Photo courtesy NOAA/U.S. Dept. of
there other forms of life in the universe? The scientific
search for extraterrestrial life forms has been bolstered by
two recent discoveries. First, the discovery of life forms in
exotic environments on Earth indicates that life is very
hearty and can adapt to the strangest and most hostile
environments. Second, astronomers found planets
orbiting stars besides our sun -- over 50 extrasolar planets
have been discovered as of 2001. Are there alien life forms on
any of these planets?
If alien life does exist, what might it be like? Would it
be simple forms of life such as bacteria, viruses
or algae, or more advanced, multi-cellular creatures, perhaps
even intelligent beings? Would aliens be animals, plants or
have characteristics of both? Would they have arms and legs
and walk upright as we do? Would they depend upon vision as
their primary sense or use another way to gather information
about their surroundings? Would they "breathe" oxygen or some
Speculation about aliens has typically been left to
science-fiction authors, science-fiction readers and Hollywood
writers and directors. In this edition of HowStuffWorks,
we will examine astrobiology, the scientific search for
extraterrestrial life. We'll apply what we have learned about
life on Earth to speculate about what alien life forms might
be like, and you can join in the discussion by submitting
ideas of your own.
Greetings, Carbon-based Bipeds!
Most of us
picture alien life the way it's portrayed in movies, where
aliens are commonly depicted as human-like forms because they
use actors either to play the roles directly in make-up or to
be models for computer-generated
animation. Also, audiences relate to human-like aliens
better than to more exotic, monster-like creatures. However,
the human body plan -- bilateral symmetry with one head, two
legs and two arms -- stems from when early amphibians and
reptiles colonized the Earth's land masses, and it seems
unlikely that such a shape would evolve on an alien world. So,
let's forget Hollywood for the moment and look closely at the
real science of astrobiology.
Astrobiology is the scientific study of life in the
universe. Astrobiologists seek to understand (among other
things) how life arose and evolved on Earth, what governs the
way life is organized and what makes a planet habitable.
Astrobiology combines the disciplines of biology,
chemistry, physics, geology and astronomy. Often,
astrobiologists must use the information learned about life on
Earth as a guide for studying life elsewhere. Let's examine
some of the things that we have learned from life on Earth.
What is Life? While it is
hard to pen a clear definition of "life," most biologists
agree that there are many characteristics in common among
living things. If an object meets these characteristics, it is
Organized -Living things are made of atoms and
molecules that are organized into cells. The
cells in an organism can be either uniform or specialized
for various functions. The cells can be further organized
into tissues, organs and systems. Living things on Earth are
quite diverse as to their organization and complexity.
Homeostatic - Living things carry out functions
that keep them in a constant, relatively unchanging state
called homeostasis. For example, your body has
systems that keep your body temperature constant -- you
shiver if you're cold, sweat if you're hot.
Reproduces - Living things make copies of
themselves, either exact copies (clones)
by asexual reproduction or similar copies by sexual
Grows/develops - Living things grow and develop
from smaller and/or simpler forms. For example, a human
begins life as a fertilized egg, developing into an embryo,
fetus and then a baby. The baby subsequently grows into a
toddler, adolescent and adult.
Takes in energy from the environment - Staying in
a relatively constant, organized state violates the second
law of thermodynamics, which states that the degree of
disorder (entropy) of all objects increases. For a living
organism to maintain organization, it must take in, process
and expend energy. The way humans and other animals do this
is by eating food and
extracting energy from it.
