I agree with others who point out that there isn't that much similarity between "Hilfinger" and "Helmin". They both start with "h" and include an "n", that's about it. I don't think they're likely to be confused.
But to your basic question, if you had a character named "Hilfinger" and a planet named "Halfamger", I could see that being confusing to the reader.
Of course there are many context where it would be obvious which you were talking about. "_____ is wearing a blue shirt today" presumably refers to the person. "_____ has a large ice cap near its north pole" probably refers to the planet. Etc.
But even sentences that might seem obvious to you when you're writing might confuse a reader. "_____ loves Sally." My first thought would be that that means the person loves Sally. But maybe the planet loves Sally, in the sense that you might say, "America fell in love with the Beatles."
Even if all context are crystal clear, a reader might still find himself routinely confused. "Wait, was Hilfinger the name of the person or the planet? Oh yeah ..."
Of course it may be that you WANT to create an association between a person and an object. You may want to name a character "Robert Stone" because you think the name "stone" creates an image of reliability and ruggedness. Maybe if literal stones play an important part in the story that would cause confusion, but generally not. The difference there would usually be obvious from context. (Like when used as a name it will be capitalized, but presumably not when used to refer to a rock. But what if it happens to be the first word in a sentence?)
All of which leads me to say: Make all names, whether people, places, ships, animals, whatever, as distinct-sounding as possible, unless you want to create an association between them. Of course I wouldn't get obsessive about it, no need to create formulas to measure how different-sounding two words are.