"Planet" - what is it, definition of the term
The term refers to a massive, approximately spherical astronomical object that orbits a star, possesses enough self‑gravity to assume hydrostatic equilibrium, and has cleared its orbital zone of comparable debris, distinguishing it from dwarf bodies and smaller minor objects; it lacks the mass required for sustained thermonuclear fusion.
Detailed information
A celestial body that orbits a star, possesses sufficient mass for self‑gravity to overcome rigid body forces, and has cleared its orbital neighborhood, is a distinct class of astronomical objects. This definition separates such objects from smaller bodies that share the same orbital region.
These objects fall into several categories based on composition and size:
- Rocky worlds composed primarily of silicate minerals and metals.
- Gas giants dominated by hydrogen and helium, with extensive atmospheres and deep fluid interiors.
- Ice giants containing large amounts of volatile compounds such as water, ammonia, and methane.
- Dwarf classifications that meet most criteria but retain neighboring debris.
Typical physical parameters include mass ranging from 10²³ to 10²⁸ kilograms, radii between 2 000 and 70 000 kilometers, and surface gravities from 0.3 g to 25 g. Density values reflect composition: rocky objects average 3–5 g cm⁻³, while gas-dominated bodies are less than 2 g cm⁻³.
Orbital characteristics are defined by the semi‑major axis, orbital period, eccentricity, and inclination. For example, an object at 1 AU from a Sun‑like star completes one revolution in roughly 365 days, with an eccentricity near 0.0167 and an orbital plane inclined only a few degrees relative to the stellar equator.
Atmospheric layers vary widely. Some possess thin, carbon‑dioxide‑rich envelopes with surface pressures below 1 bar; others retain massive hydrogen‑helium blankets where pressure exceeds 100 bars and temperature gradients span several thousand kelvin. Atmospheric composition directly influences climate, weather patterns, and potential for liquid water.
Internal structure typically consists of a metallic core, a silicate mantle, and an outer crust. Core dynamics generate magnetic fields through convective motion of electrically conductive material. Magnetic shielding protects surface environments from high‑energy stellar particles.
Natural satellites orbit many of these bodies. Satellite populations range from a single large moon to extensive systems of dozens of small companions, often accompanied by ring structures formed from debris.
Habitable conditions are confined to a region around the host star where surface temperatures permit liquid water. Factors such as stellar luminosity, orbital distance, atmospheric greenhouse effect, and planetary mass determine whether an object resides within this circumstellar zone.