ASTR 1210 (O'Connell) Study Guide 23
Are we alone?
Are there billions of advanced lifeforms
in the universe?
From the history of science, it's obvious that human beings have a
deep urge to understand nature and the universe around them. We are
the first species on our planet to have the desire and capacity to
confront questions on a cosmic scale. The first recorded serious
explorations were made over 2500 years ago, and since then we have
invested tremendous effort into investigating basic questions about
the mechanisms of nature and the place of humans in the universe.
Science has made great progress by insisting on strict standards of
evidence. And, after 500 years, we now have a very good, if still
preliminary, understanding of many aspects of the universe.
The length of time it took to get meaningful answers indicates how difficult
it is to obtain reliable knowledge on this scale.
One of the biggest unanswered questions concerns the presence of
other, non-terrestrial lifeforms in the universe. In fact, the most
powerful impact of astronomy on popular thinking concerns something
that it hasn't discovered yet and possibly never
So, in this lecture we examine the questions of whether
there is life elsewhere in the cosmos, where it might be,
and what forms it might take.
The two questions at the top of the page frame the
possibilities for the existence of other advanced species in the
When you combine the ideas of life and the universe
you reach a stunning conclusion no matter which way you argue:
Most astronomers subscribe to the latter view---that conditions
on Earth are typical (or at least not uncommon)---which implies
that extraterrestrial life is widespread and that there are many
- There are billions of stars in each galaxy and
about 100 billion galaxies within reach of our
telescopes. (See Study Guide 2.)
The total number of stars in the observable universe
is of order 10,000,000,000,000,000,000,000 (or 1022).
- If you believe Earth and humanity are special, uniquely
fortunate, or specially created (the "pre-Copernican" view), then we are
ALONE in this unimaginably vast cosmos.
- On the other hand, if you believe we are average (the
"Copernican" view), then the universe TEEMS with billions of
This is not just a modern point of view. It goes back to the ancient
Greeks and was historically advocated by many other writers,
e.g. Huygens, the discoverer of Saturn's moon Titan.
See this translation of his book Cosmotheoros.
"To consider the Earth as the only populated world in infinite space
is as absurd as to assert that in an entire field of millet, only one
grain will grow."|
--- Metrodorus of Chios (ca. 300 BC)
"What a wonderful and amazing Scheme have we here of the
magnificent Vastness of the Universe! So many Suns, so many
--- Christiaan Huygens (1698)
A revolution in prospects for astrobiology
Several remarkable discoveries lend credence to this picture:
These breakthroughs, coming after centuries of speculation,
have changed the whole character of the study of exterrestrial
- 1995+: The discovery of planetary
systems around nearby stars (see Guide 11). We have
discovered hundreds of other planetary systems in the last 25 years.
Many, if not most, stars like the Sun have planets. Despite
the technical challenges, we have already identified many Earth-size
In April 2014, the discovery of
186-f, the first Earth-sized planet (only 10% larger) in the
"habitable zone" around another star,
- 1996: Evidence suggesting the presence
of fossil microorganisms on Mars (see
Guide 17). This claim is highly controversial. But
right or wrong, it has stimulated the development of vigorous research
in the rapidly growing field of "Astrobiology."
- The discovery of water-rich environments on
Mars (in the past) and under the surfaces
and Enceladus. And the strange and
unique but promising hydrocarbon-rich surface
B. Life on Earth
Just as our everyday "common-sense" perspective provides no clue to the scale
of the real universe, our naive assumptions about the nature of life
on Earth, as they existed at the beginning of the scientific age, have
been shattered by the evidence:
Age: Life on Earth is ancient. It has thrived for at
least 3 billion years, according to the fossil record, and the
character of lifeforms has changed radically throughout that time.
Human beings, on the other hand, are brand new.
The genus Homo, which
includes modern humans, has existed for only about 2.5 million years,
or 0.06% of the age of the Earth. Homo sapiens emerged
only about 250,000 years ago.
