Planetarium rhymes with:
aquarium, barium, herbarium, honorarium, merriam, aquarium, atrium, auditorium, bacterium, barium, crematorium, delirium, deuterium, disequilibrium, emporium, equilibrium, herbarium, honorarium, merriam, miriam, moratorium, myriam, mysterium, opprobrium, tellurium, thorium, yttrium