There are currently about fifty known exoplanets with diameters that range from Mars-sized to several times the Earth’s and that also reside within their stars’ habitable zone – the orbital range within which their surface temperatures permit water to remain liquid. A “water world” is an extreme case, an exoplanet defined as being covered by a deep ocean, perhaps as deep as hundreds of kilometers, and among these fifty are several that might be candidates for this category. Astronomers note that at least two of the terrestrial planets in our solar system, Earth and Venus, may possibly also have been water worlds early in their evolution.
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