Zita the Spacegirl: Book 1: Far from Home
Reading Level: Grades 3-5
If you found a hole out in the grass while exploring, and inside the hole was a meteoroid, and poking out of the meteoroid was a button, you'd probably push it, wouldn't you? (I would. Zita did.)
You'd probably freak out when your best friend Joseph got sucked into into this weird vortex thing after you pushed the botton... and then when you pushed the button again, YOU got sucked in, plopping out onto an alien planet. (I would. Zita kind of did.)
Zita's best friend Joseph has been kidnapped by a cult of aliens who believe the world is about to end... and the button that brought both her and Joseph to this alien world is crushed accidentally by a huge alien named Strong-Strong.
So now Zita's totally stuck on an alien planet that apparently is about to come to an end, her best friend has been kidnapped, AND she has no way to get home.