The answer to this question is no, there are no aliens on earth. There have been no crashed alien spaceships despite what the Roswell conspiracy theorists say. Popular fiction and entertainment is full of references to aliens on earth. There are TV shows about ancient alien encounters. One thing these stories all have in common is that they are not true. Much like Bigfoot, the stories about aliens and alien spacecraft are all contemporary myths.
What most people don't realize is how far the stars are away from earth. The fastest spacecraft produced on earth would take thousands of years to reach the nearest star. Space is a hostile place. There is no air, water, food, or energy. Everything to keep you alive has to be brought along at great cost. Without gravity, bones weaken and muscle atrophies. Even with fusion power it would take many lifetimes to reach even close stars. Chances are slim anything could survive such a journey.
We do not even know if there is life on any other planets in the universe. Consider life on earth. There is one species from the millions that can even consider the concept of life on another planet. It has only been in the last hundred years that the technology to go to space has been developed. How would any aliens even know to look for life here on this planet? Where is the physical evidence of these beings if they are here? What makes people think that a creature from a completely alien planet could survive on earth?
It is hard at times to tell fantasy from reality. Alien life forms are in the realm of fantasy, as they have been suggested but not discovered. I am not saying that it is impossible for life to have evolved on another planet and developed the technology to travel the galaxy. I am saying there is no evidence for them at this time. Something like a visit from another planet could not be kept secret for long. The governments of the world have been using flying saucer reports to hide their secret aircraft for years. Hollywood and science fiction thrives on the concept of aliens, because aliens make money. The concept of an alien creature on earth is interesting, but there is no evidence for this actually happening.
Once again, the only aliens on earth are people from other countries. The flying saucers have not landed. Grover's Mill is not under attack by Martians. There are no little green men hiding behind the barn. The bulbous-headed little gray men on TV are latex models. Things you see in movies are made-up stories. We are not receiving messages from other planets on radio telescopes. The reports you see in the National Enquirer about celebrities having space alien babies are lies. If any aliens do come to earth and land, you'll know about it right away. They'll have a deal to sell Coke within an hour.