The decision to visit planets other than Earth is not supported by everyone. What would be a good argument against sending humans to other planets?

Sending humans to other planets may cause danger. We dont fully know what these planets are capable of doing. If there is such a thing as life on other planets then we still dont know weather or not life there is friendly or not. They could be harmful, because just as we dont know know exactly who or what they are, they dont know who or what they are either, so its a high chance they may want to defend themselves.