I'm afraid the odds are heavily stacked against your alien ships. They would have to have travelled here at way below the speed of light, taking centuries to get here. That would take technology and resourcefulness that we can only dream about. Well, unless you think they're from Mars. We also, at least for now, have to assume that they must obey the laws of physics. None of this is impossible, of course, but it's hard to see how they could have done it, let alone bothered to do it (a not unimportant consideration). Once again, there's a good deal of evidence that has not been corroborated at best or which has been trashed at worst, and nothing really concrete at all. Like me and my ball lightning you have to work very hard to find alternative and more plausible explanations, which is the best you can do, and only then adopt the Holmesian philosophy regarding the impossible and the improbable. What a pity religion doesn't follow the same path.