back tones from the musical group
Blur when it landed, though those tones
never came.)
Introduction
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NASA continues to explore Mars with robotic spacecraft
and one day should have a more sophisticated biology lab
landing on it.
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In the meantime, its Mars Exploration Rovers, Spirit and
Opportunity, and the European Space Agency’s Mars Express
have found clear signs that water flowed on Mars in the
distant past, raising hope that life could have formed there
at that time and might even have survived.
Although studies of a martian meteorite in 1996 gave
some indications of ancient, primitive life on Mars (see
Chapter 6), this idea has not been generally accepted,
though it is still causing much discussion.
Jupiter’s moon Europa and Saturn’s moon Titan (see
Chapter 7) are also intriguing places where scientists think
it is possible that life has begun.
Introduction
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Since it seems reasonable that life as we know it
would be on planetary bodies, we first discuss the
chances of life arising elsewhere in the Solar System,
as well as the kinds of stars most likely to have
planets suitable for the emergence and development
of intelligent life.
Next we explore attempts to receive communication
signals from intelligent extraterrestrials.
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We also consider a way to estimate the number of
communicating civilizations elsewhere in our Milky Way
Galaxy, or at least to see which factors most seriously
limit our ability to do so.
It is possible that humans are the only technologically
advanced civilization in our Galaxy, or one of very few.
Introduction
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Finally, we explain why most scientists do not
consider reported sightings of UFOs to be good
evidence for extraterrestrial visitations to Earth.
NASA has formed an institute of “astrobiology” and
is making a major push to investigate matters of
biology that can be important to understanding the
origin of life or to space exploration.
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The institute is “virtual,” in that it has no actual
buildings but rather is made of individuals who
communicate by e-mail and by occasional meetings.
20.1 The Origin of Life
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It would be very helpful if we could state a
clear, concise definition of life, but
unfortunately that is not yet possible.
Biologists state several criteria that are
ordinarily satisfied by life forms—reproduction,
for example.
Still, there exist forms on the fringes of life,
such as viruses, that need a host organism in
order to reproduce.
Scientists cannot always agree whether some
of these things are “alive.”
20.1 The Origin of Life
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In science fiction, authors sometimes conceive of beings
that show such signs of life as the capability for intelligent
thought (see figure), even though the being may share
few of the other criteria that we ordinarily recognize.
In Fred Hoyle’s novel The Black Cloud, for instance, an
interstellar cloud of gas and dust is as alive as (and
smarter than!) human beings.
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But we can make no concrete
deductions if we allow such wild
possibilities, so exobiologists prefer to
limit the definition of life to forms that
are more like “life as we know it.”
20.1 The Origin of Life
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Life on Earth is based on amino acids—chains of carbon,
in which each carbon atom is bonded to hydrogen and
sometimes to oxygen and nitrogen.
Chemically, carbon is essentially unique in its ability to
form such long chains; indeed, we speak of compounds
that contain carbon as being organic.
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But life is selective, incorporating only about 20 of all the
possible amino acids.
Similarly, long chains of amino acids form proteins,
though life utilizes only a small fraction of the multitude of
possible combinations of amino acids.
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The genetic code of any living creature is contained in one
extremely long and complex structure: DNA
(deoxyribonucleic acid), the famous “double helix.”
20.1 The Origin of Life
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How hard is it to build up long organic chains?
To the surprise of many, an experiment performed in the
1950s showed that making organic molecules is easier
than had been supposed.
Stanley Miller and Harold Urey, at the University of
Chicago, filled a glass jar with simple molecules like water
vapor (H2O), methane (CH4), and ammonia (NH3), along
with hydrogen gas (H).
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They exposed it to electric sparks, simulating the vigorous
lightning that may have existed in the early stages soon
after Earth’s formation.
20.1 The Origin of Life
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After a few days, long chains of atoms formed in the jar, in some cases
complex enough to include simple amino acids, the building blocks of life.
Later versions of these experiments created even more complex organic
molecules from a wide variety of simple actions on simple molecules (see
figures, left and middle).
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Such molecules may have mixed in the oceans to become a “primordial soup”
of organic molecules (see figure, right).
20.1 The Origin of Life
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Since the original experiment of Miller and Urey, most scientists have
come to think that the Earth’s primitive atmosphere was not made of
methane and ammonia, which would have disappeared soon after the
Earth’s formation.
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Instead, it may have consisted mostly of carbon dioxide, carbon monoxide,
and water, and such a mixture does not generally lead to a large abundance
of amino acids.
On the other hand, complex molecules may have formed near
geothermal sources of energy under the oceans or on Earth’s surface, or
perhaps in vents deep underground, where the right raw materials
existed.
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Also, extraterrestrial amino acids have been found
in several meteorites that had been long frozen in
Antarctic ice, as well as in some other meteorites
(see figure).
In any case, some extrasolar planets or moons in
our Galaxy may have had primitive atmospheres
similar to the mixture used by Miller and Urey, so
the results of their experiments are interesting.
20.1 The Origin of Life
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Also relevant are laboratory observations with
conditions mimicking the low density (strong
vacuum) of space.
Amino acids have been observed to form.
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However, mere amino acids or even DNA
molecules are not life itself.
A jar containing a mixture of all the atoms
that are in a human being is not the same as
a human being.
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This is a vital gap in the chain; astronomers
certainly are not yet qualified to say what
supplies the “spark” of life.
20.1 The Origin of Life
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Still, many astronomers think that since it is not difficult to
form complex molecules, primitive life may well have
arisen not only on the Earth but also in other locations.
