Stephen Hawking thinks that it's very likely alien life exists. He thinks it's a bad idea for us to try to contact them. He thinks that aliens visiting us, as our civilization stands right now, would be similar to when Columbus arrived in America.
Fear not, however, the Center for SETI (search for extraterrestrial lab) has technology in place that let's them detect signals from up to 1000 light-years away. Given how long we've been emitting broadcasts into space, it's not very likely that we have been on someone's radar long enough for someone to pay us a visit.