Author:LunineAbstract:Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water—“oceans”. Of these, the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects—Jupiter's moon Europa, and Saturn's moons Enceladus and Titan—have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa's is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5–40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan's ocean is the deepest—perhaps 50–100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration.