LEGO-compatible ring gear with 56 teeth.
Possible combinations are:
1x8 teeth, 3x24 teeth
1x16 teeth, 3x20 teeth
1x24teeth, 3x16 teeth

My first design included 40teeth sun and 8teeth planets but it didn´t turn well.

To hold the gears together I used some LEGO-rods as shown in the pictures.

Edit Nov. 2022:
I have added a version with teeth on the outside.