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). 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 involving more or all the senses of higher animals. Finally, reproduction is the last requirement for a definition of life as exhibited by life on Earth. Reproduction is the production of new cells in the growth process, and involves the division of one cell to form two new cells (Wikipedia, 2006). Only by following this definition of life can we differentiate life found on Earth from things that may seem life-like but not meet our definition of what life on Earth is. For example, while a train might move, it can not reproduce by itself. Using the accepted definition of these properties that define life, we can determine if a subject is truly a form of life, or simply life-like. There is a wide range of views about what comprises life and what significance it is given. Some of these viewpoints are biological, and some philosophical. Whether we are classified as a grouping of once primitive cells that have evolved into highly complex structures, or believe we were the created expression of the heart of a loving God, human beings are a unique species, with emotions unparalleled by any other species. Possibilities of Extraterrestrial Life in our Solar System
Human life demands an environment of respect and dignity. Other necessary ingredients for life include, air, water, food, shelter and clothing. We develop and grow in the context of family and social interactions. We need to feel loved and valued in order to thrive. Earth, one of the terrestrial planets, is the perfect planet that can support life, containing elements and components necessary for survival (Earth, n.d). This unique planet has a perfect temperature and atmosphere. It is not too hot or too cold. It has water, a requirement for life, and is found in all three forms--liquid, solid, and gas. Water covers the surface of the Earth. The atmosphere is composed primarily of carbon dioxide, oxygen and nitrogen. The ozone layer of the atmosphere shields Earth's inhabitants from harmful radiation rays (Earth, n.d.). The moon, though waterless on the surface, is believed to contain frozen water...