The planets have to be in order, and your child asks what is past the last one and has opinions about Pluto. What this shelf is really sorted by is what your child can DO with the book in their hands. That spans a two-year-old opening a fold-out on your lap and an eleven-year-old reading three hundred pages of physics alone, which is a huge distance, and it's the whole reason this page exists. If you're buying one space book for a nine-year-old, the picture book at the top is not it, and I'll tell you which one is.
Sorted by what your child can do with it, not by subject, because on a space shelf the subject is identical and the object in their hands is not. At the youngest end, Papa, Please Get the Moon for Me is a lap book. It's Eric Carle, it's a father fetching the moon for his daughter, and the pages fold out so your child works the size with their arms. There is not one fact in it, and it teaches the moon's phases anyway. A Hundred Billion Trillion Stars is the other picture book, and it's about scale rather than the solar system, which is why it survives a house that already has planet books. There's No Place Like Space is the Cat in the Hat covering the actual solar system in rhyme, and it's the one that works read aloud at four and read alone at seven. National Geographic Little Kids First Big Book of Space is the first fat reference book, built to dip into a spread at a time. Then the shape changes. The Magic School Bus is the only fact book here with a plot, and it's the one a child will read who won't read a reference book. Professor Astro Cat's Frontiers of Space is where a child goes when planet facts have run out and they want black holes and the Big Bang, written by an actual physicist behind the cartoon cat. Planetarium is a museum printed as a book, 112 oversized pages of engraved galaxies, and it asks a fluent reader for real attention with no jokes to help. George's Secret Key to the Universe is the outlier and gets its own paragraph below. The rough rule: under 5, the two picture books. 5 to 8, the Cat in the Hat, the National Geographic and the bus. 8 and up, Astro Cat, Planetarium, George.