3. Intelligent robot jellyfish
You know who loves to ruin your fun day at the beach? Jellyfish, that's who. Now imagine a planet where they're the dominant species, are smart enough to communicate, and will fly through the air.
Once your panic has subsided, deal with the fact that this might be exactly the type of alien race waiting for us out there in the abyss of space. Dr. Maggie Aderin-Pocock, space scientist and nightmare creator, contends that aliens living in a hydrogen-heavy environment would likely evolve to look like jellyfish who communicate via flashes of light and "sucked" energy from their atmosphere. Going along with the silicon-based life theory, she also suggests that they'd have metallic skin, because she hates that you sleep at night.