What Came from the Stars by Gary D. Schmidt
The Valorim are about to fall to a dark lord when they send a necklace containing their planet across the cosmos, hurtling past a trillion starsall the way into the lunchbox of Tommy Pepper, sixth grader, of Plymouth, Mass. Mourning his late mother, Tommy doesn't notice much about the chain he found, but soon he is drawing the twin suns and humming the music of a hanorah. As Tommy absorbs the art and language of the Valorim, their enemies target him. When a creature begins ransacking Plymouth in search of the chain, Tommy learns he must protect his family from villains far worse than he's ever imagined.