Last month, residents, students, scientists and newscasters were fascinated by a bright white light which burst through the evening skies in Southern California. But what lay inside that lights is still unknown.

Videos and photos overwhelmed social media with tags like #UFO and #Meteors as people proposed various alien themed theories. Some people were so concerned that they called the authorities, who later claimed the light was connected to U.S. Army operations.

Whatever the source of the sighting that afternoon– military missile test or otherwise – it definitely struck a chord with many people. Many of us are fascinated by the search for alien life and while we may be afraid of what may unfold we are also enthralled with the possibility of finding something new and exciting.

In July of this year, Russian billionaire Yuri Milner announced a new initiative towards the search for extraterrestrial life. Project Breakthrough Listen aims to find glimpses of alien vestiges by sweeping the skies on a massive scale, using the largest radio telescopes available to sift through a billion stars closest to Earth. The $100 million project has attracted the attention of researchers and astronomers internationally, including Stephen Hawking who is leading the scientific exploration.

But this isn’t the first time we’ve tried to contact life outside our planet. Shortly after the advent of the radio, Carl Sagan and Louis Friedman proposed signal exchange as a means of communicating with other life forms. The idea was that by capturing radio signals outside of our planet, alien messages could be identified by detecting aberrations in captured signals. In 1980, the Search for Extraterrestrial Intelligence (SETI) Institute was formed.

In 2009, another attempt at the search for extraterrestrial life was made. NASA launched Kepler, a space probe sent to survey the composition of the Milky Way and along the way, keep an eye out for aliens. While Kepler did not find Yoda from Star Wars, images taken by the Hubble telescope did reveal new information about our galaxy.

“We learned that the Milky Way alone contains approximately 20 billion sun-like stars from the Kepler experiments,” says Dr. Blas Cabrera, professor of Particle and Astrophysics at Stanford University. “Of those, one-fifth are estimated to have habitable planets similar in size to Earth.”

That’s four billion planets just like ours! If even 0.01% of those contain life, there would be 400,000 life containing planets within our galaxy alone – imagine how many alien planets could exist in the universe! So why aren’t the skies swarming with rocket ships, and why haven’t we detected any signs of extraterrestrial life forms? Here are some theories:

We still don’t understand how life is created

If life arose spontaneously from the basic elements that constitute a habitable planet, then there must be many other cases, besides Earth, that carry life. However, this may not necessarily be true.

As Cabrera points out, “We are looking carefully for any hints of spontaneous formation of life. For example, new findings [4] show evidence of flowing water on Mars. If living organisms were found there, that would directly support spontaneity.”

But no such organisms have been found yet. In fact, the formation of life may be more complex than originally thought. “The conditions necessary for life are still unknown. Given that Earth formed over four billion years ago under extremely harsh conditions, it could be that only recently the environment shifted to provide favorable opportunities to host life,” explains Vanessa Bailey, a post-doctoral candidate studying planets outside our solar system in the Kavli Institute of Particle Astrophysics.

If this is true, then the hypothesis that hundreds of thousands of alien planets exist within the Milky Way galaxy would be inherently false.

Intelligent life suffers catastrophe before communication signals are received

Think about sending a text message from a basement. Just as cell phone towers would find it difficult or impossible to detect the signal, the distance separating us from potential intelligent life is hard to bridge.

Early SETI investigations gave us important knowledge on estimations of some metrics of the universe. Millions of galaxies containing several hundreds of billions of stars span across more than 13 billion light years of space. In the time it would take a signal to travel from a random location to ours, it is highly likely that civilization would become extinct due to natural catastrophe. A prime example of this boom and bust cycle of life can be found in the dinosaurs. Having lived for over 120 million years, dinosaurs suddenly disappeared off the face of the Earth when environmental conditions altered. It is assumed that life forms on other habitable planets could similarly experience cataclysmic events ending their existence. We would never find out about them because they could die before a meaningful signal is delivered.

Aliens might speak a totally different language

“A large concern is that we might be looking for the completely wrong signals,” Cabrera says.

Just because humans use radio signals does not mean life outside our planet does. “Extraterrestrial life could potentially use forms of communication we just don’t comprehend, express information in senses we can’t even imagine,” adds Bailey.

Humans perceive through sound, sight, touch, smell, and taste. if aliens perceive through senses different from ours, there would be no way to know of each others’ existence.

Where we go from here

These are only a few of the dilemmas we face in our quest to find alien life. However we should not let them discourage us. After all, through the search that is just beginning, we have already learned so much about ourselves and the universe we live in. Bailey remains hopeful.

“We are only going to continue learning more.”