Snakecharma,
Agreed, the chances of earth being the only planet in the universe to harbour life....is pretty remote.
However, the chances of other life forms reaching our planet and flying though earths atmosphere....are equally remote.
It doesn't matter which planet you are from, the laws of physics remain.
The distances involved with inter solar system/galactic travel are the biggest hurdle. Speeds of at or above the speed of light would be required to cover the vast distances. The problem is that the energy required to propel an object at or even near the speed of light is astronomical in itself.
The mass of an object increases arbitrarily as it approaches the speed of light and an infinite amount of energy is actually required to reach or surpass that speed.