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UFO enthusiasts should read speculative Sci Fi like Orion's Arm or Project Rho.

Like... if aliens exist, and they have the capability to travel across galaxies to get here, they aren't octopuses in flying saucers that we can detect. They're basically omniscient and invisible. Their ships would look more like this:

https://www.orionsarm.com/eg-article/480a48b000f5b

It can redirect the path of a snowflake or rip a moon to fragments. It can accelerate a pebble into a habitat-shattering kinetic bomb or decelerate a RKKS projectile to harmlessness. It can reach down to a planetary surface from orbit and lift a sophont into space or shatter the crust of a world.

And that's a ship that wants to be seen, restricted to plausible physics humans can imagine, from a 'primitive' civilization with a mere 14,000 lightyears of expansion. That's nothing on an astronomical timescale.

Point being it's ridiculous to think we'd detect atmospheric UFOs, or that they'd even need atmospheric UFOs. It's anthropic fallacy. If they want to stay hidden (and astrophysical objects like re-arranged stars don't give them away), they can stay hidden.