So, Nintendo has updated the Star Fox:Zero japanese website with all the planets in this game. There are a total of 11 planets, named like this:
Some of these planets might be familiar to you, since they appeared in previous Star Fox titles. If we compare the number of planets to the N64 Star Fox, which seems logical since they're both similar, the N64 game featured 15 planets. Also, Star Fox: Zero features 1 level more than the previous SF title (Assault). We don't really know the length of these levels, so the length of the game could still be acceptable.
Thoughts?
Sources: https://www.nintendo.co.jp/wiiu/afxj/stage/index.html
http://nintendoeverything.com/star-fox-zero-update-planets-and-routes/