(1) The distance to the next Solar type system is too far:
Our Sun is a star, much like the billions of other stars (suns) in our Milky
Way Galaxy.
The distance to the nearest star is immense and would take an impossibly
enormous amount of energy for any spaceship to travel that distance.
The amount of energy required to travel the very much smaller distances from
Earth to Mars, Jupiter, Saturn is already large no matter what energy source you use. The distances are so large to the stars that it would also take a lot of time for the trip. Also you must remember that the speed limit for any object is the speed of light: 3 x 10^8 m/s or 3 x 10^5 km/s –
An excellent description of how tremendously far the nearest stars are is
given in the recommended book “Seeing in the Dark” by Timothy Ferris, ISBN 0-684-86580-7:
If you think of our enormous Sun as a grain of sand then
(a) the orbit of the Earth is 2.5 cm (1 inch) in radius.
(b) our Solar System is the size of a beach ball. (Almost all
our unmanned probes stay within this space).
(c) the nearest star is another grain(s) of sand over 6 km
(4 miles) away. Compare that with the radius of the orbit of the Earth.
(d) even at this very compressed scale, our Milky Way Galaxy
would be 160,000 km (100,000 miles) wide.
(2) The planets and moons of our Solar System are too
inhospitable to life.
Due to these 2 factors alone, alien sizes and alien ships as described by
much of science fiction and told by extra-terrestrial buffs, cannot have come
from outer space to be on Earth.
Our Solar System planets and moons are just too inhospitable to sustain, let
alone evolve such life. This does not mean we will not find microscopic or
even larger life forms in our Solar System, but not intelligent life, let alone
space machines.
The near stars are multiple stars systems or have other characteristics that
would prevent the evolution of intelligent life. This is on top of the
difficulty of travel with the large distances involved.
However there is the possibility that these so called Aliens that supposedly
abduct people originate on Earth — so far there is no evidence of this.
I would say the most likely source of Alien experiences is the brain of the
person claiming this kind of experience. It is much like a religious
experience, where it is very real to the person, possibly more real than the
usual sensual input from the outside world; but it is still a personal brain
experience.
There are also all those exoplanets and exoMoons – very interesting, but (1) above is still a major factor
O. Hooge, B.C., Canada
Naturalism and Religious Experience 