A quartet of curious babies wearing glasses and spacesuits explores the moon, the sun, and the planets. In speech bubbles, various babies ask questions ("Why are we floating?") or make comments ("It's so quiet"), leading to answers or information in the narrative text about gravity on Earth and sound in space. Using bright, bold colors and simple lines and dots, Kastner gives each planet personality and facial features. Mercury appears in sunglasses because it is "closest to the sun." (Caregivers will note that thanks to artistic license, Venus is depicted as magenta rather than dun.) The order of presentation is confusing, especially given its very young intended audience. One early spread shows the sun, "the center of our solar system," yet a schematic spread showing the sun surrounded by the planets occurs a few pages after information is given about Earth and its moon. Although the descriptions of the planets are simple—"Mercury is the smallest planet and closest to the sun. Venus spins backward!"—the topic is not one that is within babies' developmental grasp. The same babies (one black, one brown, and two pale) are back in companion title Ocean, exploring marine life at the surface and in the depths.The book is published in two bindings, conventional and board; given the complexity of the topic, use with preschoolers is recommended despite the "Baby" branding. (Informational picture book. 3-6)