Responds to stimuli - Living things respond to
changes in their environment. For example, if a
stimulus causes you pain, you respond by
moving away from that object. If you place a plant near a
well-lit window, the branches or shoots grow toward the
light (phototropism). For protection, some animals
change color to blend in with their surroundings
Adapted to its environment - The characteristics
of a living thing tend to be suited for its environment. For
example, the fins of a dolphin are flat and adapted for
swimming. The wing of a bat has the same basic structure as
the bones in a dolphin's fin, but has a thin membrane that
Photo courtesy NOAA/U.S. Dept. of
structures of underwater microbes called
that we've got a definition of what life is, we need to look
at how it changes over vast expanses of time. The basic rules
governing whether species arise, live, remain unchanged or
become extinct are those of evolution by natural
selection as proposed by Charles Darwin. Darwin's theory
of evolution has the following points to it:
Similar organisms reproduce similar organisms -- a dog
reproduces a dog, a dandelion reproduces dandelions and a
fish reproduces a fish.
Often, the number of offspring are overproduced such
that the number that survive is fewer than the number
In any population, individuals vary with respect to any
given trait, such as height, skin color, fur color or shape
of beaks, and these variations can be passed on to the next
Some variations are favorable, in that they make those
individuals best-suited to their environment, and some are
not. Those organisms with favorable variations will survive
and pass those traits on to their offspring; those
individuals with unfavorable variations will die and not
pass on their traits -- this is natural selection.
Given sufficient time, natural selection will accumulate
these favorable traits. The species will evolve.
Although Darwin's theory of evolution was proposed
to explain changes in Earth-based species, its principles are
general enough that it could be applied elsewhere in the
universe as well.
Life in the Extreme
Photo courtesy NOAA/U.S. Dept. of
Commerce Hydrothermal vent in
until about 30 years ago, it was believed that all life on
Earth was dependent upon energy from the sun. Furthermore, it
was thought that you would probably not find life where
temperatures were extremely hot, like in geysers or hot
springs, or extremely cold, like in the Antarctic desert.
These ideas changed when oceanographers explored
hydrothermal vents, openings in the ocean floor where
extremely hot, mineral-rich water erupts from the crust.
Hydrothermal vents are located several miles below the
surface, on the ocean floor, where the surrounding water is at
or near freezing, it is absolutely dark and the pressure is
high. In organized communities around the bases of these
vents, called black smokers, scientists found clams, crabs and
exotic, giant tubeworms measuring 6 feet (2 meters) long. The
water coming out of these vents is 230 to 662 degrees
Fahrenheit (110 to 350 degrees Celsius).
How can these animals survive so far from the sunlight,
under these extreme conditions? In the water, scientists found
species of bacteria that split hydrogen sulfide from the water
to get energy to make organic compounds
(chemosynthesis). The tubeworms have bacteria in their
tissues that help them derive energy from the water. The clams
feed on the bacteria, and the crabs feed on the tubeworms.
The discovery of hydrothermal-vent communities showed that
it is possible for life to evolve in places without light from
and in other worlds without sufficient light from the parent
view of the discovery of hydrothermal vents, it may be
possible that life exists on Europa, an icy moon of Jupiter,
which scientists believe has a water ocean beneath its icy
Photo courtesy NOAA/U.S. Dept. of
Commerce Tubeworms around a
Life has been found in other extreme environments as well.
Scientists discovered microcolonies of lichens called
cryptoendoliths in rock samples of the Antarctic
desert, where temperatures often drop to 100 degrees below
zero and there is little or no liquid water. In contrast,
thermophilic (heat-loving) bacteria have been found in
hot springs where temperatures exceed the boiling point of
Photos courtesy NASA Living cryptoendoliths (green, black,
green-blue lines) in a rock sample from Antarctica
(left) and a thermophilic, rod-shaped bacteria (about 1
micron long) from a hot spring in Yellowstone National
If life can evolve in extreme environments on Earth, it
seems possible that life may exist in the extreme environments
of other worlds such as Mars.
