You say they are assertions
Because they are, and that's all they are.
but can you say in all honesty that they can't possibly be true?
There are absolutely no grounds on which to base the possibility to begin with. It's
possible (and IMV probable, but that's a separate discussion) that there is life on other planets. We understand a great deal about life here on this planet, we can define it, we know of the existence of umpteen other planets, we know how to go about looking for life elsewhere and we would know it if we found it.
Absolutely none of those conditions apply to your string of bald assertions.