Answering the question “What is life?”
is actually why we are searching for life on
other planets! There are many definitions of
life, which will continue to change as we find and
understand more forms of life. Part of why life is
difficult to define is that there is such a wide
variety of living things on Earth that must be
classified, from amoeba to Redwood trees.
Most biologists consider life to be a process
and define it based on what living things do, not
what they are. Living things meet seven basic
- They must maintain their desired internal
environment, called homeostasis. For
example, your internal temperature stays ~98°F,
even if the weather changes.
- Living things must be organized in some
repeatable way. Cells and organs are good
- They must convert chemicals or energy into
useable forms, called metabolism. For
example, you eat and breathe to get energy to
think and move around.
- Living things must grow and change over time.
- They must respond to stimuli (like how you
blink in bright light) and adapt to long-term
changes in their environment.
- Living things must also reproduce.
Now, let’s try an example: The scientific
community is divided on whether viruses should be
considered life. They are clearly organized,
respond to stimuli, reproduce and mutate (adapt)
over time. However, they don’t control their
internal environment (which is why fevers can heat
them until they break apart), don’t metabolize,
and can’t reproduce without taking over another
organism’s cell. What do you think? Are viruses
So, when scientists are looking for life on other
planets, they are looking something to meet the
seven criteria above. They tend to look for life
like we’ve seen on Earth but try to keep an open
mind, since it is difficult to search for
something unknown. Some scientists argue that we
should loosen the definition of life to
“self-organizing and reproducing” so we don’t
exclude something like viruses.
Some efforts focused on finding life outside
Earth are searching for:
- Water (H2O) since it is
liquid in a wide range of temperatures and its
polarity allows it to separate materials (allowing
for easy organization).
- Oxygen (O2) because the
double-bond has just the right energy to be broken
apart and built easily, allowing for energy
- Simple proteins and aromatic carbon
compounds. Carbon bonds are also in the energy
sweet spot where many different compounds with
many properties can be formed and broken down
easily. We started looking for these compounds in
space when scientists found that non-biological
processes can make them, which may have lead to
the formation of cells and then biological
organisms like those on Earth.