Rare Earth Hypothesis
equation, developed by astronomer Frank Drake and
promoted by Carl Sagan, is used to estimate the number
of intelligent civilizations in the universe. In
contrast, geologist Peter Ward and astronomer Donald
Brownlee from the University of Washington have proposed
a hypothesis -- the Rare Earth Hypothesis -- that
life on Earth is unique. Their hypothesis states that a
series of chance events or situations, such as living in
the habitable zone of the sun, having a Jupiter-type
planet to clear away comet
debris and having few mass extinctions, has allowed life
to develop on Earth and would be unlikely to happen
elsewhere. See "Rare
Earth: Why Complex Life is Uncommon in the Universe"
Some Ground Rules for Alien life
Photo courtesy NOAA/U.S. Dept. of
what we have learned from life on Earth, what can we say about
alien life? While it would probably be vastly different from
life on Earth, alien life would probably adhere to certain
universal guidelines, as the widely varying life on Earth
does. These guidelines or ground rules include the following:
Alien life would be governed by laws of physics and
Alien life would be based on some type of chemistry
(eliminating the sci-fi concept of pure-energy beings).
Solvent - On Earth, the solvent for all of our
biochemicals is liquid water. Other chemicals could be
solvents as well, such as ammonia, methane, hydrogen
sulfide or hydrogen fluoride.
Temperature - Alien life may require
temperatures at which its solvent can remain liquid.
Pressure - Alien life may require environmental
pressures (and temperatures) that allow solvents to exist
in three states of matter (solid, liquid, gas).
Energy source - Living things require energy to
remain organized. This energy may come from a star or
from chemical or geothermal energy (as in hydrothermal
vents and hot springs). On any alien world, there would
have to be some source of energy to sustain life.
Complex molecules - Living things on Earth are
organized and made of complex, carbon-based molecules that
carry out biochemical functions. Carbon is a versatile atom that
can form bonds with up to four other atoms, in many
shapes, to make molecules. Although not as versatile as
carbon, silicon can also form up to four bonds with other
atoms and has been proposed as a basis for molecules of
alien life (silicon-carbon hybrid molecules have also been
proposed). It is likely that alien life forms would have
some type of complex molecule to carry out similar
Informational molecule - In Earth organisms, deoxyribonucleic
acid (DNA) is a complex molecule that carries
genetic information and directs the formation of other
molecules in order for life to reproduce and function.
Because a characteristic of life is that it reproduces, it
seems likely that alien life forms would also have some
type of informational molecule.
Alien beings that are larger than microbes would have
some equivalent of cells.
As an organism gets larger, its internal volume (cubic
function) grows faster than its surface area (square
function). This places a limit on the organism's size,
because substances from the outside of the organism must
pass into and throughout the organism by diffusion, which
depends upon large surface areas, short distances and
differences in concentrations. As an organism grows larger,
the distance to its center increases and diffusion gets
slower. To maintain workable diffusion distances, an
organism must have many small cells instead of one large
cell. So, an alien would be multi-celled if it is larger
than a microbe. (We would not expect to find a light-years
wide, single-celled organism like that portrayed in the
original Star Trek episode "The Immunity Syndrome.")
Alien life would evolve and adapt to its surroundings by
the theory of evolution as previously explained.
The physiological make-up of a multi-celled alien would
be most suited to its environment. Organ systems would be
adapted to environmental conditions such as temperature,
moisture and gravity.
The alien would have some way of bringing solids,
liquids and gases inside its body, distributing them to
every cell and removing waste products (equivalents of heart ,
blood vessels and kidneys,
The alien would be able to take in energy from its
surroundings, extract the energy and eliminate wastes.
The alien would have senses (such as sight, sound,
touch) to obtain information from the environment and
respond to stimuli (while we use vision as our primary
sense, this may not be true of aliens). They would also
have some type of brain or nervous system to process
The alien would have some means of reproduction,
either sexual or asexual.
Alien organisms would probably have similar ecological
structures to life on Earth.
Population sizes would be limited based on the
predominance of food, predators, disease and other
Alien life forms would exist in food chains and food
webs in their native environment, like life on Earth.
Producers will make food, consumers will eat producers
and/or other consumers and decomposers will recycle atoms
and molecules from dead organisms back into the
Alien life forms will be integrated with their
habitats and ecosystems, like life on Earth.
