Dd wants to do an astronomy unit but not having luck finding lesson plans. She has a refraction telescope and I can get into the observatory pretty easily. Just not finding any good project type things for her age 11 on about 7 - 8th grade level.
I would look a the Boy SCout Merit badge book for astronomy. They have a series of requirements that must be completed to earn the badge and I use those for lesson plans. They are $5 and change and really great!!
My boys actually do the merit badges but I often have DD do them for science, social studies, etc.
We are doing the elementary apologia this year on astronomy (it's a full year curriculum). You could build your own with library videos and books. Bill Nye has one on the sun and one on space or the solar system. There's a series called What's Up in Space-see if your library has it. They should have many books on planets and space as well. DS1 is very in to space right now and is getting a telescope for Christmas. I also got a couple astronomy books on Amazon-the everything guide to space for kids or something like that. I also found some star guides. NASA has a lot of info and pictures on their site.
How about going to Hobby Lobby (if you have it) and making your own solar system? I did that in 7th grade as a science project. There's paint as well to paint the planets before you attach them to the little wires.
What we have have been doing is ASTRONOMY ACADEMY My kids love it. There are two different units, one for younger and one for older kids. We are also doing lapbooks with it, but mine are a little younger, so yours may not want to. But if so, we get all of our lap books from HOMESCHOOLSHARE.COM