The term extraterrestrial life indicates an unknown life forms that might exist beyond Earth. So far it was not found in any solar system body, but may exist on planets that orbit other stars.
Life in the form of microbes is very common in the universe but complex life—animals and higher plants is likely to be far more rare than is commonly assumed.
Basic Requirements for Life: Source of Energy, Complex Chemistry, Liquid Solvent Medium, Location for life to emerge. Asking how life might emerge on other worlds focuses on what processes were most important in how life emerged here. Asks questions that get at the heart of the inner workings of life and biological evolution. Helps us to better understand what kinds of questions to ask about life’s history on Earth.
These developments, which must be considered in the search for extraterrestrial life, can roughly be grouped into three areas:
1. the early emergence of life on Earth
2. its nature and diversity
3. its toughness and tenacity
Development of the proper strategy, and definition of those conditions that do and do not support life will be key to the ultimate discovery of extraterrestrial life. With the proper strategy and approach, the question seems to be not one of whether there is life, but when we will find it.
Stephen Hawking afraid that some aliens might be intelligent and dangerous, causing a threat to humans. In his view, most of the aliens looked like microbes and simple animals - that is, forms of life as they were on the Earth during most of its existence. "If ever the aliens visit us, I think it would look like that when Christopher Columbus first landed in the coast of America, which is not exactly ended well for the Indians."
The first search for extraterrestrial intelligence began in 1960 when the astronomer Frank Drake in West Virginia put radio telescopes. Today, half a century later, Dr. Frank Drake tirelessly still listens to radio signals in order to identify the one that sent him ET.