We have been asking this question since time immemorial, and are still searching the stars for lifeforms beyond our solar system. Earth is a planet that's located in one of the arms of the Milky Way Galaxy, which contains roughly 200 billion stars. The Milky Way is just one galaxy among millions in space. Perhaps we are the only ones living in this small corner of the universe, but it's difficult to believe that there aren't other beings out there somewhere. The galaxy is too big for one species.
Let's approach this question from a biological standpoint. Water and oxygen are abundant on our planet. Those two elements are in just the right quantities and the ambient temperature is at a comfortable level to sustain our existence. The heat and light of the sun is not too harsh and not so insufficient that plants and animals, not to mention all human beings, cannot survive.
There could be another planet far out in the darkness of space that resembles Earth. If there is water, which is a necessary building block for carbon based life forms to thrive then maybe another civilization shares this galaxy with us. However, space is unimaginably large. Contact with aliens is a slim possibility given the size of the cosmos.
Consider the following possibilities. What if they are silicon, or even mechanical life forms? What kind of civilization will they have developed? It will be nothing like ours by any stretch of the imagination. What if life on another planet consists only of simple plants and animals?
Gravitational conditions would undoubtedly affect life on any habitable planet. If an alien society does exist then we can only guess how the evolutionary process has taken place on another world. It's a safe bet that the environmental conditions they are accustomed to will be different from ours. They could be immeasurably stronger than us or exceedingly frail. These conditions would almost certainly affect their ability to travel through space, if they have developed the necessary technology. Maybe they are not sufficiently advanced to do so.
Why haven't we found any evidence of alien life? Is our search futile? We may not find anyone else in this part our universe for a long time. Let's assume for a moment we did make contact with beings that come from another sector of the galaxy. Would communication be possible between us and them? It's a mistake to believe that an alien race would accept, or even understand, the human concepts of cooperation, coexistence and the exchange of information. They might have evolved to a point where they view humans as an inferior life form, in much the same way that we view insects and certain animals here on Earth. We can gladly extend our hand, but maybe extraterrestrials will not be so friendly.