Do you believe that Aliens have already visited our planet?

In my opinion probability of extra terrestrial life to be found near our solar system is very small. The star nearest to Sun is Proxima Centauri . Its distance from Sun is around 25 million million miles. If the technology with aliens is unimaginably(current scenario) advanced to travel at 1/4th of the speed of light the which would be around 46 thousand miles per second, space craft would need 36 years for the round trip from Proxima Centauri to the Sun. This period is too huge to survive in space and remain protected from continuous bombardment of all kinds of intense rays and objects.
There is evidence that shows most of the photographs of flying saucers or UFOs were either fake or that of airplanes at high speeds and unusual shape as that of the "Black bird".

Lets assume that the aliens do come to visit our planet. The reason could be that they want to evaluate earth as a place for living or for any other reason. Well if they can survive in space for this long they can make any planet there home, Earth wouldn't be as precious to them as it is for us. If they have technology as advanced as to travel at 1/4th of the speed of light they shouldn't really be afraid of human beings for so long to keep hiding from us and make designs in the farms without letting anyone see them.

All these speculations are based on the assumption that the star nearest to our solar system has some planet which had the perfect constituents and environment for evolution of life which is highly unlikely.
To me meeting extra terrestrial life in the near future looks like a rare possibility at least till the time string theorists come up with some twist to allow the aliens to come from other dimensions.