The appearance of very simple organisms in Earth rocks
that are 3.5 billion years old, and indirect evidence for life
as far back as 3.8 billion years (not long after the end of
the bombardment suffered by the newly formed Earth),
suggests that primitive life arises quite easily.
Similarly, the presence of life in what appear to be very
harsh environments on Earth (water that is highly acidic or
near its boiling point, for example) shows that life can
exist in extreme conditions.
20.1 The Origin of Life
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The discovery of indigenous life on at least
one other planet or moon in the Solar
System would provide much support for the
hypothesis that simple organisms such as
microbes and bacteria form readily.
But even if life is not found elsewhere in
the Solar System, there are so many other
stars in space that it would seem that life
might have arisen at some location.
20.2 Life in the Solar System
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Life elsewhere in our planetary system,
if present at all, is primitive at best
(single cells, or perhaps very simple
multi-cellular organisms).
Among all of the planets and moons,
only a few have nonzero odds for life.
Mars has provided the best evidence
thus far, but it is still very controversial
(see figure).
Moreover, even if real, life on Mars might not have been
independent of Earth—a meteorite from Mars containing simple life
may have contaminated the young Earth, “seeding” it with life,
though this idea is of course quite speculative.
20.2 Life in the Solar System
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Europa, one of Jupiter’s large moons,
looks like a promising environment:
Below its icy surface there is almost
certainly water slush or even an ocean.
A Europa orbiter to study this moon in
more detail is being considered by NASA,
though its recent “Vision for Space
Exploration” of “Moon, Mars, and
Beyond” is apparently slowing down all
other NASA projects, even the Europa
orbiter.
20.2 Life in the Solar System
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Titan, Saturn’s largest moon, has a thick atmosphere of
nitrogen molecules.
As we saw, in 2004, the Cassini spacecraft sent the
Huygens probe into this atmosphere.
There is evidence for substantial methane and ethane on
Titan, perhaps in the form of lakes; methane can be solid,
liquid, or gas at temperatures reasonable for Titan, just as
water can have solid, liquid, or gaseous phases on Earth.
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Huygens, as it approached Titan’s surface, imaged what
appeared to be a lake shore.
The Cassini orbiter’s radar has found a reflection that
seems lake-like, presumably with the liquid being a tarry
substance.
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The absence of liquid water, however, makes it more difficult
for life as we know it to form.
20.2 Life in the Solar System
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Io, another Galilean satellite, has conditions that may be
suitable for life resembling that found near volcanic vents on
Earth.
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Note that an intelligent alien who obtains and correctly
interprets a spectrum of Earth could deduce the presence of life
here.
The large amount of free oxygen suggests the continuous
production by a process like photosynthesis; otherwise, oxygen
would rapidly be depleted because it is so reactive.
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Again, however, the apparent absence of water is a major problem.
In addition, methane quickly reacts with oxygen, so the significant
amount of methane (largely from cows—“bovine flatulence,” to
quote the late Carl Sagan) in our atmosphere implies a steady
production mechanism—the decay of organic compounds.
Such signs have been picked up from spacecraft, such as Galileo
and Cassini, while they were near the Earth and headed for the
outer planets.
20.3 Suitable Stars for Intelligent Life
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If we seek indigenous (that is, originating locally, not from
elsewhere), intelligent life on planets orbiting other stars,
what kinds of stars have the best odds?
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Stars that are either near the beginning or end of their lives
are not very good bets.
For example, intelligent life may take a long time to form,
so very young stars are less suitable than older stars.
Stars heading to the red giant or white dwarf stages go
through rapid changes, making it difficult for complex life
to survive.
White dwarfs and neutron stars certainly have passed
through stages that would have destroyed life.
20.3 Suitable Stars for Intelligent Life
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Some types of main-sequence stars are also not especially
suitable.
The lives of O-type and B-type stars are probably too
short for the development of life of any kind.
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Planets around A-type stars might have life, but probably not
intelligent life.
Type-M and type-L stars have a small ecosphere (also
known as the “habitable zone”)—the range of distances in
which conditions suitable for life might be found.
There is unlikely to be a planet in this narrow region
around a low-mass star.
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Also, such a planet would be in “synchronous rotation”: The
same hemisphere would always face the star, so one side
would be very hot, and the other very cold.
20.3 Suitable Stars for Intelligent Life
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Main-sequence stars of types F, G, and K are the most
likely candidates.
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They live a long time, and models of their ecosphere lead to
reasonably large sizes. (In more detail, such models must
explain the “Goldilocks effect” in the case of the Solar
System: why Venus is too hot, Mars is too cold, but the
Earth is just right.)
Single stars, stars in very wide binary systems (with
planets orbiting close to one star), or closely spaced
binary stars (which planets could orbit at large distances)
are most suitable.
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Planets that move from one star to another in a binary
system (for example, like a “figure eight”) tend to have
unstable orbits and are ejected.
20.3 Suitable Stars for Intelligent Life
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Over 160 extrasolar planets have already been found (see
our discussion in Chapter 9).
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Generally these planets are gas giants, in many cases very
close to the star or on highly eccentric orbits and hence
probably inhospitable to intelligent life.
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In most cases they orbit F, G, and K stars because the
searches specifically targeted those stars.
A few of them, however, appear to have orbital properties
potentially suitable for the emergence of life; perhaps at
least simple life exists within their atmosphere or on moons
orbiting them.
The 2005 announcement of a planet, presumably rocky,
with only about 7 to 8 times Earth’s mass, gives hope of
our finding closer analogs to Earth, which encourages
those trying to discover signs of life there.
20.4 The Search for
Extraterrestrial Intelligence
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How should we look for intelligent extraterrestrial life?
Perhaps we can find evidence for such life here on Earth, in the
form of alien spacecraft that have landed here.

