In the Milky Way alone, there are estimated to be billions of planets located at an ideal distance from their stars to allow life to develop. So why have we not detected extraterrestrial life forms?
Now, a new study, published in The Astronomical Journal, takes on the Fermi Paradox, which asks the very same question.
The team of scientists believes that alien life may have come to Earth. It was so long ago though, that we weren't around to see it.
What is the Fermi Paradox?
The Fermi Paradox posits that there is no evidence for extraterrestrial life in the Milky Way — but there really should be. This is because the number of planets located in the habitable zones of their solar systems is enormous.
The Milky Way consists of up to 400 billion stars. About 20 billion of these are sunlight stars. Estimates suggest that about a fifth of these sunlight stars have an Earth-sized planet located in its habitable zone.
If only 0.1% of the planets located in the habitable zones — the areas with ideal conditions for life to develop — of sunlight stars contained life, there would be 1 million planets containing life in the Milky Way.
What's more, the Milky Way is about 13 billion years old, while the Earth is relatively young at 4 billion years of age.
The first habitable planets in the Milky Way are estimated to be about 12 billion years old. That means extraterrestrials have potentially had had a head start of billions of years to create space-traveling civilizations.
But wait, there's still more. With humanity's existing space technology, it is estimated that it would take about 2 million years to travel and colonize our entire galaxy. That is a relatively small amount of time when taken in the context of billions of years.
This is what led Physicist Enrico Fermi to ask, "where is everybody?"
Now, the new study — published in The Astronomical Journal — says that Earth may, in fact, have been visited by extraterrestrial life. Unfortunately, human beings weren't around to hold a welcoming committee.
In a 1975 paper, astrophysicist Michael Hart took on the Fermi Paradox and came to the conclusion that there are likely no alien civilizations in the Milky Way.
The new findings claim that extraterrestrials might exist, they are likely just being strategic and biding their time.
"If you don't account for the motion of stars when you try to solve this problem, you're basically left with one of two solutions," Jonathan Carroll-Nellenback, a computational scientist and the study's lead author, explained to Business Insider. "Either nobody leaves their planet, or we are in fact the only technological civilization in the galaxy."
However, the authors of the new study say that previous research hasn't taken into account this one crucial fact: our galaxy moves. In the same way that planets orbit stars, star systems orbit the galactic center. The Milky Way, for example, takes 230 million years to orbit the galaxy.
Solar systems orbit at different speeds, meaning that they occasionally pass each other at closer distances. So, according to the study, extraterrestrials could be waiting for close passes in order to hop to other star system and explore planets in their habitable zones.
If that's the case, it could take much longer to spread across the galaxy than previously estimated. As such, aliens may not have reached us. On the other hand, they may have reached us millions of years before humans evolved.
This brings up the possibility that extraterrestrials did land on Earth, found no intelligent life, and moved on.
Simulating the spread of alien life
In order to explore the scenarios in which aliens could exist, despite the Fermi Paradox, the scientists used numerical models to simulate the spread of a hypothetical civilization across the galaxy.
A variety of possibilities were factored into the study. These included the proximity to new star systems, the range and speed of hypothetical interstellar probes, and the rate at which these probes would be launched.
One problem the researchers do make sure to point out is that they are working with only one data point: our own behaviors and capabilities for space exploration.
"We tried to come up with a model that would involve the fewest assumptions about sociology that we could," Carroll-Nellenback told Business Insider. We have no real way of knowing the motivations of an alien civilization.
What we do know is that a lot more data will soon be in the hands of these researchers, and in the hands of others who wish to take on the Fermi Paradox. One example of this is NASA's Webb Telescope — set to launch in 2021 — that will map the birthplace of stars.
If there is life out there, we might be getting closer to finding it — even if it's already found us.