I found no better discussion as the first post in the Philosophy of Being and Existence. But before I answer your question, answer me this: ‘Why is the Universe the way it is?’
I am asking this from a purely philosophical point of view, not fully scientific. One may argue that the Universe is as it is, because we are here to observe it the way it is. If we weren’t here, there would be no one to marvel at the complexity and the beauty of the Universe, so no point asking such a question in the first place. I don’t fully understand the Anthropic principles, but I reckon the statement I just made forms the Weak Anthropic principle.
If our Universe, is compatible for life, then why just Earth? The Universe is infinite, that means there is a probability as high as infinity that intelligent life may develop on some other planet outside our Solar System (Exoplanet)! Let’s bring down the probability of finding life into measurable quantities by calculating numbers for the Milky Way Galaxy. As of 12th September 2014, astronomers have discovered 1822 Exoplanets so far. These are only the once which we have been able to catalogue, and it is postulated that there is at least one planet per star on average. 25% planets in our Milky Way Galaxy are sun-like. 20% of those sun-like stars have an Earth sized planet in the habitable zone. Therefore, with 300 billion stars in our Milky Way galaxy, there are about 15 billion potential planets in the habitable zone!
Habitable zone refers to the region around a star, where the atmospheric pressure of a planet can support liquid water, thereby the conditions would be similar to the Earth. Hotter stars would have their habitable zone with a greater radius than cooler stars. The distance of this habitable zone from the star is directly proportional to the square root of the luminous intensity of the star. Twice the flux of the star, 1.414 times the distance of the habitable zone, as compared to Sun-Earth system. But why do we assume that water is necessary for life? There might be forms of life which have different bio-chemistry than the life forms of Earth. This is theory is true, hence the probability of finding life in the Universe suddenly shoots up. But for the sake of this discussion, we will keep the bio-chemistry same as Earth. Below image shows the solar system, compared with another planetary system Gliese 581, and the planets in the habitable zone. Log scales have been used.
But according to the Habitable Zone diagram, the Moon and Mars…
Yes, both Moon and Mars, and even the dwarf planet Ceres fall in the habitable zone, making them probable candidates for the search for life. As would be clear from news, Mars is the subject of much discussion and study for liquid water and life. Scientists have found that in history, Mars would have liquid water flowing on its surface, just like Earth’s ocean. However, the current atmospheres of Moon and Mars have far too less atmospheric pressure to sustain liquid water, and capture greenhouse gases. Venus, when is it at the greatest distance from the Sun does touch the habitable zone, but other times it does not, and this variation in sunlight cannot support life, along with the toxic atmosphere of Venus.
Having said that, it is postulated that planetary satellites (moons) outnumber planets and is one of the emerging studies for search for life, a measure known as Natural Satellite Habitability.
So, to answer your question, giving the great odds of habitable planets (and satellites), I believe yes, there is life out there somewhere; but not within 100 light years of Earth. If there was, we would have detected radio waves from them in the past 100 years or so since we are sending and receiving radio signals. You may argue that maybe they are not intelligent enough to make contact yet, or they may have made contact 100 years ago when we weren’t detecting radio waves. In principle, yes and there seems no end to this discussion, but from a factual perspective, yes there can be life, but not within 100 light years from us.
We can only wonder…
… can you hear me?