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, Italian physicist Enrico Fermi, most famous for creating the first nuclear reactor, came up 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) Institute in Mountain View, California, 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."

Fermi reportedly made the initial remark, but he died in 1954. Publication fell to other people, such as Michael Hart, who wrote an article titled "An Explanation for the Absence of Extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975. (Some say this is the first such paper to explore the Fermi paradox, although this claim is a bit hard to prove.)

"We observe that no intelligent beings from outer space are now present on Earth," Hart wrote in the abstract. "It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy." He noted, however, that more research in biochemistry, planetary formation and atmospheres was needed to better narrow down the answer.

While Hart was more of the opinion that we were the only advanced civilization in the galaxy (he argued that in Earth's history, somebody could have visited us already unless they started their journey less than two million years ago), he outlined four arguments exploring the paradox:

1) Aliens never came because of a physical difficulty "that makes space travel infeasible," which could be related to astronomy, biology or engineering.

2) Aliens chose never to come to Earth.

3) Advanced civilizations arose too recently for aliens to reach us.

4) Aliens have visited Earth in the past, but we have not observed them.

The argument has been challenged on many grounds. "Maybe star travel is not feasible, or maybe nobody chooses to colonize the galaxy, or maybe we were visited long ago and the evidence is buried with the dinosaurs — but the idea has become entrenched in thinking about alien civilizations," wrote Fermi paradox researcher Robert H. Gray in a 2016 Scientific American blog post.

Frank Tipler, a professor of physics at Tulane University, followed up on the argument in 1980 with a paper titled "Extraterrestrial intelligent beings do not exist," also published in the RAS Quarterly Journal. The bulk of his paper dealt with how to get resources for interstellar travel, which he suggested could be achieved by having some kind of self-replicating artificial intelligence moving from star system to star system and create copies using materials there.

Since these beings aren't on Earth, Tipler argued we are likely the only intelligence out there. He also said that those who believe in extraterrestrial intelligence are similar those who think about UFOs, because both camps believe "we are going to be saved from ourselves by some miraculous interstellar intervention."

Today, the topic of extraterrestrial intelligence is a popular one, with several papers appearing every year from different researchers. It's also been fueled by the discovery of exoplanets.

Plentiful planets

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 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.

Fermi first formed his theory long before scientists found planets outside of our solar system. There are now more than 3,000 confirmed planets, with more being found frequently. The sheer number of planets that we have found outside of our solar system indicates that life could be plentiful.

Over time, with more advanced telescopes, scientists will be able to probe the chemical compositions of their atmospheres. The eventual goal is to understand how often rocky planets form in the habitable regions of their stars, which is traditionally defined as the zone in which water can exist on the surface. Habitability, however, isn't just about water. Other factors must be considered, such as how active the star is, and what is the composition of the planet's atmosphere.

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. 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.

Researchers previously focused on red dwarf stars as a possible host for habitable planets, but as the years of study continued, limitations arose. It was exciting to find nearby planets such as Proxima Centauri b and the seven rocky planets of TRAPPIST-1 in the regions of their stars where liquid water could exist on the planets' surface. The trouble is, red dwarfs are volatile and could send several forms of life-killing radiation towards the surface. More study is required to better understand these stars.

More exoplanet-hunting spacecraft are coming online in the next few years. The Transiting Exoplanet Survey Satellite (TESS) launched successfully in April 2018 to study nearby stars. NASA's James Webb Space Telescope, expected to launch in 2020, will examine planets for the chemical makeup of their atmospheres. The European Space Agency's PLATO (PLAnetary Transits and Oscillations of stars)is expected to launch in 2026. And larger ground-based observatories are also being envisioned, such as the European Extremely Large Telescope that should see first light around 2024.

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]

Life: plentiful, or rare?

The odds of intelligent life are estimated in the Drake Equation, which seeks to figure out the number of civilizations in the Milky Way that seek to communicate with each other. In the words of SETI, the equation — written as:

N = R* • fp • ne • fl • fi • fc • L

— has the following variables:

N = The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.

R* = The rate of formation of stars suitable for the development of intelligent life.

fp = The fraction of those stars with planetary systems.

ne = The number of planets, per solar system, with an environment suitable for life.

fl = The fraction of suitable planets on which life actually appears.

fi = The fraction of life bearing planets on which intelligent life emerges.

fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.

L = The length of time such civilizations release detectable signals into space.

None of these values are known with any certainty right now, which makes predictions difficult for astrobiologists and extraterrestrial communicators alike.

There is another possibility that would dampen the search for radio signals or alien spacecraft, however: that there is no life in the universe besides our own. While SETI's Frank Drake and others suggested there could be 10,000 civilizations seeking communications in the galaxy, a 2011 study later published in the Proceedings of the National Academy of Sciences suggested that Earth could be a rare bird among planets.

It took at least 3.5 billion years for intelligent life to evolve, the theory by Princeton University researchers David Spiegel and Edwin Turner said, which indicates it takes a lot of time and luck for this to happen.

Other explanations for the Fermi paradox include extraterrestrials "spying" on Earth, ignoring it altogether, visiting it before civilization arose, or visiting it in a way that we can't detect.

Recent Fermi paradox discussion

While the Fermi Paradox question has baffled scientists for decades, there are some new insights that could help researchers better understand why aliens have been so hard to find.

In 2015, a study looked at the likelihood of a world evolving with a habitable environment, using data from the Hubble Space Telescope and Kepler Space Telescope. It suggests Earth was an early bloomer. Even though the study excluded intelligent life, the study suggests that our planet's birth came very earlyin the universe's history. When Earth was formed about 4.6 billion years ago, the study said, only "8 percent of the potentially habitable planets that will ever form in the universe existed." In other words, most of the material available to form habitable planets is still around — giving lots of time for alien civilizations to form.

Or perhaps life may be too fragile to survive for long. A 2016 study suggests that the early part of a rocky planet's history can be very conducive to life, as life could emerge after about 500 million years after the planet cools down and water is available. However, after that point the planet's climate could easily wipe life out. Look at Venus (which has a runaway greenhouse effect) or Mars (which lost most of its atmosphere to space). The study was led by Aditya Chopra, who was then with the Australian National University (ANU) in Canberra.

In 2017, Space.com republished a RealClearScience article with 12 reasons why we cannot find aliens, ranging from intelligent life self-destructing to nobody being willing to transmit their whereabouts.