What is Extraterrestrial Life? - Forms & Signs

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will discuss the possible forms that extraterrestrial life might take and the signs we could look for to detect extraterrestrial life. A short quiz will follow.

Thousands of Alien Races

Avatar has the Na'vi; Star Trek has Vulcans, Klingons, Romulans and countless others; and E.T. has... well, it has E.T. Extraterrestrial life is life outside of Earth; we humans, on the other hand, are terrestrial life. Humans have dreamed up thousands of extraterrestrial races. But what are the odds of finding extraterrestrial life in the real world? And how would we know if we had?

The answer to whether such life exists has huge implications both in terms of understanding how the universe works, and philosophically -- humans have long asked the question, 'Are we alone?' The answer may help us better define not only the universe, but ourselves.

What Would Extraterrestrial Life Look Like?

One Way People Imagine Aliens
One Way People Imagine Aliens

While in sci-fi, aliens are often similarly evolutionarily advanced to us, that might not be what we find in the universe. Extraterrestrial life could be as simple as bacteria-like organisms, or could be far more advanced and complex than us. Many of our searches for extraterrestrial life so far have assumed that such life would have a lot in common with us. But what if, in the words of Spock, 'It's life, Jim, but not as we know it.'

One common example of life not quite as we know it is silicon-based life. The human body is built largely around carbon, but there are alternatives. If you look at the periodic table, carbon shares a group with a number of other elements, each with similar chemical properties. Silicon is one of these elements. What if all the carbon in the human body was replaced with silicon? How different would silicon-based life be? It is a question we cannot currently answer.

There are other possibilities for life not as we know it. Perhaps such life might use ammonia instead of water, perhaps intelligent life will be reptilian instead of being evolved from apes, or perhaps there could be reproducing life inside the Sun or in a neutron star that is nothing like we would expect.

Our very definition of life may also be limited. Our current definition says that life must maintain an internal state (homeostasis), be structurally composed of cells (organization), create energy (metabolism), grow, adapt to the environment, respond to stimuli and reproduce. But this definition and others have been debated furiously and continue to be debated to this day. Extraterrestrial life could turn out to be very different from what we expect.

Life Signs

When a doctor in a sci-fi show says there are no 'life signs' on a newly discovered planet, you've got to wonder what exactly they're looking for. In real life, how will we be able to determine whether there is life on another planet? What are the 'signs'?

Chemical Composition

One initial indicator is chemical composition. Although there are a total of 26 elements that form life on Earth, most life is built on only 6 main elements. Those elements are hydrogen, carbon, oxygen, nitrogen, sulfur, and phosphorous. Finding that these elements are present on another planet might be a first sign that life there could be possible.


Life on Earth uses copious amounts of water. The chemical reactions that occur in cells use water as a solvent, and so liquid water appears to be vitally important. Looking for liquid water has been a big part of our search for life elsewhere in the solar system.

Extrasolar Planets

Scientists are also searching for extrasolar planets (or exoplanets) -- planets orbiting around other stars. If those planets are positioned inside a star's so-called 'habitable zone', an area where it isn't too hot or too cold, it is possible that we might find life there. Extrasolar planets are detected in a number of ways: for example by watching for planets passing directly in front of the star, or by looking for the wobble of a star due to the gravitational pull of its planets.


And then there's SETI (the Search for ExtraTerrestrial Intelligence), which is a group of scientists and observatories that search the universe for radio signals that may indicate life -- whether alien weather reports, or a simple message.

A SETI Satellite
A SETI Satellite

In Our Solar System

There are several candidates for extraterrestrial life right in our own solar system.

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