Question

We keep looking for life in other planets. But what is the definition for life? What is the difference between living and non-living?

Your question strikes one of the most profound and disputed principles of astrobiological study. Since astrobiology is the science of understanding life's origins and possibilities in the universe, its investigation necessitates a definition of life. Without one, as you imply, we would not be able to recognize life in another realm outside our own. While there are many contending definitions, one that is generally accepted comes from Bruce Jakosky's book, "The Search for Life on Other Planets." NASA scientist Jakosky defines being "alive" in general terms if the object 1) utilizes energy from some source to drive chemical reactions, 2) is capable of reproduction, and 3) can undergo evolution. Of course, this definition is subject to several complications. For example, fire can reproduce itself, contains heat energy, and uses biogenic elements (carbon, hydrogen, oxygen, nitrogen, phosphorous, and sulfur). And yet, fire it is classified as a byproduct of non-biological chemical reactions in part because it cannot evolve. In fact, another definition of life by geologist Joseph Kirchvink emphasizes evolution as the only defining characteristic of living objects versus non-living ones. In short, the answer to your question is that there is no answer. Next month, you will find a feature article on the complications of defining life on our website: http://nai.arc.nasa.gov--stay tuned! Anna Lee Strachan, NASA Astrobiology Institute
May 1, 2002