I hate to disagree with someone as smarter than me as Stephen Hawking, but I just cannot share his fear of alien visitors:
He suggests that aliens might simply raid Earth for its resources and then move on: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.”
He concludes that trying to make contact with alien races is “a little too risky”. He said: “If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.”
If the aliens have the technology for interstellar travel, I doubt there is much point for them in raiding our resources. The resources you can find on Earth you can find in by mining asteroids as well, which is probably easier than dealing with the gravity well of Earth. Plus, if they have the resources to travel between stars, how much need will they have for the resources of Earth anyway?
They might decide to exterminate us just for the heck of it, but raiding our resources I'm just not buying.
As for communicating with aliens in the first place, I am not too optimistic. I'm sure they are out there, mainly as simple life, probably, but also as highly technically evolved civilizations. I just find it highly unlikely that, in the vastness of the Universe, life only evolved on Earth. But what's the chance of finding someone on roughly our level of intelligence? If they are much dumber, we won't be able to communicate with them in the first place, and if they are much smarter than us, why would they want to communicate with us?
I hope they will. There's a lot we can learn from aliens much smarter than we are, but I'm not too optimistic...