I need to emphasize on the positive aspect of the fact that someone has already worked on the topic I am including in a paper. So, I want to have a connecting sentence such as "Fortunately, the topic has already been explored...", however I know that words such as "fortunately" are not perceived well in academic writing. Any other suggestions for words or phrases that keep the positivity without sounding unprofessional?
I think this is an excellent example where it is short-sighted to condemn a word in general, and instead one has to think about why the word is problematic and whether that actually applies in the given context.
I think your supervisor’s concern stems from the fact that it is good practice to take a neutral stance on scientific results to avoid exhibiting a bias or inducing it. For example, I would indeed consider it unprofessional to write:
Fortunately, the solution of this equation is 42.
However, this is not what you are doing. Your fortunately is about scientific progress, not about the outcome of your study. If you so wish, science is inherently biased towards this anyway. If it is about the circumstances that allow you to further human knowledge¹, I see no issue with using fortunately, e.g., when writing something like:
Fortunately, we already know the solution to this equation since it has already been intensively studied on account of its relevance to the field of underwater basket weaving.
Mind that you still need to judge whether this word really improves your communication. It may still be that your specific fortunately is superfluous, since the fortunate nature of what you are describing is apparent and it is not needed for the text flow either.
Finally, since you were asking for alternatives: If somebody considers expressing your opinion on how research fits together problematic, a synonym won’t solve this.
¹ There are some rather obvious limitations to this, e.g., you wouldn’t want to find it fortunate that we know something on account of blatantly unethical experiments conducted in a darker age.
I don't think your supervisor's feedback was to indicate the specific word "fortunately" is a problem but rather that you should not express "positive aspect of the situation" or other emotional or value judgments. If the sentiment is fine to express, then "fortunately" is a perfectly reasonable word to use.
If you want a synonym, you can use a thesaurus, but I don't think that will actually be a solution here that will appease your supervisor and there is no magic list of professional and not-professional words that mean the same thing.
I don't think you need anything there. The same just without "fortunately" explains perfectly well that you don't have to do this as others have done it before. I don't buy that "I need to emphasize that it is very fortunate...".
While I wouldn't say that "fortunately" is unprofessional or really problematic (and I wouldn't complain about it too loudly), I don't like it, because I as a reader of a scientific text am interested in the facts and not in whether XXX makes you happy or not. Using it means that you bother me with something that I (in this kind of text) don't want to be bothered with.
The simple alternative is not to add any personal judgement:
"The topic has already been explored..."
Adding "Fortunately" is just a filler: Either it is already clear why you find this fortunate, in which case the statement is not needed, or it is not clear, in which case just stating so does not help. Referring to another work already has a positive connotation by default, as otherwise you would be expected to discuss any conflicts or contradictions to your own work.
If there is a specific reason why you think the existing work is positive (or negative!) for your own, then prefer to name that reason:
"We do not provide a thorough proof in this paper since the topic has already been explored..."
"The topic is of general importance for our domain, as has already been explored..."
This expresses the positive aspect in a factual way and helps the reader gauge how the referenced work ties into your own.
“Happily” is one that I use (but I also don’t really see an issue with “fortunately”).
On the topic of whether these terms are appropriate for academic writing: If you take the point of view that papers are communicating "facts", then avoiding terms that impart subjective judgement can be a reasonable view to take.
Papers are not always just listings of facts, however: the papers I find most useful are about the relationship and interplay between ideas and bodies of thought. In this case, "fortunately" (or "happily" in its "by happenstance" meaning) both convey the idea that there is no default expectation that someone not working on the writer's specific problem would have considered the subtopic at hand, but that it turns out people working on another topic do care about it and have explored its nuances.
By highlighting the idea that "this next step should be hard, but is unexpectedly easy", words like "fortunately" provide context and can help the author shape the reader's understanding of the concepts and how they fit together.
I don't think it is the word, but just how the word is used. As in "Luckily, the opposing player missed the goal" which literally says the opposing play is lucky when the intent is that the person speaking is lucky. You need to say who is fortunate, or thankful, or whatever.
Using informal grammar makes your work much harder to read by those for whom English is not a primary language.
Of course, your advisor might have meant something else.