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A number of short phrases have come into use to describe various extra-solar planets that could be termed planet types. The planets Earth, Jupiter, and Neptune (and occasionally, Mercury) are often used to designate the size as are super-Earth and mega-Earth, the latter two which may simply designate the size or may also suggest it is rocky, or mini-Neptune for a more Neptune-like medium-sized planet. The adjectives hot and warm are added to designate a planet at least as close to the star as its habitable zone. Hot typically means very close to the star, perhaps an orbit of a few days and temperatures in thousands of degrees. Warm has been used for planets with close to Earth temperatures or for considerably warmer. The terms gas planet, liquid planet, and rocky planet are used to indicate a constituent of significance. Some terms based on solar system models:
Some terms based on constituents:
Some terms based on other criteria:
Some of these (and other planet-type terms) are theoretical, some of which are theorized to exist but have not been observed, as well as some types that are theorized merely for comparison with observed planets, such as a theoretical planet made entirely of iron: comparing such a theoretical planet's mass/radius ratio with that of some observed planet hints at how much iron the observed planet is likely to include.