A planet is a large astronomical body that orbits a star or other stellar remnant and has enough mass for its own gravity to make it approximately round, while also satisfying the other criteria used to distinguish planets from smaller bodies. In the Solar System, the International Astronomical Union defines a planet as a celestial body that orbits the Sun, has sufficient mass for hydrostatic equilibrium so that it assumes a nearly round shape, and has cleared the neighbourhood around its orbit.
Under this formal Solar System definition, there are eight recognised planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The word planet ultimately comes through Greek and Latin terminology associated with a wandering object, reflecting the ancient observation that planets appear to move against the background of apparently fixed stars. A planet is therefore not simply any large object in space. Its classification depends on its physical properties, orbital circumstances and dynamical relationship with surrounding objects. The concept is particularly important because astronomy distinguishes planets from stars, dwarf planets, asteroids, comets, moons and other smaller bodies using measurable physical and orbital characteristics rather than merely apparent brightness or size.
Planetary formation is closely connected with the formation of stars and planetary systems. Most planets are thought to originate within rotating discs of gas and dust surrounding young stars. Within these protoplanetary discs, microscopic grains collide and stick together, forming larger aggregates that eventually become planetesimals. Gravitational interactions allow some planetesimals to grow into planetary embryos and, eventually, planets.
The precise evolutionary pathway depends strongly on distance from the star, available material, temperature, orbital dynamics and the amount of time available for accretion. In the Solar System, the terrestrial planets formed relatively close to the Sun from rocky and metallic materials that could remain solid at high temperatures, whereas the outer planets formed in colder regions where abundant ices were available.
Jupiter and Saturn became massive enough to accumulate enormous hydrogen and helium envelopes, while Uranus and Neptune contain substantial quantities of heavier volatile materials and are commonly classified as ice giants. Planet formation is not necessarily complete when a planet reaches its present size because collisions, atmospheric loss, migration, differentiation and interactions with other bodies can continue altering planets for billions of years.
A defining physical characteristic of a planet is approximate hydrostatic equilibrium. When an object becomes sufficiently massive, its self gravity overwhelms much of the material strength resisting deformation, causing the body to approach a rounded shape. The exact boundary between irregular small bodies and rounded larger bodies is not a single universal mass because composition, temperature and internal structure influence how readily an object deforms.
A rocky body and an icy body can reach roundness at different sizes because ice generally deforms more readily than rock under long geological timescales. Planets are consequently differentiated from many asteroids and other small bodies not merely by diameter but by whether their gravity has substantially shaped their overall form. Even a planet is not necessarily a perfect sphere.
Rotation causes equatorial bulging, while variations in internal density and geological structure can produce smaller deviations from an ideal spherical shape. Earth, for example, is more accurately described as an oblate spheroid and, for high precision, as a geoid. Rapidly rotating giant planets exhibit particularly noticeable flattening.
The third part of the Solar System definition, orbital neighbourhood clearing, refers to dynamical dominance rather than literal removal of every other object from an orbital region. A planet can share its broad orbital zone with asteroids, comets, dust and small bodies while still being dynamically dominant. Earth's orbital neighbourhood, for example, contains near Earth asteroids and other objects, yet Earth's mass allows it to gravitationally control a substantial region and strongly influence the trajectories of smaller bodies.
Jupiter similarly exerts enormous gravitational influence throughout the outer Solar System. This criterion is what separates the eight recognised planets from dwarf planets such as Pluto, which is round and orbits the Sun but has not become gravitationally dominant in its orbital neighbourhood. The distinction is therefore fundamentally dynamical. A planet is not required to be the only object in its orbital path. Instead, it must have become the principal gravitational body in the relevant region through its mass and long term orbital interactions.
Planets can be classified in several different ways according to composition, size, location and physical structure. The four inner planets are terrestrial planets because they possess predominantly rocky and metallic interiors with relatively thin atmospheres compared with the giant planets. Jupiter and Saturn are gas giants because hydrogen and helium constitute most of their bulk composition, while Uranus and Neptune are generally called ice giants because water, ammonia and methane rich materials make up a much larger proportion of their interiors.
Exoplanets have produced additional categories and terminology, including super Earths, mini Neptunes, hot Jupiters and ultra hot planets, although these terms do not always correspond to sharply defined physical classes. A planet can also be described by its orbital position, atmospheric properties, temperature, geological activity or potential habitability. These classifications are useful descriptive tools rather than replacements for the formal planetary definition. They reflect the enormous diversity discovered among planets both inside and outside the Solar System.
Planets have complex internal structures that depend strongly on their mass and composition. Rocky planets generally have metallic cores, rocky mantles and crusts, although the precise layering varies considerably. Earth's core is predominantly iron and nickel, with a liquid outer core and solid inner core, while convection in the liquid outer core contributes to the generation of Earth's global magnetic field. Giant planets possess much deeper atmospheres and high pressure interiors where familiar materials can exist in unusual physical states.
Jupiter and Saturn are dominated by hydrogen and helium, with pressure increasing enormously towards their centres. Uranus and Neptune contain substantial water, ammonia and methane rich material beneath their atmospheres. Planetary atmospheres are likewise highly variable. Venus has an extremely dense carbon dioxide dominated atmosphere, Mars has a thin carbon dioxide dominated atmosphere, Earth has a nitrogen and oxygen dominated atmosphere, and the giant planets possess hydrogen and helium rich atmospheres. Atmospheric chemistry, internal heat, magnetic fields, solar radiation and geological processes all influence planetary environments.
A planet's surface and environment can evolve dramatically over geological time. Impacts can create enormous craters and basins, volcanism can build mountains and volcanic plains, tectonics can reshape crustal plates, erosion can modify landscapes, and atmospheric circulation can transport heat and material around the globe. On Earth, liquid water, plate tectonics, biological activity and atmospheric processes have profoundly altered the surface. Mars preserves evidence of ancient rivers, lakes and volcanic activity despite its present cold and dry conditions.
Venus shows extensive volcanic and tectonic features beneath its dense atmosphere. Gas and ice giants do not have solid surfaces comparable to Earth's, but they exhibit powerful atmospheric circulation, storms, cloud layers and internal dynamics. Planetary magnetic fields can also influence interactions with stellar winds and charged particles. Understanding planets therefore requires integrating astronomy, geology, atmospheric science, chemistry, physics and planetary dynamics.
The discovery of planets around other stars has greatly expanded the scientific meaning of the term. An exoplanet is a planet outside the Solar System, and thousands have been confirmed using methods such as transits, radial velocity measurements, gravitational microlensing and direct imaging. Transit observations can reveal a planet's approximate size by measuring the small reduction in starlight when the planet passes in front of its star, while radial velocity observations detect the star's motion caused by gravitational interaction with an orbiting planet.
Some exoplanets orbit extremely close to their stars, while others occupy much wider orbits. Scientists study their masses, radii, densities, atmospheres and orbital periods to understand planetary formation and evolution. The modern concept of a planet therefore extends far beyond the eight Solar System planets. Nevertheless, the exact classification of unusual objects can remain scientifically debated because different contexts may use different operational definitions.
