Scientists said Thursday that extraterrestrial environmentalists could become so angry about what we have done to the planet that they may see us a threat to the intergalactic ecosystem -- and take action by attacking Earth.

The scenario was devised in a joint study by Penn State and the NASA Planetary Science Division. The title? Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis.

The analysis divided projected close encounters into the categories neutral, ones that cause mankind intentional harm, and one in which aliens do us intentional harm.

The study warned that extraterrestrial intelligence could attack and kill us, enslave us, or potentially even eat us. ETI could attack us out of selfishness or out of a more altruistic desire to protect the galaxy from us. We might be a threat to the galaxy just as we are a threat to our home planet. 

Humanity may just now be entering the period in which its rapid civilizational expansion could be detected by an ETI because our expansion is changing the composition of Earth's atmosphere (e.g. via greenhouse gas emissions), which therefore changes the spectral signature of Earth, the study added.

According to UFO Digest, extraterrestrial aircrafts may be predatory or hostile and might be testing our readiness now. 

Should a hostile group choose open large-scale invasion, we wouldn't see just a few over a city exercising military operations, I believe we would see not just thousands, but tens of thousands of craft landing simultaneously worldwide in order to drop large numbers of their own troops on our soil for military purposes, UFO Digest reported.

On the bright side, the study also said that finding extraterrestrials -- or being found by extraterrestrials -- could help our civilization avoid its likely demise.