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The exoplanet known as GJ 1214 b is one of the best-studied planets in its mass and size class. Its mass is about 6.4 Earth masses and its radius is about 2.69 Earth radii, which puts it in the regime of a super-Earth. However, the implied average density of GJ 1214 b is very low, only about 0.35 times that of Earth, and its chemical composition is ambiguous, so it could be a mini-hot-Neptune. The difference would be mainly in its physical state: rocky, liquid, or gaseous. It is not known. There has been talk of the planet being a waterworld, or ocean planet.
The star GJ 1214 is about 42.4 light years from Earth, and its mass is only about 15% of our Sun's mass. Its radius is also small, only about 21% of our Sun's radius (it is known as an “M dwarf”). The orbital period is very short, 1.58 Earth days, corresponding to a maximum star-planet distance (semimajor axis) of just 1.4% of the Earth-Sun distance (0.014 astronomical units (AU)).
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Advanced Readers: For a good introduction to the literature, see de Mooij et al. (2012), Optical to near-infrared transit observations of super-Earth GJ1214b: water-world or mini-Neptune?, and references therein. The results of the study in this paper, which attempted to determine if GJ 1214 b is a waterworld or mini-hot-Neptune, are inconclusive.
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File under: GJ 1214 b; Is GJ 1214 b a close-in super-Earth, waterworld, or mini-Neptune.