Functional basis: random chemical interactions, governed by natural physical
In contradiction to all expectations through about 1900, no
force" is required. The laws of physics and chemistry that
govern the structure and functioning of organic systems
are exactly the same as those that govern inorganic
Unity: Despite extraordinary external diversity, there is only
one type of terrestrial life at the molecular level
- A small set of basic chemical building blocks (large molecules
constructed from abundant atoms like H,C,N,O, and a few others)
- The key constituent is carbon, which offers 4 chemical bonds as
the basis for forming complex molecules
- Nucleic acids (DNA, RNA): These long strand
molecules provide an enormous capacity for information storage and
chemical control for molecular reproduction
350 million years of evolution on Earth: characteristic terrestrial lifeforms
from the Cambrian (500 Myr
ago, left) to the Jurassic (150 Myr ago, right) periods
The proliferation and diversification of lifeforms is produced by evolution through natural selection for
better adapted types (Darwin, Wallace 1858)
The basic principle of evolution is very simple: those
organisms that are best adapted by their genetic makeup to a given
environment thrive and pass those favorable genetics on to subsequent
generations. Dispersion in genetic material in each generation is
guaranteed by chemical mutations. Mutations that are accidentally
favorable are amplified by natural selection and propagate forward
into modified species. On Earth, there have been hundreds of millions
of generations preceding the familiar lifeforms we see around us
today. The consequent adaptation to an immense variety of
environmental niches is profound.
Despite the raging "creationism," "intelligent design," and other
controversies surrounding Darwinian evolution that get prominence in
the media, and sometimes state legislatures, biological evolution is
as well established a basic fact of science as any other,
e.g. that Earth is a planet or that the Sun is a star.
All the basic "predictions" of Darwinian evolution have been
thoroughly confirmed in the last 150 years of biology, physics,
astronomy, geophysics, and paleontology. The evidence is
fossil record is now fabulously rich (250,000
species)---vastly more so than in Darwin's day---and clearly shows the
radical change of lifeforms on Earth through time. Huge fossil
collections exist for epochs between 10,000 and 500,000,000 years ago.
E.g.: geological age-dating by radioactive isotopes is now
highly accurate and graphically reveals the enormous stretch of time
over which evolution on Earth has been working.
E.g.: genetics, the mechanism by which inherited characteristics are
passed from one generation to the next, was mysterious in Darwin's time
but is now thoroughly understood, as are the causes of the
mutations that engender changes in species.
E.g.: DNA mapping has recently demonstrated astonishing chemical
similarities between humans and other higher lifeforms. We share
98.8% of our genes with chimpanzees.
The astronomical evidence for evolution of the universe and its
contents over a period of 10-15 billion years is as strong as, but
entirely independent of, the biological & paleontological evidence for
evolution of life on Earth. See Study Guide 2
and links therein.
It is now clear that humans are part of a
continuum of life on Earth, not a special class.
This is probably the most stringest test of evolutionary biology ever
made, and it passed with flying colors. Darwin, Huxley, and the other
leading biologists up to 75 years ago could not have imagined how
precise or conclusive such chemical tests of the evolutionary
principle could be.
E.g.: an apparent contradiction between the age of the Sun, in
Darwin's time thought to be only about 30 million years, and the
geological/fossil time-scale (much older) was resolved when physicists
realized (ca. 1940) that the source of the Sun's energy was nuclear
fusion, for which there is abundant fuel to maintain the Sun's
enormous luminosity for much longer periods. The Sun's age is now
estimated to be about 5 billion years, and the solar and
geological time-scales are now in good agreement.
E.g.: detailed measurements of the brightnesses & colors of stars in
star clusters permit us to age-date their formation times. These
range from about 10 million years for clusters in active star-forming
regions to 13 billion years in the case of "globular clusters" like M80.
E.g.: by using powerful telescopes to study the Hubble Deep Field and other distant regions, astronomers
can observe the universe as it was billions of years ago. No
other field of science is able to make such direct observations of
the distant past. Not only can we see the past, we also can
determine how different the contents of the universe were at earlier
they change with "lookback time." There is no doubt that the universe
E.g.: spacecraft observations of the cosmic background radiation
from the Big Bang have recently determined the age of the universe to
be 13.7 billion years.
Controversies over the reality of evolution are confined to political,
religious, & education circles.
are not important among active scientists, who accept evolution
as a foundation of modern science. Scientists involve themselves in
these controversies only insofar as they try to protect the integrity
of their disciplines and of science education from political
In these debates, evolution is often mistakenly conflated with the
question of the origin of life on Earth. Scientists do not
presently have a good understanding of how (or even whether) life
originated on Earth. That discussion is hypothetical. The evidence
for evolution (that is, for a change over time) of lifeforms on
Earth, however, is entirely independent of the question of life's
origins. You do not have to know where a sapling came from in
order to know that a tree is growing.
