> The assumption problem can go both ways though, if one can not provide evidence to disprove an extraordinary claim.

So far there is an awful lot of evidence of no extra-terrestrial life on earth: every single microbe shares the same basic cellular building blocks, and often whole chunks of dna/rna. Lots of cameras, radar, satellites and other sensors everywhere that keep not detecting aliens day in day out. We are incredibly certain that space travel is very difficult and will take a very long time (relativity is an extremely well-tested theory, so we're very sure that nothing can travel faster than light). We are actively looking for it and have not seen a single sign of life outside of our atmosphere. So the claim that there is no extra-terrestrial life on Earth has a lot of evidence to support it.

The claim that extra-terrestrial life has visited Earth goes against much of our scientific understanding, it would require a bunch of well-tested theories in physics to be outright wrong. So it is this claim (and not the opposite) which is extraordinary.

> I think such discussion invites at least accepting the possibility of the extraordinary

As far as I know most scientists accept the possibility of alien life, and more often than not are actually hoping that this is the case. But to be convinced that this is the case, rather than it being an unlikely or hypothetical scenario, would take a lot more evidence than a few hundred (currently) unexplained phenomena. For example, some samples of alien tech or biology.