After all, Pioneers 10 and 11 and Voyagers 1 and 2 are even now
carrying messages out of the Solar System in case an alien
interstellar traveller should happen to encounter these spacecraft
(see figures).
20.4 The Search for
Extraterrestrial Intelligence
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The odds, however, seem very small, given the vastness of space.
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A potentially more fruitful approach is to search for electromagnetic
signals, but some waves are more suitable than others.
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Although spaceships can, in principle, travel between the stars, even rather
quickly according to Einstein’s special theory of relativity, such journeys are
difficult, expensive, dangerous, and probably rare compared to
communication with electromagnetic waves.
For example, x-ray and gamma-ray photons have high energy and are
therefore expensive to produce, and typical atmospheric gases block them.
Ultraviolet photons are absorbed by interstellar dust in the plane of our
Milky Way Galaxy.
At optical wavelengths, the signal from a planet orbiting a star is very
difficult to distinguish from the bright light of the star itself, unless a large
amount of energy is concentrated into a narrow range of wavelengths or
into a short pulse.
And at infrared wavelengths, Earth’s warm atmosphere makes the sky
bright.
20.4 The Search for
Extraterrestrial Intelligence
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For many years, radio waves have seemed to be the best
choice: They are easy and cheap to produce, and are not
generally absorbed by interstellar matter.
Also, there are few sources of contamination—although
the increasing number and strength of radio and television
stations is a threat to radio astronomers in the same
manner that city lights brighten the sky at optical
wavelengths.
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At least initially, it seems too overwhelming a task to listen
for signals at all radio frequencies in all directions and at all
times.
One must make some reasonable guesses on how to
proceed.
20.4 The Search for
Extraterrestrial Intelligence
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A few frequencies in the radio spectrum seem especially
fundamental, such as the 21-cm line of neutral hydrogen
(as described in Chapter 15).
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This wavelength corresponds to 1420 MHz, a frequency over
ten times higher than stations at the high end of the normal
FM band.
We might conclude that creatures on a far-off planet
would decide that we would be most likely to listen near
this frequency because it is so fundamental.
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The “water hole,” the wavelength range between the radio
spectral lines of H and OH, has a minimum of radio noise
from background celestial sources, the telescope’s receiver,
and the Earth’s atmosphere, and so is another favored
possibility.
20.4 The Search for
Extraterrestrial Intelligence
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Humans have conducted several searches for extraterrestrial radio
signals.
No unambiguous evidence has been found, but the quest is worthwhile
and continues.
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A telltale signal might be an “unnatural” set of repeating digits, such as the
first 100 digits of the irrational number  (the Greek letter “pi”).
We must verify that the signal could not be produced by a natural, nonintelligent source (such as a pulsar), and also that it is not of human
origin (either unintentionally or intentionally).
Though there is agreement to verify the veracity of the supposition that a
signal received is indeed a message or a sign of extraterrestrial life, and
to do so before making a public announcement, the discovery of such an
extraterrestrial signal would be so momentous that it would surely be
released to the public without major delay.
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A protocol has been accepted by researchers in the field as to how to
announce any believable signal from extraterrestrials, so as not to cause fear
and panic.
20.4 The Search for
Extraterrestrial Intelligence
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In 1960, Frank Drake conducted the first
serious search for extraterrestrial signals.
He used a telescope at the National Radio
Astronomy Observatory (in West Virginia)
for a few months to listen for signals from
two of the nearest stars—tau Ceti and
epsilon Eridani.
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He was searching for any abnormal kind of
signal—a sharp burst of energy, for
example.
He called this investigation Project Ozma
after the queen of the land of Oz (see
figure) in L. Frank Baum’s stories.
20.4 The Search for
Extraterrestrial Intelligence
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A NASA-sponsored group based at the Search for
Extraterrestrial Intelligence (SETI) Institute in California
began an ambitious effort on October 12, 1992, the 500th
anniversary of Columbus’s landing in the New World.
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It made use of sophisticated signal-processing capabilities of
powerful computers to search millions of radio channels
simultaneously in the microwave region of the spectrum (see
figure).
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Consisting of both a sky survey and a
targeted study of individual stars, in its
first fraction of a second it surpassed the
entire Project Ozma.
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But Congress cut off funds anyway, after
about a year.
20.4 The Search for
Extraterrestrial Intelligence
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The targeted search of the project, known
as Project Phoenix, is continuing, backed
by funds contributed by private individuals.
Now led by astrophysicist Jill Tarter (see
figure), a real-life model for actress Jodie
Foster’s character Ellie Arroway in the
movie Contact, it examines about 1000
