What gets me a little angry is how many scientists use the basis for comfort relating to alien species is what is good for humans and Earth biota. Oxygen and nitrogen in the atmosphere, about one AU away from the star, and liquid water. Remember War of the Worlds, and how those aliens died from our diseases? It seems to me that nobody ever thinks that what aliens might find on this planet may kill them.
There are aquatic organisms that live on the bottom of the ocean, where arsenic is found and guess what... they’re still alive. For all we know, aliens could live on a planet like that, and then breathing an oxygen atmosphere could subsequently kill them; not because arsenic isn’t present necessarily, but maybe because they’d have an allergic reaction to any elements in our atmosphere.
But almost everything is indeed possible. Just because we’re not yet capable of creating a Dyson sphere or a space elevator, doesn’t mean that other species haven’t done it. There’s also the issue of routine space travel and even space warfare. Just because everything before being sent into space is static clean, doesn’t mean it has to be for anything else. I frankly think the effort into cleanliness is a little much.
That’s why the ETs are always wearing similar-looking encounter suits when they visit our surface.