ASTR 1210 (O'Connell) Study Guide 23
Are we alone?
Are there billions of advanced lifeforms in the
The most powerful impact of astronomy on popular thinking in the last 150 years
concerns something that it hasn't discovered yet and possibly
never will: aliens.
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.
- If you believe we are average (the "Copernican" view), then
the universe TEEMS with billions of advanced lifeforms.
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 18 years.
many stars like the Sun have planets. Despite the technical
challenges, we have already identified many Earth-size candidate
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.
Functional basis: random chemical interactions, governed by natural physical
No special "vital 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 systems.
Unity: Despite extraordinary diversity, there is only
one type of terrestrial life at the molecular level
- A small set of 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, 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. Rich fossil
collections exist for ages between 10,000 and 500,000,000 years.
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 50 years ago could not have imagined how
precise or conclusive such chemical tests of the evolutionary
principle could be.
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 times and trace how they change with "lookback time." There is
no doubt that the universe has evolved.
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
raw materials. Energy sources: solar radiation, volcanic vents,
- The plausibility of early molecular evolution was demonstratead
in a classic laboratory simulation:
experiment. Sugars, amino acids, and DNA bases were generated
in only a few days from pre-biotic ingredients. This showed
how easy it is to start a chemical synthesis sequence that can lead to
- Note that organic molecules thrive even in ostensibly hostile
environments such as meteorites or interstellar clouds. Bio-evolution
could well have begun elsewhere than on Earth.
- Also 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.
- 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
simple organisms, entirely unlike the profusion of advanced types seen
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.
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
For more information on panspermia, click
Size of the "habitable zone" for Earth-like planets
surrounding four different types of stars;
"F" stars are more
massive than the Sun, "K" and "M" are less massive.
The volume of the habitable zone increases for hotter stars.
E. Life Elsewhere in Our Solar System
Are there plausible biospheres elsewhere in the Solar System?
- Raw materials;
- Dense medium (preferably water, but other liquids/dense gases are possible);
- Protected environment, maintained in the appropriate temperature/pressure range;
- Energy source; primarily sunlight, but volcanic vents, lightning, etc. are alternatives;
- Sufficient time (100 Myr-1 Byr?)
For a particular type of parent body (terrestrial planet, Jovian
planet, comet), we can define a habitable zone as those
distances from the parent star for which that type can offer
a comfortable 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 (see above) for Earth-like
planets ranges from about 0.9 AU to 1.4 AU, covering (of course) the
Earth's orbit but not reaching to either Venus or Mars. The habitable
zone for Jovian-type planets would be much larger, as would the zone for
icy satellites of planets. Whether life could thrive in such
environments, however, is unknown.
Possible biospheres on the outer satellites: (Left) The
icy surface of Europa (pseudocolor, Galileo mission); (Middle) The
water vapor geysers of Enceladus (pseudocolor, Cassini mission);
(Right) The hydrocarbon-rich surface of Titan, as viewed by the
descending Huygens probe.
Overall: primitive lifeforms are possible in several settings,
but remote detection is unlikely. We must search "in
- Venus: no! High temperature, pressure, and corrosive atmosphere are sufficient
to sterilize surface of all Earth-like life.
- Mars: plausible evidence for biosphere > 1 Byr ago with abundant
water; SNC meteorites provide some evidence
for microorganisms. Too cold and dry now for life? Absence of ozone
in atmosphere allows damaging solar UV flux at 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 Mars.
- Titan's (S)
extraordinary hydrocarbon-rich atmosphere is a possible
Cassini mission has 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 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 an apparent organic molecule coating,
could act as "portable reservoirs" of organisms; however, temperatures are normally
F. Intelligent Life Elsewhere
This is a fascinating but virtually 100% speculative subject. There
is a paucity of facts, understanding, and imagination, and this leads
to a wealth of conjecture and controversy.
There is also the danger of carbon or planetary
chauvinism. E.g. intelligent life need not be confined to
planets or even planetary systems. Why not an sentient interstellar
gas cloud? This idea was explored in the famous science fiction
novel The Black Cloud by astronomer Fred Hoyle.
The Drake Equation
The Drake Equation, named after
astronomer Frank Drake, was a first attempt to estimate the number of
advanced technical civilizations in our Galaxy capable of undertaking
interstellar communication. Here are the elements in the Drake
estimate based on our current understanding:
The quantities entering the Drake calculation under items (3) and
(4) are highly uncertain and controversial. But the point is that no one
can presently exclude the possibility that a large number (10,000!) of
advanced civilizations currently reside in our Galaxy. The number is large
because the Galaxy is huge.
- There are 100 billion stars in our Galaxy
- Assume 0.001% - 5% of the stars have Earth-like planets.
Support? The recent detection of thousands of extra-solar
planets. Even though most of these are Jupiter-class, 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 Earth-sized planet
candidates and has verified one to date in the habitable zone.
- Assume all develop life leading to advanced civilizations
(i.e. assume Earth is average)
"Earth is average" is the Copernican assumption, which has
proved so successful in studies of the structure of our inorganic
universe. However, we have very little intuition here, and some
biologists would argue that the chances of developing technological
species are small.
- Assume the communication phase lasts 10,000 years.
Note: we have only recently entered this phase. It has been 80 years
since we developed commercial radio stations that could be detected
over interplanetary distances. Our artificially generated EM
radiation is the most definitive marker of advanced lifeforms on
this planet. Human EM signals are now propagating out through the
Galaxy, with the most distant signals being about 80 light years
- Combine all above. ===> 10 - 10,000 communicating civilizations
in our Galaxy
- ===> Distance to nearest: 10,000 - 1000 light years
The estimated separation between advanced civilizations, 1000-10000
LY, is obviously a large distance. 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.
"Where are they?"
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. At 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: "Why aren't they here?"
If interstellar expansion is possible for thousands of Galactic
species, there ought to be aliens cluttering up the Solar System. We
don't see them---there is no credible evidence for alien
visitation---so are the estimates for the number of advanced
civilizations and our expectations regarding interstellar travel
badly wrong or naive?
All answers to the question are sheer
speculation, but I think the best is: "They are here, but we don't
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
listening, radio search programs are active. 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
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. UFO'S, Alien Artifacts
Some people, most not active scientists, claim that there is actually
good evidence for spacefaring aliens in our solar system.
Naturally, there is much more space on the Internet devoted to these
claims than to more sober, evidence-regarding work:
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 & similarity of reports. Publicity can
even provoke mild mass hysteria (e.g. alien abduction claims).
- Alien artifacts?
- There have been a number of claims of physical evidence for
an alien presence. One of the most
interesting is the face on Mars, a
partially-illuminated feature in the Cydonia region of Mars imaged
(poorly) by the Viking spacecraft (1970's) and looking like a carved
human head. One of the original Viking images is shown below (left
panel). This elicited speculation about civilizations on Mars, some
of which is
- 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 interesting features in the original data, 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 are credible either.
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 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 evolution.)
- 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
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).
October 2014 by rwo
Text copyright © 1998-2014 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.