stars.
No unexplained signals have been found
so far, though there have been some
exciting false alarms, as in 1997 when an
intriguing signal turned out to be from the
SOHO spacecraft (which was described in
Chapter 10)!
20.4 The Search for
Extraterrestrial Intelligence
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A well-known project is “SETI@home,” whose operation
is based at the University of California, Berkeley (see
http://seti.berkeley.edu).
It has established a way for the general public to
contribute: During otherwise unused time on your
home computer, a special program can automatically
analyze data from the giant Arecibo radio telescope in
Puerto Rico (see figure, top) for signs of extraterrestrial
signals (see figure, bottom).
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Thus, there is a very small
but nonzero chance that the
first unambiguous evidence
of intelligent life elsewhere in
the Universe would be found
by your computer, should
you choose to participate!
20.4 The Search for
Extraterrestrial Intelligence
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The SETI@home effort already has nearly 6
million participants in over 200 countries,
effectively forming the Earth’s largest
supercomputer, and the amount of computing
time contributed has been over two million
computer-years.
The scientists conducting the project have
taken the most suspicious signals from the
widespread data analysis and gone back to
Arecibo to observe those sources in detail.
Obviously, no confirmation was obtained.
20.4 The Search for
Extraterrestrial Intelligence
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Although most searches for extraterrestrials have been
conducted at radio wavelengths, some optical and nearinfrared searches are also underway.
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Because the plane of the Milky Way Galaxy absorbs visible
light, we can’t expect to survey as many stars as at radio
wavelengths, but there are still plenty of them.
If we search for short, very intense pulses of light emitted
by lasers, we can actually see quite far in the plane of our
Galaxy, increasing the number of available stars.
More importantly, the laser pulses outshine the light from
the star that a planet is orbiting.
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Finally, such laser pulses are very difficult or impossible to
produce by any natural phenomena other than life; detection
of them could provide strong evidence for the existence of
extraterrestrials.
20.4 The Search for
Extraterrestrial Intelligence
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Only some scientists think that there is a reasonable chance of
detecting signals from extraterrestrial beings, at least in the
near future.
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Of course, now that we have looked and listened for a while,
that possibility seems to be reduced.
There are other reasons to think that there may not be any
extraterrestrials at all in our neighborhood within the Milky Way
Galaxy, or perhaps even in our entire Galaxy.
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But it would be a shame if there were abundant signals that we
missed just because we weren’t looking.
For example, the odds are that life would have arisen elsewhere
than on Earth first, so why aren’t the aliens already here?
Maybe they aren’t here because they don’t exist.
Even if we don’t get messages from outer space, there are
many scientific spin-offs of the search.
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Investigating thousands of stars in detail, and mapping the sky in
different parts of the spectrum, gives us bits of information that
can lead to scientific breakthroughs of other kinds.
20.5 Communicating with Extraterrestrials
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All of the searches described above are
passive—astronomers are simply looking for
signals from intelligent, communicating
extraterrestrials.
If such a signal is ever found and confirmed by
several cross-checks, we might choose to
“reply”—but not until some global consensus has
been reached about who will speak for Earth and
what they will say.
Nevertheless, we have already intentionally sent
our own signals toward very distant stars that
may or may not be orbited by planets containing
intelligent life.
20.5 Communicating with Extraterrestrials
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For example, on November 16, 1974, astronomers
used the giant Arecibo radio telescope in Puerto Rico
to send a coded message about people on Earth (see
figure, right) toward the globular star cluster M13
(see figure, below) in the constellation Hercules.
The idea was that the presence of over 200,000
closely packed stars in that location would increase
the chances of our signal being received by a
civilization in the planetary system of one of them.
20.5 Communicating with Extraterrestrials
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But the travel time of the message (at the speed
of light) is about 24,000 years to M13, so we
certainly cannot expect to have an answer before
twice 24,000, or 48,000 years, have passed.
If any creature is observing the Sun at the right
frequency (2380 MHz) when the signal arrives,
the radio brightness of the Sun will increase by
10 million times over a 3-minute period.
A similar signal, if received from a distant star,
could be the giveaway that there is intelligent life
there.
20.5 Communicating with Extraterrestrials
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In retrospect, M13 may not have been the best
choice as a target, despite its large number of
stars.
The problem is that the stars in globular clusters,
being very old, were formed from gas that did not
have a large proportion of heavy elements; it had
not gone through many stages of nuclear
processing by massive stars and supernovae.
Rocky, Earth-like planets are thus not as likely to
have formed there as they would have around
younger stars.
20.5 Communicating with Extraterrestrials

