planet

Planets are celestial bodies that orbit stars, have sufficient mass for gravity to shape them into nearly spherical forms, and have cleared their orbital neighborhoods of other debris. The term originates from the Greek planētēs, meaning wanderer. In the International Astronomical Union definition adopted in 2006, a planet must orbit a star, be in hydrostatic equilibrium (nearly round), and have removed most other objects in its orbital zone. Objects that fail the clearing criterion are classified as dwarf planets; Pluto is a well-known example of a dwarf planet.

Planets are commonly grouped into terrestrial planets with rocky surfaces (Mercury, Venus, Earth, Mars) and giant planets, which include gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune). They form from the protoplanetary disk that surrounds young stars; through accretion and collisions, material differentiates into cores, mantles, and, where present, atmospheres and magnetic fields. Their sizes range from smaller bodies to gas giants many times the mass of Earth.

Most planets orbit stars, and many reside in planetary systems with multiple worlds. In our solar system, eight planets orbit the Sun. Beyond it, thousands of exoplanets have been discovered around other stars, largely by transit and radial-velocity methods, with space missions such as Kepler and TESS expanding the known diversity of planetary systems.

Habitability depends on factors such as location in the habitable zone, atmospheric composition, and geologic activity. A planet in the habitable zone may permit surface liquid water, but long-term habitability requires favorable conditions. The study of planets, known as planetology or planetary science, seeks to understand formation, structure, dynamics, and potential for life across the universe.