planet

The term planet is used for major objects orbiting stars other than binary companions. Detected extra-solar planets are so-far sufficiently large that there is no need to consider whether they qualify as "major". For the solar system, the IAU distinction is that the object be sufficiently massive that gravity draws it into a spherical shape (shaped by hydrostatic equilibrium) and also massive enough to dynamically clear any planetesimals out of the path of its own orbit. This latter distinction (termed clearing the neighborhood) distinguishes it from a dwarf planet, and disqualifies Pluto from being considered a "normal planet" (and incidentally disqualifies Ceres, also once considered a planet), leaving the solar system with eight known planets out of the hundreds of thousands of objects known to orbit the Sun.

Also, the object must not be so massive as to trigger fusion and thus qualify as a star. The IAU has chosen a nominal threshold of 13 Jupiter-masses, above which an object orbiting a star is presumed to be a brown dwarf companion, based upon the masses likely to be capable of triggering deuterium fusion, a threshold that may be revised as astronomers learn more. For any specific object, there may be other reasons to conclude that it could or could not burn deuterium.

Planetary science covers the study of solar system planets and exoplanets, study in which there is some overlap but obviously a lot more detail regarding the former. The term also includes study of solar system moons, minor planets, and comets. There is an overall general classification of planets into gas planets versus rocky planet (versus planets of liquid or frozen volatiles), with numerous planet types indicating finer distinctions.