With the popularity of "Avatar" it seems like a good time to once again address the question of the possibility of intelligent life beyond Earth. This question has been studied scientifically ever since the formulation of Drake's Equation in 1960. Drake attempted to define the astronomical variables associated with intelligent life and assign estimated values to them in order to calculate the number of extraterrestrial civilizations in our Galaxy. Since it's inception Drake's Equation has been elaborated upon, but the values of all the described variables have been very speculative and estimates of the number of intelligent life forms in the "Milky Way" has varied from 1 (ourselves) to the thousands or millions.
With the advent of astronomical techniques that has allowed detection of a multitude of planets orbiting other star system in our galaxy, Drake's Equation can now be refined. In particular, based on an inventory of recently discovered exoplanets, new calculations estimate that at least 15% of stars within our galaxy have a planetary system similar to our own. Regardless of other considerations it means that of the roughly 200,000,000,000 stars in our galaxy over 300,000,000 may have an earth-like planet orbiting in the star's habitable zone. If we extrapolate to the rest of the universe, with its estimated 100 billion galaxies, the number of potential earth-like planets becomes truly astronomical (for an edifying take on the scale of the universe relative to our existence check out Yosef 52's recent diary "The Human Scale of Meaning...").
Another take on the likelihood of intelligent life beyond our planet is based on the Anthropic Principle. The Anthropic Principle posits that the universe is fine tuned for life as we know it. Theists take this as evidence that the universe was designed by a deity to suit human existence (aka The Strong Anthropic Principle or SAP). Others attempt to rationalize the Anthropic Principle by falling back on controversial ideas such as the multiverse, which hypotheses an infinite number of self-contained universes each with their own physical characteristics. Given the possibly infinite number of possible universes it becomes unremarkable that ours exists and that we evolved to observe it (aka The Weak Anthropic Principle or WAP).
Both the intelligent design and multiverse solutions to the Anthropic conundrum are beyond scientific verification. Intelligent design advocates postulate a conscious supernatural agency that modeled the universe in such a way as to allow for intelligent life. Supernatural agents are obviously beyond testing by scientific methods. The multiverse at present is also beyond scientific testing as alternative universes are by definition self-contained and beyond observation by outsiders.
Many argue that the Anthropic Principle is a tautology or truism (the universe is the way it is because that’s the way it is). It can be argued that conditions that allow for life on earth are not necessarily unique because we have a sample size of only one. Other physical parameters that we are unaware of may also permit intelligent life. Another consideration is that the universe we live in may be a continuation of an infinite regression. We do not know what preceded the Big Bang but we can rest assured that "something" existed beforehand. Non-existence is an oxymoron. Something can not come from nothing because nothing would then be something. If we accept existence as an infinite probability function then our universe is bound to exist sometime and somewhere in the infinite vastness of time and space.
A more delimited Anthropic Principle favored by creationists is that not only are the physical properties of the universe fine-tuned for our existence, but that intelligent life as exemplified by the human species, could have emerged only on our home planet Earth because of a set of unique circumstances, such as the binary system of the Earth and Moon, the presence of Jupiter as a shield against extra-terrestrial impacts, our position in the Milky Way Galaxy, etc. This is best exemplified by the creationist video "The Privileged Planet," which elaborates on the Rare Earth Hypothesis. The latter argues that "the emergence of complex multicellular life (metazoa) on Earth required an improbable combination of astrophysical and geological events and circumstances". The problem with this idea is that there may very well be a multitude of special circumstances in the universe that could also allow for intelligent life to evolve. What exactly are the necessary rather than proximate criteria for intelligent life to emerge? No one has any real knowledge of what they may be. Thus an intelligent species elsewhere in the universe would a priori attribute its existence to the special circumstances surrounding itself.
When we look at the immensity of the universe in both time and space with its potentially 300,000,000,000,000,000 (300 quadrillion) earth-like planets to suppose that we are the only intelligent life form in existence is the height of shortsighted arrogant conceit.