Life as we know it requires three ingredients: energy, organic molecules, and liquid water. Our search for life beyond Earth is a search for planets, dwarf planets, and moons that harbor substantial liquid water. We call these places “ocean worlds.” We’re learning that ocean worlds could be ubiquitous in the galaxy. Just in our solar system, we have found evidence of oceans on Saturn’s moons Titan and Enceladus; Jupiter’s moons Europa, Ganymede, and Callisto; Neptune’s moon Triton; and on Pluto.