This site is for “science fiction enthusiasts”, not for science experts. So you can't expect answers about science to be reviewed by people who could spot mistakes.
That being said, some scientific background is necessary to understand the fine points of hard science fiction works. So it's inevitable that there will be discussions about science.
I'm going to tentatively lay out some guidelines on how much scientific discussion is on-topic. I expect these guidelines to be heavily revised from other people's input and experience from questions on the topic.
- As per the site's topic, science questions should be related to science fiction. Questions that are purely about science as we know it are off-topic.
- Questions about how the science of a particular work relates to science as we know it are ok.
- It is understood that answers may not represent the upmost in scientific expertise.
How to best defend the Earth given futuristic technology. (Must fit into current laws of physics)
For a specific futuristic technology, maybe. It's a bit open-ended, but it might fit the six guidelines for great subjective questions (as long as it's understood that since this isn't a site for science experts, answers are not expected to be fully-researched science and are likely to contain speculative inferences, hence are subjective).
What is the most efficient way to get to another star?
As asked, too broad and imprecise. If you give some context, it can be ok. For example, if you give the technological, political and economic context of a book you're writing, and ask how to justify an interstellar journey on that basis, I think it could be ok. (Though some of the planned Stack Exchange sites on popular science would be a better fit.)