Which is the smallest planet?
Solar System
Mercury (0.4AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055M⊕). Mercury has no natural satellites; besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history.[74] Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind.[75] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact; or, that it was prevented from fully accreting by the young Sun's energy.[76][77]
Gas dwarf
The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density (2.1(+2.2/-1.2) grams per cubic centimetre) that indicates either a substantial water content or possibly a thick gas envelope.