It's likely a lenticular cloud...nothing more.
Face it, if there were alien travelers, earth would be the LAST place they'd want to visit!!
What would we appear to them? Like babling infants? Pets? Two legged animals unable to think telepathically or understand their language? If they were intelligent enough to build a vessle to travel through space to here, we'd likely be so primitive we'd seem more removed than chimps to them. (maybe...who knows...) Maybe we're just a forgotten experiment from ages ago.
