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Are we alone? - As human beings, we are curious about our existence and the world around us. As we discover more and more about the universe we live in, we increasingly feel the need to question whether or not we are alone in it.

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Are we alone? - Many projects have existed for decades in an attempt to find other intelligent life in the universe, but we have yet to make contact with anything (or anyone) else.

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Fermi's paradox - Italian-American physicist Enrico Fermi posited his now-famous paradox in the early 20th century.

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Fermi's paradox - Fermi asked how it could be possible to not have any evidence of other intelligent life in the universe if the probability of its existence was so high. 

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Probability of life - Our galaxy alone is estimated to contain between 100 and 400 billion stars, with many of them potentially surrounded by planets.

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Probability of life - Scientists have reported that there could potentially be billions of planets orbiting the habitable zone around their stars in the Milky Way galaxy, meaning that they could contain the conditions necessary for life to exist.

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Search for life - Organizations including NASA, the European Space Agency, and the Search for Extraterrestrial Intelligence (SETI) have invested in projects to search the stars for signs of life.

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Search for life - Giant radio antennas and telescopes are used to send out and listen for potential radio transmissions.

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Search for life - Other methods include looking for bright flashes of light that could indicate laser beams.

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Radio waves - However, there are many possible problems with using radio frequencies to determine whether intelligent life exists beyond Earth.

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Radio waves - These issues include not having the proper technology to interpret the transmissions sent out by other intelligent life, simply not receiving the signals because they are not directed at us, and being located too far away, meaning the signals have yet to reach us.

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Radio waves - We have been listening for radio frequencies for less than a century, so we still would not have received signals from any intelligent life located farther than 100 light years from Earth.

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Great Filter - One theory that attempts to explain why we have not achieved contact with other intelligent life is known as the Great Filter.

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Great Filter - This theory refers to an event that prevents a civilization from reaching the level of technological advancement necessary for interstellar travel or communication.

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Great Filter - The event in question may involve a cataclysmic disaster that wipes out existing life or could just represent the cosmic limits of technological advancement.

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Zoo hypothesis - Another explanation for the lack of evidence of extraterrestrial life is known as the zoo hypothesis.

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Zoo hypothesis - The hypothesis proposes that extraterrestrial life forms are aware of our existence but are intentionally avoiding contact in order to study us from a distance and not disturb our "primitive" society.

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Gaian Bottleneck - This theory proposes that life has existed elsewhere in the universe but that it has never had the same opportunity to grow and evolve as it has on Earth.

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Gaian Bottleneck - Unstable and shifting planetary environments could cause life to die off before having the chance to evolve any sort of advanced intelligence.

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Deep oceans - Scientists have discovered oceans trapped under thick layers of ice on some of Saturn's and Jupiter's moons.

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Deep oceans - Life could hypothetically exist in these deep oceans but, even if evolved enough to be intelligent, it would face enormous obstacles trying to reach the surface of its own planet, let alone be able to send out signals.

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Too young - Another theory postulates that due to the relatively young age of our Solar System and Earth, aliens have already visited our corner of the galaxy and we've just missed them.

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Already here? - Not to be ruled out is the theory that aliens are already on Earth and living among us.

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Too impatient - Astrophysics student and researcher at Cornell University, Evan Solomonides, wrote a paper outlining his research on the probability of extraterrestrial life, in which he suggested that we just need to be patient.

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Too impatient -

He points out that we have barely explored our own galactic neighborhood, let alone our galaxy, and that if we hope to hear from extraterrestrial life, we may need to wait another 1,500 years, give or take.

See also: Probing deep space: journeys into darkness

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