行星（英语：planet；拉丁语：planeta），通常指自身不发光，环绕着恒星的天体。其公转方向常与所绕恒星的自转方向相同（由西向东）。一般来说行星需具有一定质量，行星的质量要足够的大（相对于月球）且近似于圆球状，自身不能像恒星那样发生核聚变反应。2007年5月，麻省理工学院一组空间科学研究队发现了已知最热的行星（2040摄氏度）。

随着一些具有冥王星大小的天体被发现，「行星」一词的科学定义似乎更形迫切。历史上行星名字来自于它们的位置（与恒星的相对位置）在天空中不固定，就好像它们在星空中行走一般。太阳系内肉眼可见的5颗行星水星、金星、火星、木星和土星早在史前就已经被人类发现了。16世纪后日心说取代了地心说，人类了解到地球本身也是一颗行星。望远镜被发明和万有引力被发现后，人类又发现了天王星、海王星，冥王星（2006年后被排除出行星行列，2008年被重分类为类冥天体，又称「矮行星」）还有为数不少的小行星。20世纪末人类在太阳系外的恒星系统中也发现了行星，截至2013年7月12日，人类已发现2000多颗太阳系外的行星。

A planet (fromAncient Greekἀστήρπλανήτης astēr planētēs, or πλάνης ἀστήρ plánēs astēr, meaning "wandering star") is an astronomical object orbiting a star or stellar remnant that

The term planet is ancient, with ties to history, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human pe****tion of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.