Most anti-evolution arguments are conceptually medieval. You
can disregard evolution only if you are prepared to disregard
the rest of modern science and scientific thinking.
Beware of those who urge you to do this.
D. Origin of Life on Earth?
As just noted, we do not have a good understanding of how life
originated on Earth. Given what we do know, however, it is plausible
that the chain of life began at the molecular level with molecular
evolution from simple, abundant, pre-organic chemicals.
An alternative to a terrestrial origin for life
is panspermia: the seeding of Earth from an external
source, accidental or deliberate.
- Probably in oceans, a dense medium with vast quantities of basic
- Biosynthesis requires sufficient sources of energy, but
these are abundant in the early Earth environment: solar
radiation, vulcanism, ocean vents, lightning, etc.
- The plausibility of early molecular evolution was demonstrated in
a classic 1953 laboratory simulation:
experiment. Starting from common pre-biotic ingredients
thought to be characteristic of the early Earth, sugars, amino acids,
and DNA bases were generated in only a few days. This showed
how easy it is to start a chemical synthesis sequence that can lead to
- Note that once it begins, molecular evolution is subject
through natural selection to the same kind of acceleration toward
survivable and proliferating forms as are biological organisms.
autocatalysis probably plays an important role here.
- Discoveries of
--- organisms thriving under unexpectedly harsh conditions of
temperature, pressure, or acidity --- demonstrate how robust
simple life forms can be.
- For 3/4 of its history, life on Earth consisted only of
organisms, entirely unlike the profusion of advanced types seen
today. Numerically, the simple organisms, starting with bacteria and
vastly outnumber the more complex types even now.
- We have discovered that organic molecules, produced by natural chemical
processes, are present in large
quantities even in ostensibly hostile cosmic environments such as
clouds. These could be the building blocks for biogenesis throughout our
- An obvious way for lifeforms to propagate in a given planetary system
is through comets or meteoroids, as the "SNC" meteorites from Mars vividly demonstrate. Meteoroids could
have spread Mars life to Earth or vice versa. We could be
- The notion of panspermia simply pushes the question of how life
originated back one level. Panspermia has occasionally been very
controversial, but at present there is no way to decide whether life
originated on Earth itself or came here from elsewhere.
Introducing the possibility of panspermia doesn't, unfortunately,
resolve the question of whether early terrestrial conditions were
favorable to the origin of Earth-like lifeforms. If life came here
from elsewhere, it could have been because the Earth was hostile
to generating lifeforms. Or, Earth could have been perfectly
hospitable to biogenesis, but the external seeding occurred before
it could take place.
- For more information on panspermia, click
The "habitable zones" for Earth-like planets surrounding
four different types of stars are shown in light blue;
are more massive and hotter than the Sun, "K" and "M" are less massive
The volume of the habitable zone increases for hotter
E. Life Elsewhere in Our Solar System
Are there plausible biospheres elsewhere in the Solar System?
- Raw materials;
- A dense medium (preferably water, but other liquids/dense gases are possible);
- A protected environment, maintained in the appropriate temperature/pressure range;
- Energy sources; primarily sunlight, but volcanic vents, lightning, interior heat, etc. are alternatives;
- Sufficient time (100 Myr-1 Gyr?)
For a particular type of parent body (terrestrial planet, Jovian
planet, Jovian satellite, comet), we can define a habitable
zone as those distances from the parent star for which that
type can offer a comfortable, long-lived biosphere, neither too hot
nor too cold, for life to develop. For Earth-like life, we believe
that biospheres require the presence of liquid water.
In the solar system, the habitable zone for Earth-like planets
with open bodies of liquid water on their surfaces
ranges from about 0.9 AU to 1.4 AU, covering the Earth's orbit
(of course) but not reaching to either Venus or Mars. See the diagram
is a brief discussion of the factors entering the determination of an
Earth-like habitable zone and a chart that shows where a number of the
exoplanets now known fall. (From Sky & Telescope, March
For stars hotter or brighter than the Sun, the zone would be farther
from the star and larger in volume; and the converse for cooler
or fainter stars.
