Photo courtesy NOAA/U.S. Dept. of Commerce
Hydrothermal-vent tubeworms
Are 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 other gas?

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 considered alive:

  • 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 reproduction.
  • 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 (camouflage).
  • 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 enables flight.

Photo courtesy NOAA/U.S. Dept. of Commerce
Club-shaped structures of underwater microbes called stromatolites
Now 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 reproduced.
  • 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 generation.
  • 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 the ocean floor
Up 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 the sun, and in other worlds without sufficient light from the parent star. In 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 crust.

Photo courtesy NOAA/U.S. Dept. of Commerce
Tubeworms around a hydrothermal vent

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 water.

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 Park (right)

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

The Drake 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 and asteroid 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" for details.

Some Ground Rules for Alien life

Photo courtesy NOAA/U.S. Dept. of Commerce
Using 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 chemistry.
  • 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 functions.
    • 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, for instance).
    • 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 information.
    • 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 environmental factors.
    • 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 environment.
    • 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.

Speculation: What Might Aliens Be Like?
Alien Speculation References
With 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 creatures.

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.)

At HowStuffWorks, 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 carbon dioxide.

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 animal

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 whole.

The Nirba, an alien predator

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.

Now It's Your Turn
Enter the Contest!
HowStuffWorks is giving you the chance to a dream up your own alien in our Create-An-Alien contest. We've told you what we think an alien might look like. Now its your turn to tell us what you think!
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 your alien.

Chemical Formulas

World-creation Parameters
Parameter Extreme 1 Earth-like Extreme 2
Sunlight Bright Moderate Dim
Force of Gravity (x Earth's gravity) 100 1 0.1
Temperature Desert heat Moderate Arctic cold
Water Coverage (%) 100 75 10
Atmosphere Reactive
(H2O, CO2, NH3 and CH4)
Air mix
(60% inert gas, 40% O2)
Pure gas
(NH3, CH4 or O2)
Energy Source Geothermal Sunlight and geothermal Sunlight

Alien-creation Parameters
Parameter Condition 1 Condition 2 Condition 3
Creature Plant Plant/animal combination Animal
Primary Sense Sight Hearing Chemical
(smell, taste)
Locomotion Rapid Slow Stationary

Try to make your alien as adapted to its environment as possible (for references on physiological adaptations, consult "Eckert Animal Physiology: Mechanisms and Adaptations," "Animal Physiology: Adaptation and Environment," "Environmental Physiology of Animals" and "Comparative Animal Physiology").

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Astrobiology Education

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