Thousand of students still want to know, how to find a profitable niche. See, If you still want to operate in a niche you know nothing about, I will suggest you don’t focus on that. Go to the Google Keyword Planner and select “Search for keyword and ad group ideas.” Next, input a very broad seed niche and click “Get Ideas.” Then select “Keyword Ideas” and not “Ad group ideas.” The more closely related to your own passions, interests, personal experiences, and/or problems… the more beneficial.
A good place to start would be niches that involve money (finance, medical, insurance..etc) and niches that are done mostly online (web hosting, web design, seo..etc).. These both tend to have lots of searches and high payouts because of stiff competition.
Yeah Keyword Planner Tool is good for getting ideas/inspiration from but it's not anywhere near accurate. Best to check the trends for the keywords you're targeting with Google Trends Tool.
In my opinion, the profitability of a niche will depends on how it will be market. I believe that it is not how marketable a niche is, but also how it will delivered to the market. If I were to choose a niche, I would choose something that interests me the most. Something that I can work on effectively.