The interest in searching for exoplanets around "cool dwarf" stars,
such as the "K" and "M" stars shown in the diagram above, arises from
the fact that their close-in habitable zones are more easily probed
with current planet detection techniques than those of brighter
Study Guide 11).
Because their atmospheres or interiors can be much warmer than their
surfaces and harbor large volumes of liquid water, the habitable zone
for Jovian-type planets or their icy satellites would be much
larger. Whether life could thrive in such environments, however,
Possible biospheres on the outer satellites: (Left) The
icy surface of Europa (pseudocolor, Galileo mission);
water vapor geysers of Enceladus (pseudocolor, Cassini mission);
(Right) The hydrocarbon-rich surface of Titan, as viewed by the
descending Huygens probe.
The existence of advanced lifeforms anywhere else in our Solar System
would astonish most scientists but is not completely impossible. On
the other hand, primitive lifeforms are certainly possible in
several settings, but remote detection is unlikely. We must
search "in situ," which implies complicated and costly space
- Venus: no! The high temperature, pressure, and corrosive
atmosphere are sufficient to sterilize the surface of all Earth-like
- Mars: There is plausible evidence for a biosphere > 1 Gyr
ago with abundant water; SNC meteorites provide
some evidence for microorganisms. Too cold and dry now for life?
The absence of ozone in the present-day atmosphere allows damaging
solar UV flux to penetrate to the surface.
- Jupiter, Saturn atmospheres: Results from the Galileo probe (1995), which sampled the
outermost layers of Jupiter's atmosphere, were not promising
but don't exclude a biosphere.
- Europa (J) and Enceladus (S)
each have evidence for a liquid water reservoir/ocean lying
beneath the visible crust of ice. These are probably the most
promising sites for bio-exploration after
Jupiter's largest moon, has just recently been shown to contain a
- Titan (S)
has an extraordinary hydrocarbon-rich atmosphere and
surface, which is a possible biosphere.
mission demonstrated the presence of liquid hydrocarbons (methane,
ethane) on Titan, and some scientists think "methanogenic" lifeforms
might exist there. Most believe the very low temperatures (-180
degrees C) would preclude living organisms on the surface. As in the
case of Europa, Ganymede, and Enceladus, however, deep reservoirs with
more favorable temperatures are possible.
Here is a
prospectus for exobiology on Titan.
- Comet nuclei: these icy bodies, often with a
coating of organic molecules, could act as "portable reservoirs" of
organisms; however, temperatures are normally very low.
F. Intelligent Life Elsewhere
This is a fascinating but virtually 100% speculative subject. There
is a paucity of facts, understanding, and imagination, and that leads
to a wealth of conjecture and controversy.
The Drake Equation
The Drake Equation, named after
astronomer Frank Drake, was an early (1961) "rough order of magnitude"
estimate of the number of advanced technical civilizations in
our Galaxy capable of undertaking interstellar communication.
"Communication" implies the ability to transfer information but not
necessarily to undertake travel between stars. The most basic form of
communication involves electromagnetic radiation.
Here is a
simplified list of the elements in the Drake estimate based on our
The quantities entering the Drake calculation under items (3) and (4)
are highly uncertain and controversial. You can find elaborate
treatments of our understanding of the important factors, and gaps
therein, in the books listed under "optional reading" below.
- There are 100 billion stars in our Galaxy
- Assume 0.001% - 5% of the stars have Earth-like planets
in the habitable zone.
Support? The recent detection of thousands of extra-solar
planets. Even though many of these are Neptune-class or larger,
rather than Earth-size, most astronomers expect that the fraction of
stars with Earth-like planets will prove to be of order 5% or larger.
This is at the high end of the range considered plausible over the
last 50 years.
The Kepler mission has identified many planet candidates with sizes
near Earth's, most of them slightly larger. Some of these are in or
near the habitable zones of their parent stars. Although the volume
of the habitable zones of stars cooler than the Sun ("K and M dwarf
stars") is smaller than ours, such stars are much more common in the
Galaxy than are Sun-like stars, so these are receiving increased
scrutiny. The TESS mission,
launched in April 2018, should discover many more planets hosted by small
stars near the Sun. The latest results are discussed in
- Assume all develop life leading to advanced civilizations
(i.e. assume Earth is average)
Support? "Earth is average" is the Copernican assumption,
which has proved so successful in studies of the structure of
our inorganic universe on the scale of the Solar System, our Galaxy,
and extragalactic space. However, we have very little
intuition here, and some biologists would argue that the chances of
developing technological species on a given Earth-like planet are
- Assume the communication phase lasts 10,000 years.
