I can't understand what one is supposed to write in the "statement of research interests" (1 or 2 pages), which is requested in all the phd applications.
If the topic is completely new and unknown, what can I write? (besides "i'm interested in studying this subject")
Should I go through a lot of pubblications of the research group, understand the topic, and propose a sort of research plan? this could take months..

Yes it could take months but my opinion is the purpose for this is 1) to gage how much you know about the field and 2) what you are interested in looking at. I dont imagine they expect fireworks but enough to warrant an interview.

Hope this helps

B

I think that an important thing is to outline why the topic you intend to explore is relevant and important for your discipline i.e. how does it fill a "gap" in knowledge or how you intend to give a new perspective to an already explored area.

And no topic is isolated - it would be a good thing to show your knowledge of the existing bibliography in related areas, at least to demonstrate that you are proposing something new which hasn't been covered before.