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The concept of a planet made entirely of matter in the liquid state has been considered, i.e., a liquid planet analogous to a gas planet made entirely of gas. As a model, a pure liquid water planet is often used for density comparison with discovered extra-solar planets. For an actual planet, temperature variation seems likely to result in some evaporation and/or freezing, making a pure liquid planet unlikely. The planet may need an atmosphere to "seal" it, and if the atmosphere is thick (e.g., half the diameter as viewed externally), then it is a matter of semantics whether it is a gas planet or a liquid planet.
Jupiter has been described as a liquid planet: some current theories consider it to be liquid, possibly from the center, possibly up to the surface we view.