Note: we have only recently entered this phase. It has been only 90
years since we developed commercial radio stations that could be
detected over interplanetary distances. Our artificially generated
electromagnetic radiation is the most definitive marker of
advanced lifeforms on this planet to an exernal observer. Human EM
signals are now propagating out through the Galaxy, with the most
distant signals being about 90 light years away.
- Combining all of these factors ===> 10 - 10,000 communicating civilizations
in our Galaxy
- ===> Distance to nearest: 10,000 - 1000 light years
One obvious source of bias is the danger of carbon or
planetary chauvinism. For instance, the numbers we used above
assume that intelligent life can develop only on terrestrial planetary
surfaces. But there's no compelling reason that intelligent life
would be confined to planets or even planetary systems. Why not a
sentient interstellar gas cloud? This idea was explored in the famous
novel The Black Cloud by astronomer Fred Hoyle. In the story, the
super-being in the Cloud expresses surprise
that intelligence could develop in so adverse an environment as the
surface of a planet.
The bottom line is that, at present, no one can exclude the possibility that
a large number (10,000!) of advanced civilizations reside in our
Galaxy. The number is large because the Galaxy is huge.
The estimated separation between advanced civilizations, 1000-10000
LY, is obviously tremendous. But it could be traversed
with foreseeable technology on cosmically short time scales.
(Remember that 10 million years is a "short time" in the cosmic
context!) Even at propagation velocities far below the speed of
light, stable civilizations could "rapidly" explore the Galaxy.
The Fermi "Paradox"
Here's a relevant, even amazing, fact: We human beings managed to
launch four interstellar spacecraft in the first 20 years of the space age!
and Voyager spacecraft, sent to study the Jovian planets, will
all leave the Solar System, as will the New Horizons Pluto mission.
The most distant manmade object, the Voyager 1 spacecraft, is now 11.6
billion miles (125 Astronomical Units) from Earth and officially
entered interstellar space in September 2013.
There is a growing community
of scientists and engineers making preliminary plans for interstellar
travel. Above right is a painting of a popular starship design, a
"Bussard interstellar ramjet."
A famous question, now called the Fermi
Paradox because it was first raised by physicist Enrico Fermi in
the 1950's, is therefore "Where is everybody?"
What Fermi meant was that if interstellar expansion is possible for
thousands of Galactic species, they should have reached here long ago.
There ought to be aliens cluttering up the Solar System. We don't see
them -- there is no credible evidence for alien visitation (see
below) -- so are the estimates for the number of advanced
civilizations and our expectations regarding interstellar travel badly
wrong or naive?
Entire books have been written on possible solutions (all sheer
speculation) to the problems raised by Fermi's question. I give my
favorite answer below.
SETI = "Search for Extraterrestrial Intelligence"
The best alternative to interstellar travel to assay the prevalence of
life in the nearby universe would be EM communication, probably
in the radio or optical EM bands
Several passive listening, radio search
programs are ongoing. Most sophisticated are those operated by
the SETI Institute, once a
well-financed NASA program but killed by Congressional scepticism and
now running on private contributions.
When SETI programs were first conceived 50 years ago, it was assumed
that we might most easily detect "leakage" radiation from
civilizations employing radio/TV broadcasts for communication. But we
ourselves have backed away from radio/TV broadcasts, and today orders
of magnitude more information is being conveyed over cable and fiber
optic systems that don't radiate significantly. Therefore, if we do
detect ETI through EM communications, it is more likely to be in the
form of beacons deliberately established to attract attention rather
G. Alien Artifacts?
Most people assume that if spacefaring aliens are active today in the
vicinity of the Earth, or had arrived anytime over the last few 100
million years, there should be concrete evidence of that visitation --
extraterrestrial intelligence. In fact, you can find many
gigabytes on the Internet devoted to claims that there is "good"
evidence for visitations. But, so far, these have not stood up under
Here are two prominent examples of a supposed alien presence in the Solar
Astronomers have also considered the possibility of detecting alien
artifacts other than deliberate signals beyond the Solar
System. For instance, an advanced civilization might have created a
mega-structure to capture a large fraction of the energy output of
their host star. Such a structure is called
Sphere, and its presence could be indicated by an unusual
near-infrared flux excess not typical of normal stars. Deliberate
searches among databases from infrared sky surveys have not yet turned
up any viable candidates.
