Everyone sometimes wonders about aliens, and are we alone in the universe. Set's say we are alone. For now of course, because there might be planets similar to Earth that have just begun developing life ( but that is a discussion for another time). If we are the only sentient beings in the universe we have to start acting like it, take some responsibilities. we are the only ones that are aware of space and all it's wonders. but we are polluting it. I'm not saying that we shouldn't explore space on the contrary, I'm thrilled that we are even trying to understand space and how it works. but we have to be cautious. the junk we leave will be there forever ( or at least until that hypothetical species  on that hypothetical other Earth-like planet doesn't develop the technology to come to our system).