A quarter century later, in 1999, a new message was sent by
Canadian astronomers toward several relatively nearby (50 to
70 light-years away) Sun-like stars, including 16 Cygni B, which
is known through Doppler measurements to have at least one
planet orbiting it (see our discussion in Chapter 9).
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The complete message, which is much larger in size, duration, and
scope than the one sent in 1974, was transmitted three times over
a 3-hour period in the direction of each star.
Still later, starting on July 5, 2003, some much more
complicated messages were sent out toward five nearby stars
(32 to 46 light-years away) from a 70-m radio telescope in
Evpatoriya, Ukraine.
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One would hope to get answers in fewer than 100 years instead of
tens of thousands, but nobody is betting on a return message.
20.5 Communicating with Extraterrestrials
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Even had we not sent these few directed messages,
during much of the 20th century (and so far in the 21st
century) we have been unintentionally transmitting radio
signals into space on the normal broadcast channels.
Aliens within a few tens of light-years could listen to our
radio and television programs.
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A wave bearing the voice of Winston Churchill is expanding
into space, and at present is about 60 light-years from Earth.
And once a week or so a new episode of Fear Factor is
carried into the depths of the Universe.
Do you think aliens would get a favorable impression of us
from most of what they hear?
20.5 Communicating with Extraterrestrials
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Is it reasonable to expect that there are any
signals out there that we can hope to detect
with projects such as that at the SETI
Institute?
What if no intelligent creatures exist
elsewhere in our Milky Way Galaxy or even in
the observable parts of the Universe?
20.6a The Drake Equation
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Instead of phrasing an all-or-nothing question about
extraterrestrial life, we can use a procedure developed by
Frank Drake, then of Cornell University, and extended by
Carl Sagan and Joseph Shklovskii, among others.
The overall problem is broken down into a chain of simpler
questions, the results of which are multiplied together in
what is called the Drake equation to give the final
answer.
The formulation is unusual foran equation because it is
really a guide to thought rather than a real way of
calculating; there is no right answer or solution to the
Drake “equation.”
20.6a The Drake Equation
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In the standard formulation of the Drake equation, we
first estimate the rate at which stars form in our Galaxy.
Second, we consider the probability that stars at the
centers of planetary systems are suitable to allow
intelligent life to evolve.
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For example, as we have already discussed, the most
massive stars evolve rather quickly, remaining stable for too
short a time to allow intelligent life a good chance to evolve.
Third, we ask what the probability is that a suitable star
has planets.

