This is a long-lost treasure for homeschooled students! "My favorite book as a kid," says one Amazon reviewer. Another says, "I remember reading this fascinating book, a selection of Scholastic Book Services, when I was hospitalized at age 9. I still remember the nurse coming into the room way after midnight to demand that I turn off the light. I couldn't do it. Planet X had me."
"Planet X" isn't science fiction, but true science writing for kids (roughly upper elementary through middle school). It's about, first, the search for Neptune; because something was throwing the orbit of Uranus out of whack, and the odds were that the something was a planet. But after confirming Neptune's existence, astronomers thought there must be something more dragging it out of its normal pattern. And after years of searching...there was our ninth planet.
Or our ninth planet-that-was.
But I think its primary appeal would be to children who can still appreciate the romance of star-searching. Yes, "Planet X" has been demoted to a dwarf planet; but that doesn't diminish the story of its discovery. (And the Neptune story is interesting too.)
This would be a great addition to a study of astronomy. The book is long out of print, but inexpensive copies are still available through online sources.
*The scientists in my family want me to mention that Pluto is no longer "Planet X," first of all because it has a name, but second because it lost its planet status. So the search for "Planet X" goes on.