The nearest possibly inhabitable solar system to our own is Proxima Centauri. It's almost 4.3 light years away. Gotta wonder how these aliens pack enough food in these tiny space vehicles.
Maybe they are local.
If they are real, here would be my theory.
Not a belief. But if we knew they were real and were betting who they were, what would I wager on?
Mars had oceans up to about 3.5 billion years ago.
Life appears on Earth about 3.7 billion years ago.
Before life appeared on Earth, Mars was still a warm, wet, habitable planet.
We know ejecta from Mars impacts occasionally land up upon the Earth. It isn't common, but not unheard of. Some of those may have had Martian critters on them.
https://www.space.com/33690-allen-hills-mars-meteorite-alien-life-20-years.htmlWe know that simple life can survive in extended periods in the vacuum of space and be revitalized.
Perhaps all life on Earth was just inoculated from Mars.
Perhaps in the 200 million years after the Earth getting knocked-up from Mars splatter, an advanced Martian civilization flourished. Perhaps they were ocean dwellers. I'd bet life always starts in the water first. I bet most often intelligent life starts there. We may be an outlier.
Perhaps over millions of years they watched as their planet slowly began to die. Not enough iron in their core generate sufficient magnetic field to prevent their atmosphere and oceans getting scoured away by solar wind. Perhaps they advanced to space travel. 200 million years is a long time to develop. Perhaps they had come to Earth and realized that life on there derived from Mars. Maybe that was very habitable for them. Maybe they look kinda like us because we have a shared DNA linage.
Perhaps they moved in and took up shop deep in our fathomless oceans. Perhaps they just keep to them selves as we mostly only barely use the oceans surface and almost nothing below. Maybe our deep ocean was a perfect new home for refugee Martians.
That might explain why they are so anthropomorphic.
That might explain how they can walk around here without space suits. Maybe Martian life was at home here because it isn't too different than Mars environment 3.5 billion years ago.
That might avoid the whole massive galactic distance problem.
That might explain why they seem so tightly associated with the oceans.
That might explain why it took us so long to start bumping into them regularly. We've only been ocean going and especially air traveling species recently in evolutionary terms.
So if they are real and turn out to be non-human origin, that would be my bet in the pool when we try and guess who they are.
Bottom line: They're Martians. But the joke is, so are we.