In Star Trek, they have a planetary classification system and sensors advanced enough to tell them whether or not it is safe very quickly. Any "M Class" planet is completely safe for humans and human-like aliens.
They have also virtually eliminated their own diseases, and star ships are rather sterile environments. The risk of contaminating a planet would be virtually non-existent. There have been a few episodes dealing with such situations, such as "Unnatural Selection", and they make it out to be that this happening is extremely unlikely.
In our real universe, the likelihood that we would ever find a planet like Earth is essentially non-existent. To explore another planetoid we would need full survival gear. Chances are not good that we will ever have manned expeditions beyond our own solar system anyway.
Fantasy and science fiction shows take an element of suspension of disbelief. Earth like worlds and aliens don't exist (especially ones that look very much like humans, even to the point that they can mate with us naturally to produce fertile offspring, which by current scientific standards would make us the same species). If I can pretend that there are thousands of human-like aliens in our galaxy living on worlds nearly identical to Earth, then I can also pretend that our sensors are sophisticated enough to quickly determine that these planets are safe to explore unaided.
They have also virtually eliminated their own diseases, and star ships are rather sterile environments. The risk of contaminating a planet would be virtually non-existent. There have been a few episodes dealing with such situations, such as "Unnatural Selection", and they make it out to be that this happening is extremely unlikely.
In our real universe, the likelihood that we would ever find a planet like Earth is essentially non-existent. To explore another planetoid we would need full survival gear. Chances are not good that we will ever have manned expeditions beyond our own solar system anyway.
Fantasy and science fiction shows take an element of suspension of disbelief. Earth like worlds and aliens don't exist (especially ones that look very much like humans, even to the point that they can mate with us naturally to produce fertile offspring, which by current scientific standards would make us the same species). If I can pretend that there are thousands of human-like aliens in our galaxy living on worlds nearly identical to Earth, then I can also pretend that our sensors are sophisticated enough to quickly determine that these planets are safe to explore unaided.
(But given whom I believe Archer was partially modeled on, it shouldn't have come as any surprise, really. I won't get into specifics, because this isn't the place for an argument about it, save to say that it was someone political and relevant to the time the show was on. 
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