Impossible to predict. It would depend on what sort of aliens these were, and why they came here. Science Fiction writers have imagined hundreds of possible results from The Twilight Zone's classic horror episode "To Serve Man" (spoiler: "It's a COOKBOOK!") to Isaac Asimov's more humorous story "Buy Jupiter", to the more martial "invasion from space" scenarios like the movie "Independence Day" and the Niven & Pournelle classic SF novel "Footfall".
One thing is certain; it would answer the questions "Are we alone in the universe" and "Is star travel possible" in a very graphic way.
If Mars had living beings they would not be aliens on Mars but, if they came to earth they would be aliens on earth. Alien actually means not from here.
No one knows.
Is there any reason to think that aliens are all cereal killers?
the earth would blast
The moon might crash into the earth
It would get to hot. We would die.
I dont think it is likely to happen.
Aliens are more advanced then us if they came to earth but if they found out to late that the only way to kill them is to shoot them in Head then it is a fair fight.
Depends if they can cloak their ships or if they can use stealth or not. Etc etc.
people would be in the snow until summer
Both animals came to earth from aliens.