However, in 2015, a fascinating candidate for a Dyson-like structure
emerged from the Kepler Mission
See Guide 18 and links therein. There
is no credible evidence that an intelligent species (other than our
own) is involved in the UFO phenomenon. An overwhelmingly strong media influence
governs the number and similarity of reports. Publicity can
even provoke mild mass hysteria (e.g. alien abduction claims).
- The face on Mars
This is a strange and interesting feature in the
Cydonia region of Mars that was imaged (poorly) by the Viking
spacecraft in the 1970's. In a partially-illuminated state, it looked
like a gigantic, carved human head. One of the original Viking images
is shown below (left panel). This elicited speculation about
civilizations on Mars, the more responsible of which can be sampled
In April 1998, the Mars Global Surveyor was retargeted to image the same region at
much higher resolution.
Several good images were returned, showing the fully illuminated "face"
to be an unambiguously natural feature. See the right panel
below. (Some of the intriguing features in the original image, like
the "headdress," were actually data drop-outs.) Yet
better images have been
more recently obtained by the Mars Express orbiter. No other candidates
for alien artifacts on Mars or other planets are credible either.
You can find trenchant commentary by Phil Plait (the "Bad Astronomer")
about the (needless) continuing controversy over this subject
Evidence of alien technology, whether in the form of a communication
signal or an artifact, would now be called a technosignature,
and a 2018
conference reviewed the status of work in this area.
- Tabby Boyajian's Star
Tabby's Star is an outwardly inconspicuous star in the large Kepler
survey field. It shows a
strange, non-repeating pattern of eclipses some of them much too
large to be from planet-sized bodies. The star has received intense
scrutiny, and it has recently been discovered to be declining slowly
in (uneclipsed) brightness in a way that normal stars do not.
It is very unlikely that an alien mega-structure is responsible for
the weird properties of Tabby's Star, but it must be admitted
that none of the more conventional explanations (e.g. debris
from a planetary collision or a swarm of comets) explain
the observations either. So far, the star is an astronomical
H. The Recognition Chasm
There is a much more fundamental problem in communicating with
alien civilizations than their distance from us. It presents
major obstacles to even recognizing them in the first place.
- The mean age difference between two
Galactic species is likely to be 100's of millions of years.
- Given favorable conditions, successful advanced lifeforms may
have continued evolution and intellectual/technical development
for a large fraction of that time. (Once they reach a certain level
of development, they become immune to the more serious astronomical
hazards for lifeforms, such as asteroid impacts and stellar
- The age separation is much more important than the
====> us : them ~ goldfish : us
- Since recognition and communication is possible only for
cultures in close intellectual proximity,
====> Aliens would appear to be natural
So, my favorite answer to Fermi's question, "Where is everybody?"
"They're here, but we don't recognize them."
Reading for this lecture:
Study Guide 23
Bennett textbook, Chapter 24
Are We Alone? by James Trefil & Robert Rood (Clemons Lib: QB 54.R55)
The Biological Universe: The Twentieth Century Extraterrestrial
Life Debate and the Limits of Science by Steven J. Dick (SciEngr Lib:
QB 54.D47 1996).
Rare Earth: Why Complex Life is Uncommon in the Universe
by Peter Douglas Ward & Donald Brownlee. (QB54.W336.2000).
Lonely Planets: The Natural Philosophy of Alien Life by
Life in the Universe by Jeffrey Bennett and Seth Shostak (SciEngr
Lib: QH327.B45 2007).
Astrobiology: A Multi-Disciplinary Approach by Jonathan Lunine (SciEngr
Lib: QH325.L86 2005)
July 2019 by rwo
Text copyright © 1998-2019 Robert W. O'Connell. All rights
reserved. Movie poster captured from
the Internet Movie Data Base.
Habitable zone drawing copyright © Brooks/Cole-Thomson. Starship
painting by Don Davis. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 1210 at the
University of Virginia.