Mysterious flying objects across the US probably aren't aliens, but experts believe life is out there. Here are 13 reasons why we haven't made contact yet.

"I can't believe we are the only living entity in the whole universe. There's just way too many planets, way too may stars ... the chemistry that led to life has to happen elsewhere," Didier Queloz, a physicist from the University of Cambridge, said in a talk at the Science Media Center in London in 2019.

Queloz had just won the Nobel prize in physics for his discovery of the first exoplanet orbiting a sun-like star. He said that his work has led him to become "absolutely convinced" that humans will detect alien life in the next 100 years.

Arguably, Fermi said, in the 4.4 billion years it took for intelligent life to evolve on our planet, the rest of our galaxy should have been overrun with similarly smart, technologically advanced aliens. But scientists have been monitoring radio waves for signs of alien life in the universe for decades, and they haven't found anything or anyone.

It's possible that aliens don't use radio waves as a means to communicate. They could be reaching out using a technology that we don't know about yet. Walsh compared the situation to one in which modern-humans would try to chat with a caveman on a cell phone (we're the cavemen in this analogy).

Walsh refers to our absence of extraterrestrial contact as "The Great Silence."

One answer to the Fermi Paradox, he says, could be called "The Great Indifference" — perhaps aliens just don't care what a sub-intelligent race has to say.

Astrophysicist Frank Drake sent out the first deliberate interstellar radio message on November 16, 1974 — 168 seconds of a two-tone sound were beamed toward the star system Messier 13 (or M13) in the Hercules constellation.

Encoded in the message were the atomic numbers of basic Earth elements, the numbers 1 to 10, and a graphic of our solar system to indicate where the message originated from. But M13 is roughly 21,000 light-years away, according to the SETI Institute, so Drake's message will take about the same number of years to get there. Then it would take any similar return signal the same amount of time to get back to us.

Hawking told the Times of London: "I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach."

Some researchers have suggested that intelligent life in the galaxy may have the same concerns that Hawking did about making contact, so therefore elect to remain silent.

In "End Times," Walsh puts forward a hypothesis in this vein: Perhaps Earth is being treated like a zoo and humans are a remote group of indigenous galactic dwellers that are being intentionally left undisturbed.

The 1951 Hollywood blockbuster "The Day The Earth Stood Still" explores such a theory. In the film, an alien spaceship lands in Washington, DC to deliver a message: live peacefully or be destroyed as a danger to other planets.

The intelligent civilizations we're trying to contact could be in a state of dormancy that may last for billions of years, he says.

Walsh explores the idea that it may just be hard to reach us way out here, especially if other intelligent civilizations have, like us, not yet figured out an efficient way to travel between star systems.

If aliens were traveling at one-tenth the speed of light, it would take them 10 million years to cross the entire Milky Way. That's less than 0.1% of the age of the galaxy. 

According to this potential answer to the Fermi Paradox, intelligent civilizations could exist in other parts of the Milky Way, but they die out or destroy themselves before they're able to find us or we're able to contact them.

As philosopher Nick Bostrom has explained, this concept suggests that life on an Earth-like planet has to achieve several "evolutionary transitions or steps" before it can communicate with civilizations in other star systems.

But an obstacle or barrier — a "Great Filter," as it's called in this line of thinking — makes it impossible for an intelligent species to progress through all of those steps before collapsing.

The study put forth four scenarios that a civilization could follow as it develops. One of those pathways leads to sustainable existence. But in the other three, civilizations overuse resources and collapse or die off as a result.

So a possible answer to the Fermi Paradox, the study authors posited, is that environmental transformation (whether that involves using up necessary resources or irreversibly changing a climate) inevitably prevents civilizations from surviving long enough to travel to distant stars.

Walsh calls these clues "necrosignatures." Nuclear holocausts, biological weapons, even disappearing planets leave detectable signs in space, he writes, and humanity should be ready to find and identify them.

Some worlds' liquid water is located under the surface, in the planets' interior. That appears to be the case for Saturn's moon Enceladus and Jupiter's moon Europa. Those underground oceans could be habitable.

"If technological civilizations can actually develop in these interior ocean worlds, they would naturally be cut off from us because of the shell of rock and ice above their ocean," planetary scientist Alan Stern previously told Business Insider. "We wouldn't see their city lights. We wouldn't be able to hear their communication. They wouldn't maybe even know that there was a universe out there to communicate with."

One study, published in The Astronomical Journal in 2019, posits that intelligent aliens could be taking their time to explore the galaxy, harnessing star systems' movements and orbital shifts to make star-hopping easier.

The study authors suggested that aliens might wait for stars to move closer to one another before spreading across the galaxy, and that other civilizations could have already been here and left no evidence of their visit.

Astrophysicist Michael Hart explored this question formally in a 1975 paper; he argued that there had been plenty of time for intelligent life to colonize the Milky Way in the 13.8 billion years since the galaxy formed. Since nobody on Earth had heard anything, Hart concluded, there must be no other advanced civilizations in our galaxy.

More recently, a 2018 Oxford University study suggested that there's a roughly two-in-five chance that we're alone in our galaxy and a one-in-three chance that we're alone in the entire cosmos.

But the more astronomers learn about conditions that make a planet suitable for life, the more it seems our galaxy could be more hospitable to life than previously thought. 

In an interview with the Nobel prize press office, he said that the detection of the first exoplanet merely served as "the trigger" for humanity's renewed hunt for alien life in the cosmos. 

Technology that enables astronomers to detect Earth-like planets has improved significantly since Queloz discovered the first exoplanet. 

That's in part why Queloz is convinced that we will find proof of aliens in the next century, if not sooner. (He suggested that in just 20 years, we may have the equipment needed to detect extraterrestrial life.)

In 2013, astronomers reported that based on Kepler data, there could be up to 40 billion planets comparable in size to Earth that exist within the "Goldilocks zone" of their respective star — where conditions might enable liquid water to pool on the surface. (Earth and Mars fall within our sun's "just right" zone.)

Even if just .01% of those Earth-like planets host life, that would still total to 40 million planets.

Another orbiting telescope, the Transiting Exoplanet Survey Satellite (TESS) is now scouting the sky for alien worlds.

So scientists from the SETI (Search for Extraterrestrial Intelligence) Institute point radio telescopes at portions of the sky to collect data. The researchers analyze that information for unusual patterns that might indicate an intentional or accidental transmission from an intelligent civilization.

But those monitoring efforts have thus far been unsuccessful.

The equation is made up of seven variables that, when multiplied together, yield a calculation of the possibility that humanity might someday hear from an intelligent alien civilization (one that has achieved the ability to transmit radio signals that we can detect on Earth).  

The problem is that we don't know the value of many of the Drake equation variables with any degree of certainty. Scientists have a good handle on the first three: the rate of star formation, the number of those stars with planets, and the number of those planets within the stars' Goldilocks zone. But the rest are still a mystery.

This post has been updated with new information. It was originally published on October 9, 2019.