For questions about any large astronomical bodies classified as planets, and their effects on their systems.

A planet, according to the International Astronomical Union, has enough gravity to become spherical, has "cleared its neighborhood" of smaller bodies, and does not undergo nuclear fusion. Questions using this tag should be related to the various types of planets and their characteristics, including, but not limited to: mass, size, speed of rotation, orbit, orbital period, axial tilt, magnetic field, internal activity (tectonic), and albedo.

Other subjects of interest include, but are not limited to: atmospheric composition, surface composition, surface temperature, geology, geography and climate. See also , and .

This tag does not tend to fit well on questions about lifeforms on a planet, unless the planet's characteristics have a direct impact on the survivability of the species: such questions should instead use a tag such as .