Intelligent life outside of earth has been debunked many times. For example, the great pyramids that ancient Egyptians built. One of the bases of alien theories is the ability for the Egyptians to build large shaped triangles in a little bit of time. The pyramids were not always so tall. The earliest pyramids were rectangular tombs called mastabas that were stacked on top of each other.
The only thing that has been proven to exist on planets outside of earth is water and bacteria. Europa, Jupiter’s moon, is covered in ice. Scientists believe that it could be a great place to land if people were to evacuate earth.
Bacteria has yet to be found on other planets and moons. However, a few of bacteria found on earth has grown on a spaceship, giving way to theorize that bacteria could survive in space.
Aliens have never come to earth. Humans were smarter than they seemed. Without machinery, humans had to become extremely innovative. Plus, Some UFO sightings have been investigated and debunked as based on sightings of Venus or known atmospheric phenomena such as mock suns (sunlight reflecting on a layer of ice particles below an aircraft).