Introduction
Mankind has always wondered, "Are we alone in the Universe? Isn't there life out there somewhere?" With an estimate around 300 billion stars in the Milky Way galaxy alone, many with their own satellites, the possibilities seem likely that some form of life should exist somewhere in the Universe. This search for life begins with a definition of life itself, followed by possible candidates both within and outside our solar system, and also includes what measures are being taken to find these life sources and justify their existence.
Properties of Life on Earth
There are certain properties that are common in all life on Earth. All life on Earth requires water, which is a basis for our biochemistry. Life on Earth uses carbon chemistry, which can also be called Organic or Bio-Chemistry. Life is DNA- based, and is a self contained entity with the following traits: moves, consumes, grows, responds to stimuli, replicates, dies, and evolves (Serri, 2006).
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The Earth seen from Apollo 17.
English: View of the crescent moon through the top...These listed properties can be described as 'Earth Like", since "Earth Like" describes all life on Earth.
Scientists offer a list of criteria for a conventional definition of "life". Living things have one or more cells, which are the basic units of life. Living things require energy to maintain homeostasis (internal organization) and to produce other phenomena associated with life. Living things will exhibit growth; in a growing organism, this will be an increase in size in all its parts. Living things will also show adaptation, which is the accommodation of a living organism to its environment. Adaptation is fundamental to evolution, and is determined by heredity. Response to stimuli is also a characteristic of living things, and this response can take many forms. This may be observed by the contraction of a unicellular organism, to more complex reactions...