Terrestrial Solar System planets either have high mean molecular weight atmospheres, as with Venus, Mars, and Earth, or no atmosphere at all, as with Mercury.
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Extrasolar Planets: May 2018
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet which was first discovered by the K2 mission and later confirmed with Spitzer Space Telescope observations.
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