What Is Life? (astrobiology approach)

It seems obvious when something is alive. But it turns out that the answer to this question is not quite as obvious as it seems. We might wonder why we would even want to ask that question. 

So let's have a look at some of the characteristics that define living things:

1. life is complex and exhibits complex behaviours

2. life grows

3. life replicate (divides)

4. life metabolizes (eats food) and makes energy

5. life contains a system for storing information

6. life evolves by Darwinian (or artificial!) selection  

It is important to remember that each characteristic we can apply for non-living things.

1. tornado exhibits complex behaviors

2. crystals grows

3. virus cannot replicate, but computer program can

4. fire metabolize

5. computer store information

6. computer programs can evolve

Here is a definition by Gerald Joyce, a NASA scientist, who described in 1994 life as, a "self-sustaining chemical system capable of undergoing Darwinian evolution."

Just one example of a type of written definition. 

And what definition of life do you like most? 

Please, share in comments.

I want to share two best online courses about life:

Astrobiology and the Search for Extraterrestrial Life.  Professor Charles Cockell and The University of Edinburgh

Introduction to Biology - The Secret of Life. Professor Eric Lander and the MITx Biology

--Anna