With the new detection of planets orbiting other stars, most
astronomers think that the chances are likely to be pretty
high.
20.6a The Drake Equation
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Fourth, we need planets with suitable conditions for the
origin of life, so we multiply by the average number of
such planets per suitable star.
A planet like Jupiter might be ruled out, for example,
because it lacks a solid surface and because its surface
gravity is high. (Alternatively, though, one could consider
a liquid region, if it were at a suitable temperature, to be
as advantageous as the oceans on Earth may have been
to the development of life here. Or a moon of the planet
could provide the solid surface.)

Also, planets probably must be in orbits in which the
temperature does not vary too much.
20.6a The Drake Equation
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Fifth, we have to consider the fraction of the suitable
planets on which life actually begins.
This is a very large uncertainty, for if this fraction is zero
(with the Earth being the only exception), then we get
nowhere with this entire line of reasoning.
Still, the discovery that amino acids can be formed in
laboratory simulations of at least some kinds of potential
primitive atmosphere, and the discovery of complex
molecules (such as ammonia, formic acid, and vinegar) in
interstellar space, indicate to many astronomers that it is
relatively easy to form complicated molecules.

As mentioned previously, amino acids, much less complex
than DNA but also basic to life as we know it, have even
been found in some meteorites.
20.6a The Drake Equation

Moreover, life is found in a wide range of extremes on
Earth, including oxygen-free environments near
geothermal sulfur vents on the ocean bottom or
surface hot springs, under rocks in the Antarctic, and
deep underground in some other parts of the Earth
(see figures).


These bacteria do not survive in the presence of
oxygen; their existence supports the idea that life
evolved before oxygen appeared in Earth’s atmosphere.
So environments on other planets may not be as
hostile to life as we had thought, even though they
couldn’t support the types of animal and plant life with
which we are most familiar.

It is these hardy examples of life on Earth that give us
hope that the possible discovery of primitive life on Mars
will someday be confirmed.
20.6a The Drake Equation

Sixth, if we want to hear meaningful signals from
aliens, we must have a situation where not just
life but intelligent life has evolved.


We cannot converse with algae or paramecia, and
certainly not with the organic compounds reported
on Mars.
Furthermore, the life must have developed a
technological civilization capable of interstellar
communication.

So we consider the fraction of stars with life on
which intelligence and communication actually
develop.
20.6a The Drake Equation

Finally, such a civilization must also live for a fairly long
time; otherwise, its existence would be just like a
flashbulb going off in an otherwise perpetually dark room.


Humans now have the capability of destroying themselves
either dramatically in a flurry of hydrogen bombs or more
slowly by, for example, altering our climate, lessening our
ozone shield (which keeps harmful ultraviolet radiation out),
or increasing the level of atmospheric pollution.
Natural disasters must also be avoided.

That an Earth-crossing asteroid or comet will eventually
impact the Earth with major consequences seems
statistically guaranteed on a timescale of a few hundred
million years, unless we take preventive action.
20.6a The Drake Equation

It is a sobering question to ask whether the typical
lifetime of a technological civilization is measured in
decades, or whether all the problems that we have—
political, environmental, and otherwise—can be overcome,
leaving our civilization to last for millions or billions of
years.


So, to complete our calculation, we must multiply by the
average lifetime of a civilization.
We can try to estimate answers for each of these simpler
questions within our chain of reasoning, though some of
them are actually more like wild guesses. (Consequently,
some scientists don’t find the exercise useful.)

We can then use these answers together to figure out the
larger question of the probability that communicating
extraterrestrial life exists.
20.6a The Drake Equation

Fairly reasonable assumptions can lead to the conclusion
that there may be thousands, or even tens of thousands,
of planets in our Galaxy on which technologically
advanced life has evolved. (Handy “Drake equation
calculators” can be found at several websites, including
that of the SETI Institute, http://www.seti.org).


Perhaps overly optimistically, Carl Sagan estimated that
there might be a million such planets.
On the other hand, adopting more pessimistic (but
possibly more realistic) values for the probabilities of
intelligent and technologically advanced life, and for the
typical lifetime of such a civilization, gives a much bleaker
picture.

Indeed, humans may be at the pinnacle of intelligence and
technological capability in our Galaxy, with few (if any)
comparably advanced civilizations.
20.6b Where Is Everyone?

One interesting argument is as follows: If there are
many (say, a million) intelligent, communicating
civilizations in our Galaxy, why haven’t we detected
any of them?


Where are they all?
Surely most must be far more advanced than we
are, and have sent signals if not spacecraft that
have reached Earth, yet there is no evidence for
them.

This reasoning, promoted among others by the late
Enrico Fermi of the University of Chicago, suggests
that advanced creatures such as us might be very
rare, even if primitive life is abundant in our Galaxy.
(Note that various surface features claimed by some
people to have been built by extraterrestrials are
either nonexistent, such as the “face” on Mars—see
figure—or have more conventional explanations, like
the huge drawings on the Peruvian desert known as
the Nazca lines.)
20.6b Where Is Everyone?

Indeed, a more extreme version of this argument points
out that there could be self-sustaining colonies voyaging
through space for generations.


