The famous astrophysicist Stephen Hawking believes that the only rational response to "Are we alone" is a resounding "No:"

In a series for the Discovery Channel the renowned astrophysicist said it was "perfectly rational" to assume intelligent life exists elsewhere.

But he warned that aliens might simply raid Earth for resources, then move on.

Hawking feels this way based on our own experience on earth:

"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said.

Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact.

He explained: "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."

In fact, even if the aliens had the best of intentions, it is still likely they would end up destroying us. In his Pulitzer Prize winning book "Guns, Germs, and Steel," Jared Diamond shows how the Europeans use of steel, more deadly germs, and our advantage of firearms devastated the peoples of America. Smallpox may have killed 80% of the Indian population of North America before De Soto's original exploration of the lower Mississippi in the 1530's, barely 40 years after Columbus discovered the mainland. The incredibly complex trading networks at the time doomed Indians from Florida to California. Even before Europeans made their way very far inland, our potent diseases had wiped out a significant portion of the Indian population.

There is a school of thought that even more than for this reason, we should keep a very low profile. In fact, if our galaxy was teeming with intelligent, advanced life, we would probably have picked up some evidence by now via the SETI project. One theory why we haven't is because there are some very bad alien civilizations out there who make a point of attacking and annihilating new contacts.  

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."

All the more reason not to advertise ourselves. It would be infinitely safer to assume that aliens want to destroy us than that they would greet us with open arms. If the latter, it's a mistake that we can't take back if the former turns out to be true.