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 requirements:

- 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 examples.

- 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 alive?

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 release/storage (metabolism).

- 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.

At some level, life is poorly defined. Life can be considered to be a system that uses energy to make similar or identical copies of itself. Living things do this through DNA which contains the information that is passed down from parent to child. Though there are different definitions of what life is and sometimes something like a virus may not be considered to be alive even though it has a lot of the same behavior of living things. “Life as we know it” is based on carbon compounds that make up the 4 major groups of bio-molecules: nucleic acids (like DNA), proteins, carbohydrates, and lipids (like fats). All living things on planet Earth have a common ancestor meaning that there was a species that was the “father” of all living things. Because of this, living things have a number of things in common with each other. For instance, there are some genes that are shared by all living things because they code for something important like copying DNA. If life existed on another planet, it may be very different because it wouldn’t share a common ancestor with living things on Earth. For example, it’s possible that life on another planet could use silicon instead of carbon to form all of its other compounds. A life form like this would be “life that we don’t know.” However, even though it would be different than Earth life, it may have similar behaviors which is why we’d classify it as life.

Life as we know it means life of the sort that we would recognize as life if we found it. All life that we know about (that exists on Earth) uses nucleic acids (e.g. DNA), proteins, lipids, and water as its solvent. We presume that life elsewhere would have the same basic chemistry, and be made of the same materials.

It may be that there are other ways that life can exist than what we know, but having never seen such life, we don't know if it's possible or not. It's also quite plausible that if such life did exist and we encountered it, we might not recognize it as being alive.

I'm awestruck by the question. Very deep and a beautiful question: if we want to understand life in the universe, why not start with earth?

I think the correct answer is that, no one knows exactly. To put a definition to "life" is to ask the question, "what do you consider living and what is considered dead?".

I will try to define it, but may fail at it:
"Living is anything that has a biological process for sustaining itself to exhibit certain functions". This can really be ambiguous in viruses, where they are "dead" for ages, doing absolutely nothing, until they find a host to replicate in. So we don't know exactly what comes under the definition of "life on earth", let alone in the universe.

On earth, life as we know it is based largely on Hydrogen, Carbon and Oxygen. But this need not be the case elsewhere. One common proposition is that there could be life forms elsewhere that are based on Silicon instead of Carbon. It depends on how the environment is in that planet. Any life form evolves to survive in its environment and has to have a biological process suitable for it. If the environment is vastly different from that of earth, life will also be vastly different. It is extremely hard to fathom how complicated life could be in other planets.

Hydrogen, Carbon, Oxygen and other elements that make us, have their relative proportions in the universe similar to ours. It could be that other life forms are also similar to ours, therefore. But, it is not a necessity.