They need not travel close to the speed of light if families
are aboard.
Even if colonization of space took place at a rate of only 1
light-year per century, our entire Galaxy would still have
been colonized during a span of just a hundred million
years, less than one per cent of its lifetime.

The fact that we do not find extraterrestrial life here
indicates that the Solar System has not been colonized,
which in turn may imply that technologically advanced life
has not arisen elsewhere in our Galaxy.
20.6b Where Is Everyone?


Another possibly relevant fact suggesting that life
having our capabilities is very rare in our Galaxy is
that more than 1 billion species have ever lived on
Earth, yet apparently we are the only species to
have developed space communication or even
acquired technology.
Similarly, it is sobering to realize that a
communicating civilization developed on Earth only
in the last century, despite evidence for primitive
life on Earth for the past 3.5 to 3.8 billion years.
20.6b Where Is Everyone?


Recently, astronomers and other scientists have carefully
considered the many factors that affect complex life on
Earth.
For example, the stability of the Earth’s axis of rotation
depends on the presence of our rather massive Moon.


If we had no moon, or only a small one, then Earth’s axis
would undergo rather rapid, random changes in its
orientation, causing large variations in the seasons and
climate, and presumably making it more difficult for complex
life to develop.
Similarly, the presence of the very massive planet Jupiter
in the Solar System, yet fairly far from Earth and having a
nearly circular orbit, is a blessing: Jupiter’s gravitational
tugs have cleared out the Solar

System, making collisions between Earth and large “killer
meteoroids” infrequent.
20.6b Where Is Everyone?


There are many other relevant factors.
The presence of plate tectonics on Earth is very important, for instance,
because it allows carbon to be recycled in a steady manner through
Earth’s atmosphere, oceans, interior, and surface.



This “carbon cycle” is important for maintaining global climate stability and
for certain aspects of life itself.
Having an abundant supply of heavy elements during the formation of
the Solar System was also crucial; most regions of our Milky Way Galaxy
were quite deficient in such elements billions of years ago, when the
Sun formed.
These arguments, and others, have led many astronomers to conclude
that Earth-like planets capable of developing complex, technologically
advanced, communicating civilizations really are rare in our Milky Way
Galaxy, though primitive life such as bacteria might be very common
indeed. However, other astronomers (such as those at the SETI
Institute) argue that there are major potential flaws in the reasoning for
this “rare Earth” hypothesis, and that communicating civilizations may
be common.
20.6b Where Is Everyone?


It is not clear, for example, whether the apparently
“special” conditions of Earth are essential to the
development of intelligent life; maybe our view has been
too highly skewed by the single example we know—
ourselves.
Moreover, is it necessarily true that if other intelligent
creatures evolved, they would choose to colonize space,
or have the ability to do so?


The rather late appearance of technologically advanced life
on Earth may also be a statistically unlikely fluke.
Finally, some of the arguments relied on the unproven
assumption that life elsewhere is quite similar to that on
Earth in its properties and evolutionary path.
20.6b Where Is Everyone?


In any case, unless we make the effort to
actually look for signs of intelligent
extraterrestrial life, we might never know
whether humans are indeed alone in our
Galaxy, or simply one of many such creatures.
Thus, many astronomers support the search
for extraterrestrial intelligence, especially
using telescopes that are simultaneously
doing other, more conventional types of
research projects.
20.6b Where Is Everyone?

Some major radio-telescope projects are being built for
SETI purposes, with more ordinary astronomy to be
carried out as a bonus (see figure).

Perhaps someday we will be scanning the skies with radio
telescopes on the far side of the Moon, shielded from Earth’s
radio interference by the Moon’s bulk.

The chances for success might be
slim, but all agree that the actual
detection of extraterrestrial signals
would be one of the most important
and mind-blowing discoveries ever, if
not the greatest discovery of all time.
20.7 UFOs and the Scientific Method

If some or many astronomers believe that
technologically advanced life exists elsewhere in our
Galaxy, why do they not accept the idea that
unidentified flying objects (UFOs) represent
visitations from these other civilizations?


The answer to this question leads us not only to
explore the nature of UFOs but also to consider the
nature of knowledge and truth.
The discussion that follows is a personal view of the
authors, but one that is shared to a greater or lesser
extent by most scientists.
20.7a UFOs


The most common UFO is a light that appears in the sky that seems to
the observer unlike anything made by humans or any commonly
understood natural phenomenon.
UFOs may appear as a point or extended, or may seem to vary in size
or brightness.


Most of the sightings of UFOs that are reported can actually be
explained in terms of natural phenomena.


But the observations are usually anecdotal, are not controlled as in a
scientific experiment, and are not accessible to study by sophisticated
instruments.
The Earth’s atmosphere can display a variety of strange effects, and these
can be used to explain many apparent UFOs.
When Venus shines brightly near the horizon, for example, we
sometimes get telephone calls from people asking us about the
“UFO”— especially if the crescent moon happens to also be in that
direction.


