Definition
A planet is a celestial object that orbits a star or stellar remnant, without producing its own light through nuclear fusion. It has enough mass to reach hydrostatic equilibrium, forming a spherical shape. It does not share its orbit with comparable bodies, except its own natural satellites or captured small objects.
Structure
The internal structure of a planet depends on its composition. Terrestrial planets have a differentiated structure: metallic core, silicate mantle, outer crust. Gas giants have dense atmospheres of hydrogen and helium, surrounding a possibly solid or fluid core. Some planets contain additional layers such as magma oceans, icy mantles, or dense structures under high pressure.
Orbital Functioning
Planets follow stable elliptical trajectories around their star according to the laws of celestial mechanics. Their orbital speed, inclination, and eccentricity depend on their distance from the gravitational center. Gravitational interactions with other bodies can create resonances, perturbations, or orbital migrations.
Classification
Planets are classified based on their composition and position: terrestrial planets (rocky, close to the star), gas giants (rich in light gases, farther out), icy planets or ice giants (high proportion of water ice, ammonia, or methane). Other criteria include the presence of moon systems, rings, or complex atmospheres.
Evolution
Planets form in protoplanetary disks by accretion of matter around cores. Their evolution depends on many factors: initial mass, position, collisions, atmospheric loss, internal cooling, and long-term gravitational interactions. Some become "dead" planets, others may be ejected from their system.
Limits
The limits of a planet are defined by its minimum mass to achieve spherical equilibrium and its ability to clear other objects in its orbit. Otherwise, it is a small body or substellar object. The boundaries with brown dwarfs or trans-Neptunian objects can be unclear depending on the criteria used.