“Lights in the Sky and Little Green Men” by Hugh Ross, Kenneth Samples, and Mark Clark is a good primer for the statistical probability. Silly title but a good laymen read for basics in how the physics, chemistry, and biology play a huge role into the statistics of extraterrestrial aliens flying a space ship across the galaxy.

Now, if you want to talk about portals and extra-dimensional beings, that’s a whole different can of worms.