It is not well known that a bright star or planet low on the horizon can
seem to flash red or green because of atmospheric refraction.
Atmospheric effects can affect radar (radio) waves as well as visible light.
20.7a UFOs



For many of the effects that have
been reported, the UFOs would
have been defying wellestablished laws of physics.
Why haven’t we heard the
expected sonic booms, for
example, from rapidly moving
UFOs?
Scientists treat challenges to laws
of physics very seriously, since our
science and technology are based
on these laws and they seem to
work extremely well.
20.7a UFOs


Most professional astronomers feel that UFOs can be
so completely explained by natural phenomena that
they are not worthy of more of our time.
Although most of us do not categorically deny the
possibility that UFOs exist (after all, the Voyager
spacecraft might someday pass by a planet orbiting
another star), the standard of evidence expected of
all claims in science has not yet been met.


Some individuals may ask why we reject the
identification of UFOs with flying saucers, when—they
may say—that explanation is “just as good an
explanation as any other.”
Let us go on to discuss what scientists mean by
“truth” and how that applies to the above question.
20.7b Of Truth and Theories


At every instant, we can explain what is happening in a
variety of ways.
When we flip a light switch, for example, we assume that
the switch closes an electric circuit in the wall and allows
the electricity to flow.




But it is certainly possible, although not very likely, that the
switch activates a relay that turns on a radio that broadcasts
a message to the president of the United States.
The president then might send back a telepathic message to
the electricity to flow, making the light go on.
The latter explanation sounds so unlikely that we don’t
seriously consider it.
We would even call the former explanation “true,” without
qualification.
20.7b Of Truth and Theories

We usually regard as “true” the simplest explanation that satisfies all
the data we have about any given thing.



Science is based on Occam’s Razor, though we don’t usually bother to
think about it.



This principle is known as Occam’s Razor; it is named after a 14thcentury British philosopher who originally proposed it.
Without this rule, we would always be subject to such complicated doubts
that we would accept nothing as known to be true.
Sometimes something we call “true” might be more accurately described
as a theory (see Chapter 1).
An example of a theory is the Newtonian theory of gravitation, which
for many years explained almost all of the planetary motions.
Albert Einstein’s 1916 theory of gravity, known as the general theory
of relativity, provided an explanation for a nagging discrepancy in the
orbit of Mercury, as we described in Chapter 10.
20.7b Of Truth and Theories

Is Newton’s theory “true”?


Is Einstein’s theory “true”?


We may say so, although one day a newer theory may come along that is
more general than Einstein’s in the same way that Einstein’s is more general
than Newton’s. Indeed, as we discussed in Chapter 19, superstring theory is
a leading candidate for the unification of general relativity and quantum
physics, and hence may someday be a more complete theory than
Einstein’s.
How does this view of truth tie in with the previous discussion of UFOs?


Though we know it is “false,” it is a good approximation of the truth in most
regions of space; it is generally an accurate model for what we observe.
Scientists have assessed the probability of UFOs being flying saucers from
other worlds, and most have decided that the probability is so low that we
have better things to do with our time and with our national resources.
We have so many other, simpler explanations of the phenomena that
are reported as UFOs that when we apply Occam’s Razor, we call the
identifications of UFOs with extraterrestrial visitation “false.”
20.8 Conclusion


You have covered a lot of material in
this book, and learned much about the
Universe.
We, the authors, hope that this new
knowledge increases your awe and
fascination for nature.


The understanding of a phenomenon
should enhance its beauty, not detract
from it.
The magnificence of the Universe
comes in part from its logical
structure—and the foundation is
perhaps unexpectedly simple.

Indeed, Einstein remarked that “The
most incomprehensible thing about the
Universe is that it’s comprehensible.”
20.8 Conclusion

The actual consequences of the basic laws can be
extraordinarily complicated and varied.



The best example is life itself: It is the most
complex known structure.
Even the simplest cell is more difficult to
understand than the formation of galaxies or the
structure of stars.
Our highly advanced brains and dexterity are what
sets humans apart from other forms of life.

We are able to question, explore, and ultimately
understand the inner workings of nature through a
process of observation, experimentation, and logical
thought.
20.8 Conclusion


It is almost as if the Universe has found a way to know
itself, through us.
We are the observers and explorers of the Universe;
we are its brains and its conscience.


This makes each one of us special to the Universe as a
whole.
Perhaps it need not have been this way: Alter the
physical constants ever so slightly and the Universe
may have been stillborn, with no such complexity.

But here we are, enjoying life in this beautiful, amazing,
mind-blowing Universe.