////I've often wondered if we're more 'tied' to this planet than we imagine. Is it possible that humans could never thrive away from the planet Earth or that aliens could never thrive away from their homeworlds?///

In order for humanoids to form on another world requires not only an earth-like planet--which isn't enough by itself--it requires a virtual exact duplicate. Think of all the things that happened on this planet to make human life possible. If even one of them didn't happen or had occurred differently, we wouldn't be here.

This planet would have to have our gravitation for which we are specifically evolved (look what happens to the bones of astronauts who spend too much time in weightless conditions). They have to have our sun and be the same distance from it. Their planet has to be 3/4 saltwater, it must have similar landmasses and polar caps in order to regulate its temperatures and climates as does our oceanic conveyor belt--just the destruction of the Greenland ice cap is threatening the belt which will pretty much destroy us if it ever stops.

Then there's marine volcanic activity which is vital to our planet. In spite of all the trouble our underseas volcanic and tectonic activity causes, be grateful we have it because our planet is a dead one without it.

Then there's stuff like asteroids and comets hitting earth that made human life possible. This other planet would need the same type cataclysms of the same size causing the same effects. Too much or too little would eliminate our chances of our ever appearing on earth.

Then there's the moon. We always talk about how important the sun is to life but the moon is just as important. It determines the 23.5 degree tilt that our planet must have in order for us to live and which this other hypothetical planet would have to have. That means that planet must also have our moon. Here's the problem: our moon appears to be a complete fluke. It is 1/4 the size of earth--no other planet we have ever observed has a moon that large in relation to its own size. Jupiter's largest moon is like 1/80th of that planet's size. Mars's two moons are dinky compared to Mars. Moreover, our tilt is constant because our moon's orbit is almost perfectly circular--something not observed in any other moon which have very elliptical orbits. If our moon's orbit was elliptical our planet's tilt would be constantly changing due to the moon's gravitational pull which would make human life impossible. Then there's the issue of the tides. This other planet has to have our tides which we are evolved for.

This hypothetical earth-like planet would have to have an ozone layer which is essential to our continued survival. It would have to have our flora and fauna. It would also need our microbes (just one example is the bacteria that live in our gut without which we cannot digest out food).

These are just a tiny smidgin of just how earth-like this other planet would have to be in order for human life to have any chance of appearing there. THEN, you're talking about this planet having formed at the same time and general locale as ours so that these denizens could visit our own when, in fact, even two such planets forming anywhere in the known universe (of which we have cataloged more than 80 billion galaxies) at any time would be wildly coincidental to put it very mildly.

Yes, we are obviously tied to this planet. For a scientist to conjecture that humanoid life is visiting this planet is basically thumbing his nose at evolution. We are specifically evolved for this planet's circumstances. It could hardly be any other way. Those who think we are being visited by humanoids have simply grown up watching too much Star Trek and too much Lost In Space.