There are many definitions of life, but most of them are based only on the origin of life on Earth. But what will life look like on other planets?
Science approaches this topic in different ways. A geneticist who works with organisms and their genomes has a very different view of what life is than an astrophysicist, for example, who has a broader, expansive, and universal view of the same issue. Earth-based definitions of life mean that what intelligent and conscious alien life can be like almost always reflects in essence how we imagine it based on our humanity.
However, much about how life arose on Earth is still unknown. Researchers still don't know if DNA or RNA was the first genetic material, or if life came from what is known as the primordial soup. This "soup" was thought to be a mixture of organic and inorganic molecules found on the Earth's surface, or it comes from energy moving from deep-sea hydrothermal vents billions of years ago.
However, even when using the five criteria of life, namely: it requires energy, is bound by a membrane, is able to store genetic information, grows and reproduces many times, not everything on Earth is labeled as "living" and "inanimate."
Since we are currently locked at home due to this COVID-19 pandemic, let's take viruses for example. A virus falls into these categories, but it is not live because it depends on other living cells to replicate.
If our definition does not even work with organisms that we have on OUR PLANET, it is likely that numerous systems will exist in the wider universe that also do not follow this definition.
Moreover, although there is such a huge variety of life forms on Earth, we can only trace its origin from one source: one random combination of events gave birth to this first cell or this first organism.
Numerous studies have shown that there are many alternative biochemistry and biology that could arise from the conditions of a young earth. The researchers hypothesized that if Martian life existed, it could metabolize carbon dioxide, similar to how terrestrial life forms do. However, this concept did not work, probably because they were especially looking for life that metabolized carbon, just like life on OUR PLANET.
Our definition of life is extremely imperfect because it is based on the limited evidence we have about an infinitely large universe. As far as we know, the Earth is the only "living" planet, which is why we assume that our path to life is the only path.
NASA's definition of life is worthy when used to establish what living things mean on Earth, as it covers most of the basics. However, any concept of life, as applied to aliens, must be limitless, and it must not make any assumptions about other life systems simply because it worked in our terrestrial mindset to find a copy of terrestrial life.
The universe is infinite, and therefore our concept of "alien" life must also be open.