颗 as the special literary way to count hearts. XD
I like it ;) The list says 心 rather than 心脏, so maybe biological hearts are counted differently? (Also the "kernel" translation makes me think of the Young Wizards books, did you ever read those? I wonder if there's a Chinese version.)

and thanks for looking up 恒星 and 星球！ I guess celestial objects are always small no matter how you study them.

(I know 粒 from Japanese, where it's used like Mandarin to mean actual grains, but can occasionally be used poetically for things like pearls...)