Advices for a research interests letter for an astronomy PhD

Hi

I have to apply for an astronomy PhD student position in a foreign coutry.
For the application, I have to provide, among the other things, a letter of "Research Interests".
This is the first time that I heard about it, and I don't know how to start.
I read other research interests letters on the web, but they are always from post-docs, or grown scientists, so their letter is suitable to apply for a "free" fellowship, and they promote their skills.

Instead, the PhD position is different from my work before, as my master thesis, workshops, or internships I have done...

Well, here I would like to know HOW to formulate this letter: how to start, what I have to write, how much, if it's better to subdivide the letter in sections or not, and other stuff like this.

Please, I don't really know how to plan it.

Thank you in advance :)

judging by the title 'research interest', I believe it is similar to Statement of Purpose, which is basically an essay saying how you have come to develop your interest in any particular areas which you wish to conduct your PhD research in, and what you wishto achieve at an academic or career level. Astronomy is something I am not too sure, but I think you should not look at the papers that you stated (from post-docs, scientists etc), but rather focus on yourself, and perhaps have a look at the SOPs posted on this website. It is about yourself, not other people to compare to. At the end of the day, if your interest does not match the university you are applying, you will have a difficult time studying there even if you are admitted.