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The term water world (or water planet or ocean planet or ocean world) is used for planets with substantial water, a characteristic that generates much interest because liquid water is a requirement for the kind of life we know about. The term water world or water planet is sometimes suggested as a possibility if water vapor is detected in an extra-solar planet's atmosphere, suggesting there is a possibility of liquid water on the surface. Any of the four terms might be used if the planet is thought to have "ocean size" amounts of water on the surface, or is totally covered with water, or has a substantial layer of water underneath the surface (perhaps a surface of ice, which would make it much like the Earth arctic). Earth fits a lot of these criteria and has been labeled with these terms, but the terms definitely would also apply to planets with far more water than Earth's 0.02% by mass.