Source: http://www.npr.org/blogs/13.7/2015/02/11...

There are four conditions that need to be satisfied for us to start fearing our distant alien neighbors. The aliens must: 1. Have developed technologies that allow them to travel or send targeted probes across interstellar distances; 2. Have spotted us as a desirable destination; 3. Have intent to explore space around them (they may not share the same kind of wanderlust); 4. Have evil intents.

The search for extraterrestrial intelligence is a must. We will only find it if we look. However, given the arguments above, we should be very confident that, even if they do exist — and that's a huge if — they are very remote. From what we know now of space travel and all its difficulties — fuel, speed, radiation shielding — had the aliens the technologies to overcome all this, they would probably also have technologies that would have spotted us a long time ago, for example, by analyzing our atmospheric composition or, if closer than around 50 light years, by detecting our radio and TV broadcastings.

We are safe, at least from this kind of menace. Best we concentrate on other, more pressing ones.