Солнечная система и ее тайны

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Common Definitions

What is life? How do we recognize life when we see it? What signatures indicate the presence of life? These questions are important to both the exploration of space and to the search for life’s signatures in Earth’s past.

There are different ways to think about life, and to make matters even more complicated, life is a word with multiple meanings in colloquial speech. We can think of “flesh-and-blood” life, or of life as a machine or an automaton. We can think of the building blocks – the molecules of life - or we could think of the information stored in some of these molecules.

Scientists have had enormous success in understanding the main attributes, or characteristics, of life. Despite this, as the discussion later in this section will emphasize, a perfect definition of what counts as life has not been found. Many scientists contend that we should not yet attempt to define life, as there is so much that we do not know about its nature, and there is much we can learn about life’s nature without having a general definition to begin with.

A good starting point in exploring the basic nature of life is to list its main attributes, which we know them to be shared by all Earth’s life. They are:

Attribute #1 states that life is a phenomena based on molecules and their chemical reactions. This requirement means that life cannot exist in many of the environments studied by astronomers. Most of the normal matter in the Universe is found in stars, and in gas between stars and galaxies. Stars are too hot for molecules to exist. Gas clouds are too dilute and too cold for most chemical reactions. Life on Earth is also based on a very specific set of molecules. Among these are four basic types called proteins, nucleic acids, sugars, and lipids.

Attribute #2 relates to the need for harvesting, storing, and using energy. We’ll discuss that later when we talk about metabolism.

Attribute #3 is sometimes used as the defining characteristic of life and encapsulates what we call Darwinian evolution or the Darwinian paradigm. Attribute #4 is often considered necessary for accomplishing attribute #3, though it has a much broader scope.

Finally, attribute #5 seems very Earth-centric, given that water is not the only liquid that could play a similar role. It is worth noting however, that from the perspective of the astronomer, water (H2O) is an extremely common molecule in the universe in all kinds of environments, in large part because H and O are, respectively, the first and third most abundant chemical elements in the cosmos. (Helium is number two in abundance, but being a noble gas, it does not engage in any chemical bonding and molecules.)

This list of attributes could be summarized succinctly in the so-called NASA definition: “Life is a self-sustaining chemical system capable of Darwinian evolution.” The definition got its name from a committee NASA gathered in 1994 to come up with a general definition. It is simple and encapsulates most of the attributes we listed above. We will use it as a guide when we try to find planets and environments where we could look for signatures of alien life.

Солнечная система и ее тайны