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One way of course to confirm the existence of life beyond Earth is to search for it (SETI). An early advocate and practitioner of this approach was Frank Drake, author of the well-known Drake Equation, which purports to estimate the number of extraterrestrial civilizations that we could expect to come in contact with.
This viewpoint is quite common amongst scientists today, although a significant minority advocate the so-called Rare Earth Hypothesis, that states that the emergence of life here on Earth is the result of an improbable combination of astrophysical and geological events and circumstances.
Speculation on extraterrestrial life, intelligent or otherwise, is not a recent development: some scholars suggest (and some dispute) that the priest and scholar Giordano Bruno was burned at the stake (at least in part) for advocating the pluraIity of worlds, that there are many worlds like ours in the Universe.
Exobiology, also know as astrobiology, is the study of the feasibility of life existing beyond the boundaries of the Earth. Such possible life is referred to as extraterrestrial.
Early on, SETI was supported by the federal government, but today it is privately financed. Thus far, the only seemingly intelligent radio signal received is the "Wow" signal received in the 1970s by the Big Ear radio telescope of Ohio State University.
While it is certainly clear that, other than human beings, and some animals here on Earth, there is no intelligent life in our Solar System, it is far from clear that there is no life (and there never was any life) in the Solar System other than that occurring in abundance on Earth. Currently there is much debate about (possibly life affirming) methane gas emanating from below the surface of Mars, and whether simple living organisms could exist below the ice-covered surface of the jovian moon Europa.
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Ironically, one question that scientists find hard to answer is this: what is life? One short and probably inadequate answer is this: a living organism is a self-replicating organism that grows by interacting with its environment in a spontaneous way.
Another important question is "What ingredients are necessary for an environment to produce and sustain life?" Studies of very primitive microbial life (called extremophiles) found at thermal vents on the ocean's floors suggest strongly that the minimum ingredients are "food, water and a temperature gradient". Of course, what food means depends on the chemistry of the living organism. Notice that light is not an absolute requirement.
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Astrobiologists can learn much about possible life beyond our celestial shores by studying life here on Earth. It's pretty well established that life began in earnest on Earth very shortly after the end of the late heavy bombardment period, about 3.85 billion years ago, but that the evolutionary development of sentient beings like ourselves took billions of years, and with the help of a few timely cosmic catastrophes. It is therefore very fortunate that we orbit a G type star whose main sequence lifetime is a good nine to ten billion years.
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In conclusion, it is far from certain, but seemingly probable, that life exists elsewhere in the Universe. That that life is also intelligent, and can communicate with us in a reasonably timed back and forth fashion, is much less likely. Put another way, we are probably not alone, but will stay lonely for the foreseeable future. And should we establish contact with, or be contacted by, extraterrestrial intelligent beings, it would be the greatest development in the history of humankind.
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