Spherical object consisting of molten or solid rock and various fractions of volatiles including gases, liquids and ices, larger than planetesimals and smaller than stars or brown dwarfs. Planets are too small to generate energy by nuclear reactions. One of six categories of matter that make up solar systems, nebula, and galaxies, and can be harvested for interplanetary or interstellar development (the other categories are interstellar gas, dust, planetisimals, brown dwarfs, and stars).
Planets are often found orbiting stars, but significant numbers exist as independent objects in deep space. A small number of worlds have developed biospheres of various kinds.