For so long, the existence of aliens has been a myth, something used to scare children and whip Ancient Alien theorists into a frenzy. Like the Loch Ness Monster and Bigfoot, aliens are just creatures created by wild imaginations — or are they?Adam Frank, an astrophysics professor at the University of Rochester, wrote an editorial for The New York Times about aliens being real figures from the past. He presents a really intriguing argument, stating that aliens once inhabited the galaxy.He, along with astronomer Woodruff Sullivan, published a study that shows "while we do not know if any advanced extraterrestrial civilizations currently exist in our galaxy, we now have enough information to conclude that they almost certainly existed at some point in cosmic history."Frank says that discovering planets and stars is easier for humans than ever before, so we have way more of an understanding about cosmic galaxies than ever. This means that "interstellar communication," or being able to communicate with extraterrestrials, depends on how many extraterrestrial civilizations exist.Here's the math: scientists have found that every star likely hosts at least one planet; we know how many stars are born each year, and we know that between 20 and 25% of all planets can host life. That makes it much easier for Frank and Woodruff to determine if Earth is the only technological civilization in the galaxy.Their theorem dictates that it's not. According to Frank, other technological civilizations definitely existed in the past. The scary part? "According to our finding, even if you grant that level of pessimism, a trillion civilizations still would have appeared over the course of cosmic history," he says.A trillion aliens? Now, that's worth an Ancient Aliens episode.