Between government document dumps and a Spielberg movie about extraterrestrial life, it’s a good week to believe in aliens. It’s also a good week to reality-check that belief.

About a third of the public in the U.S., Australia, and elsewhere now believes aliens are already on Earth, fueled in part by the government’s recent UAP document releases and the new Steven Spielberg film Disclosure Day. Carol Oliver, Professor in Science Communication and Astrobiology at UNSW Sydney, writing in The Conversation, has three reasons why that belief is probably wrong.

Space Is Much, Much Bigger Than You’re Imagining

Proxima Centauri, the closest star to our Sun, sits roughly 40 trillion kilometers away, 268,000 times the distance between Earth and the Sun. At the speed of our fastest spacecraft, the Parker Solar Probe, getting there would take approximately 6,650 years. That’s just the nearest neighbor.

Traveling near the speed of light introduces its own problem. Einstein’s theory of relativity tells us that time moves more slowly for objects in motion, a phenomenon called time dilation. NASA astronaut Scott Kelly returned from a year on the International Space Station milliseconds younger than his identical twin for exactly this reason. For aliens making an interstellar round trip at necessarily higher speeds, the effect would be far more pronounced. They’d return home to a planet potentially centuries older than the one they left. Time exiles, essentially.

The Energy Requirements Are Effectively Impossible

Even setting aside the distance problem, the energy required for interstellar travel is staggering. A spacecraft’s mass increases with velocity, which means the energy needed to accelerate it keeps climbing. At the speed of light, mass becomes infinite, and so does the fuel bill. Space also isn’t a perfect vacuum—scattered hydrogen atoms become intense radiation at near-light speeds, capable of destroying both passengers and hull. Physicist Miguel Alcubierre has theorized that faster-than-light travel could be possible, but the energy requirements are currently nowhere near achievable.

Our Atmosphere Might Be Actively Hostile to Them

Earth’s oxygen-rich atmosphere, which cyanobacteria spent 2.4 billion years building, is perfectly suited for us and potentially corrosive to anything that evolved elsewhere. Oxygen is reactive. For alien biology, it could be toxic. And as Oliver points out, reports of alien visitors never seem to include descriptions of protective suits, which any lifeform from a non-oxygen environment would almost certainly need.

Of course, this doesn’t mean that aliens aren’t out there (firm believer that they most certainly are). With over 100 billion stars in our galaxy alone, the odds of Earth being the only inhabited planet seem low—according to some scientists, anyway.