There is no concrete evidence that any extraterrestrial life has ever visited Earth. However, that doesn’t mean it’s impossible that lifeforms from beyond the stars have come to our planet. Science has answers for how, strictly, a sufficiently advanced race could cross the vast distances between Earth and even its closest neighboring planets.
Here are a few ways a sufficiently advanced species could make contact with Earth.
Interstellar distances are so impossibly vast that travel between stars would have to take place throughout lifetimes. No one lifeform could complete the journey: even the closest star to the sun is four lightyears away, meaning that it would take thousands of years to reach it using even the fastest engines available.
As such, it’s much more likely that humans will encounter alien robots than alien life forms. An advanced species interested in communicating with its neighbors would likely rely on spacefaring machines to send their messages across the stars.
This has led many speculative scientists to presume that humanity’s first contact with an alien culture will be with that culture’s robots. It’s simply the most likely outcome given our current understanding of physics and space travel laws.
Humanity’s current understanding of the laws of physics is that it is entirely impossible to travel faster than the speed of light. Due to the nature of our universe, that’s the upper limit of speed. If it was possible to go more quickly, the theory goes, then the light would go that fast instead.
If we’re wrong, though, and there is a way to move faster than the speed of light, then a sufficiently advanced spacefaring society would be able to visit us if they knew we were here. We encounter the issue that even an advanced society would need just to guess to find us. There are trillions of stars in our galaxy, and many of those stars have Earth-like planets orbiting them.
A spacefaring civilization would need to get pretty close to the planet to listen for radio signals and look for artificial satellites that could indicate the presence of intelligent life. Sadly, the odds that Earth gets lucky and is detected by a passing alien probe are extremely low.
None of this is to say that humanity is fated never to encounter another intelligent species. Assuming that no extinction events occur any time soon, our species has plenty of time to spread across the solar system and search the stars with our telescopes and probes for signs of life.
If we have neighbors out there, we’ll find them eventually. The question is whether we’ll meet alien life forms or their machines first.