As you can see, life of any kind is
intimately tied to its environment, so the characteristics of
the planet would be extremely important in determining the
characteristics of the life form.
these ground rules in mind, and since no extraterrestrial life
forms have been conclusively discovered, alien physiology lies
in the realm of our imagination. Science-fiction authors,
especially the 'hard" ones who try to adhere strictly to real
science, have been doing this for years. They first design or
build a world, carefully working out its physical,
astronomical and ecological characteristics. Next, they work
out what type of aliens could exist in that world. An example
of one such world-building exercise can be found at the Epona
Project, where several science-fiction writers came
together to create a world called Epona, complete with
planetary, geological and ecological data. One artist, Steven
Hanly, created Epona
For his novel "Mission
of Gravity," Hal Clement created a world called Mesklin
that circles a double star. Mesklin rotates once every
eighteen minutes and has a flattened shape caused by its
rotation. The gravity of Mesklin ranges from three times
Earth's gravity, at the equator, to seven hundred times at the
poles. Mesklin has a hydrogen atmosphere and methane oceans.
Mesklinites, one of the planet's life forms, are small,
centipede-like creatures made of an insect skeletal protein
called chitin. They have 18 pairs of legs that end in
sucker-like feet, forward pinchers for grasping, a strong
circulatory system and absorb hydrogen right through their
shells. They are immensely strong -- a result of living on a
high-gravity world, yet they have a fear of being picked up
because a fall from a small height could be fatal on such high
gravity. (See "Barlowe's
Guide to Extraterrestrials" and "The
Science of Aliens" for descriptions of Mesklinites and
other alien life.)
we've envisioned an alien world and alien life forms. In our
world, the planet orbits a bright star. Only 10 percent of the
world is covered with surface water, but throughout the land
mass there are pockets of water that collect under the sands
from the sparse rainfall. The environment is hot and arid and
the sunshine is bright. The planet is massive and has gravity
that is one-hundred times stronger than that of Earth. The
atmosphere is an Earth-like air mixture of helium, oxygen and
The two alien life forms that we envision for this world
are animals -- mobile predators that live around the planet's
few small bodies of surface water. Both aliens are short,
about 1 foot (30 centimeters) tall, with thick limbs to
support their weight against the immense gravity. Both have
thick coverings or skins to minimize evaporation and conserve
water. To gather information, one relies primarily on vision,
while the other uses chemical senses (taste and smell).
The Lashlarm, an alien
The Lashlarm is our first alien predator. It looks
like a walking toilet bowl. The mouth portion is supported by
three stalky legs connected to a flat pedestal. Underneath the
pedestal are many scales, so the pedestal glides across the
surface of the sand much like a snake moves along the ground.
It has several sensory appendages that allow it to locate prey
by chemical means. It hunts near the small bodies of surface
water, feeling along the water's edge and tasting the sand and
water for other animals. Upon locating prey, the Lashlarm
crouches down and glides up to it. The Lashlarm then opens its
large mouth and springs down upon the prey, swallowing it
The Nirba, an alien
The Nirba is is slightly larger than the Lashlarm.
It lives in the water, near the edge, much like a crocodile or
alligator but is not fully aquatic. The Nirba comes out to
prey on other animals that come down to the water,
particularly the Lashlarm. It has a large head with nostrils
located on top of its nose so it can breathe while mostly
submerged. The Nirba has thick skin, to prevent dehydration
while out of the water in the hot sun, and big, muscular front
legs with large claws for killing its prey. A long tail helps
it swim in the water, and the "arrowhead" end assists in
hunting and territorial defense.
We had fun
applying some principles about biological adaptations to our
alien design. Now, it's your
turn to try!
Below are two tables where we've established some
parameters about the world and the alien organism. Although
the table is arranged in three columns, you do not have to
follow all of the choices in any given column. For each
criteria (each row), take one of the choices. Once you have
your world and alien criteria, it's time to design