Third grade?!? Sheesh! I sympathize, actually. As a librarian, I've seen parents trying to find age-appropriate information for kindergarteners on things like pineapples. I'd advise (surprise, here) going to the library and getting a kid-level book on Saturn. I don't know what country you're in, but if it's the USA, Seymour Simon's book would be my first choice, even though it's about fifth grade reading level. Trouble is, if scientists don't know something like this, the author/s of this sort of book usually don't say that we don't know yet. If the information isn't in a more or less age appropriate book, then let the teacher know, either in a note or in the child's acutal report.
After all, isn't the reason kids get report assignments, besides finding out about Saturn, to teach them how to find out stuff? (Call it developing research skills, if you have to be formal about it) You're a good, loving parent, but the child should do the work . After all, you went to third grade a while ago, it's the kid's turn now.
I don't mean to grump at *You*, but I've seen too many report assignments where the teachers didn't find out if the kids could actually find what they needed for the report. And I've seen too many moms who got into the habit of finding the stuff for the kid - not just picking up a book or two, but doing the research, and sometimes the actual writing - and this habit continued into high school, and even into college, for some. The kids didn't learn anything, except perhaps how to have an executive secretary. I know you're too smart to end up with That fate, Nancy! (Thanks for letting me erupt.)