The UFOs as aliens hypothesis is only as believable as the most a priori believable for how it could be true. When I tried to find a story like that, I ended up relying heavily on the idea of panspermia siblings. And now that I’ve given that idea a bit more thought, I’ve realized that it is somewhat harder to arrange than I’d realized, and thus somewhat less believable. Making UFOs as aliens less likely, though still possible.
The scenario, if you recall, is that there are aliens visiting Earth today who have not expanded much to colonize and remake the universe, aliens who were born at a planet around a star that is a sibling to our sun. That is, this alien’s star was born in the same stellar nursery as our sun. This scenario requires three key elements:
Old Non-Expansionist Aliens – A substantial fraction of advanced civilizations choose not to expand and visibly remake the universe, but do choose to go visit their sibling stars that develop advanced life, and these civilizations last for longer than the typical differences between when advanced life would appear when grown from simpler life at the same level four billion years before. Thus a substantial fraction of alien civilizations must last for several hundred million years. (Oh and they choose do all these apparently-useless glow-buzzings of our treetops.)
Easy Earth Filter – In order for there to be at least two advanced civilizations both born from the same stellar nursery, it can’t be too hard to evolve advanced life from the sort of life that Earth starts with. The time of the origin of life on Earth and the time now remaining suggest 3-9 hard steps happened on Earth, if this whole time was take up by hard try-try steps. So we need some combination of a large nursery, fewer such hard steps, much of Earth history being taken up with delay steps instead of hard try-try steps, and the “hard” try-try steps being that hard. So, for example, in a nursery of ten thousand stars, there might be just three try-try steps each only a factor of ten hard, and perhaps half of Earth history was taken up with delay steps.
Panspermia or Huge Try-Once Step – In order for life to spread across a large fraction of a stellar nursery, that life would have to appear within roughly a hundred million years after that nursery formed. So either life appeared from nothing very fast, mainly via some very hard try-once steps, or our nursery was seeded by life from an Eden at some other passing star, either just as our nursery was forming, or via a prior seeding of the molecular cloud which collapsed to form our nursery. (Which requires life to survive a long time in a molecular cloud.) On average stars pass within 5 parsecs of such clouds 50-100Myr.
While this prior Eden would have had a similar number of hard steps as Earth, those steps would on average be much harder, so that most of the total great filter would have happened at Eden. Very hard steps might include the very first life, and the transfer from Eden to a stellar nursery.
A 2012 in Astrobiology works out details of this scenario for life moving between star systems in a stellar nursery, where many stars are crammed together and many rocks are flying between them.
We don’t know when life first appear on Earth, but current best guess is 400Myr, with a range 200-800Myr, after the Earth and Sun formed together. They were formed together with ~1K-10K other stars, all packed close together.
Earth had water to support life within ~160–290 Myr, while our cluster took ~135–535 Myr for sibling stars to drift away from each other (the largest value is for the largest star clusters). During this early period there were a lot of rocks smacking into Earth kicking up a lot more rocks. Maybe the top kilometer of rock across Earth was kicked up.
About ~1% of these rocks were ejected from Earth with a weak enough impact shock to let life survive, and rocks of >10 kg seem like they could protect life from radiation and impact over the 3-5 million years it would take to drift to the closest star system in this cluster during this period. Some kinds of life could last that long.
About 2 * 10^11 such rocks would escape our solar system at a slow enough velocity to be captured by a neighboring star. Given such assumptions, if the nearest star were also Sun-like, then the number of such rocks ejected from Earth in this period that would land on an Earth-like planet around that nearest star is about 3*10^4. If that star had half the sun’s mass, this number falls to just 10^4.
Thus if our Sun’s stellar nursery were big enough, and if life appeared early enough in this cluster, then life might have spread to many stars in this cluster. And thus aliens could have evolved before us at one of those stars, and then came here to be the UFOs we see. But this is a lot of ifs, and so the a priori unlikeliness of this scenario has to be against the a priori unlikeliness of: secret Earth orgs with really advanced tech, a vast conspiracy to create the false appearance of UFO encounters, or mass delusions widespread enough to create the same.