Hawking said of a potentially habitable alien world, Gliese 832c:
One day, we might receive a signal from a planet like this, but we should be wary of answering back.
Meeting an advanced civilization could be like Native Americans encountering Columbus.
That didn't turn out so well.
It's worth pointing out that the Earth is pretty noisy – we've been sending out radio and TV signals into space for hundreds of years now.
Some astronomers believe that any alien civilisation advanced enough to travel through space fast enough to invade earth with a occupying force would have noticed us by now.
There's also the fermi paradox, which suggests there's nothing out there – that we're alone.
Or there's the petri dish or conservation theory – that Earth has been left to its own devices by more advanced life until we reach a certain level of advancement or development, that our planet is essentially a wildlife reserve.
In which case said aliens can't be happy with our global warming efforts and are probably thinking about starting all over again.