What Is the Fermi Paradox?

By Elizabeth Howell, Space.com Contributor | April 02, 2014 04:19am ET

The Fermi Paradox seeks to answer the question of where the aliens are. Given that our star and Earth are part of a young planetary system compared to the rest of the universe and that interstellar travel might be fairly easy to achieve the theory says that Earth should have been visited by aliens already.

As the story goes, Enrico Fermi (an Italian physicist) first came out with the theory with a casual lunchtime remark in 1950. The implications, however, have had extraterrestrial researchers scratching their heads in the decades since.

"Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire galaxy," the Search For Extraterrestrial Intelligence (SETI) said on its website.

"Within ten million years, every star system could be brought under the wing of empire. Ten million years may sound long, but in fact it's quite short compared with the age of the galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise."

It is true that the universe is incredibly vast and old. One estimate says the universe spans 92 billion light-years in diameter (while growing faster and faster). Separate measurements indicate it is about 13.82 billion light-years old. At first blush, this would give alien civilizations plenty of time to propagate, but then they would have a cosmic distance barrier to cross before getting too far into space.

The sheer number of planets that we have found outside of our solar system, however, indicates that life could be plentiful. A November 2013 study using data from the Kepler Space Telescope suggested that one in five sun-like stars has an Earth-size planet orbiting in the habitable region of its star, the zone where liquid water would be possible. That zone is not necessarily an indication of life, as other factors, such as the planet's atmosphere, come into play. Further, "life" could encompass anything from bacteria to starship-sailing extraterrestrials.

A few months later, Kepler scientists released a "planet bonanza" of 715 newly discovered worlds, pioneering a new technique called "verification by multiplicity." The theory essentially postulates that a star that appears to have multiple objects crossing its face or tugging at it would have planets, as opposed to stars. (A multiple star system at such close proximity would destabilize over time, the technique postulates.) Using this will accelerate the pace of exoplanet discovery, NASA said in 2014.

Our understanding of astrobiology (life in the universe) is just at a beginning, however. One challenge is these exoplanets are so far away that it is next to impossible for us to send a probe out to look at them. Another obstacle is even within our own solar system, we haven't eliminated all the possible locations for life. We know from looking at Earth that microbes can survive in extreme temperatures and environments, giving rise to theories that we could find microbe-like life on Mars, the icy Jovian moon Europa, or perhaps Saturn's Enceladus or Titan.

All of this together means that even within our own Milky Way Galaxy the equivalent of the cosmic neighborhood there should be many Earth-size planets in habitable zones that could host life. But what are the odds of these worlds having starfarers in their bounds? [Countdown: 13 Ways to Hunt Intelligent Aliens]

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What Is the Fermi Paradox?