planetoidien

Planetoidien is a proposed astronomical term for a class of small planetary bodies intermediate between asteroids and dwarf planets. Typically defined as objects that are gravitationally rounded (in hydrostatic equilibrium) but have not cleared their orbital neighborhood, placing them between asteroids and dwarf planets in size and dynamical behavior. The term is infrequently used and not part of the official IAU taxonomy.

The name derives from planetoid with the suffix -ien to denote a class; it has appeared in speculative astronomy discussions and some science-fiction contexts; not widely adopted in formal nomenclature.

Planetoidien bodies are expected to be primarily rocky or mixed rocky-icy with diameters from a few hundred kilometers up to around a thousand or more, depending on composition. They would likely exhibit rounded shapes due to self-gravity, possible internal differentiation, and surfaces shaped by impact, tectonics, and sublimation processes. Some may retain tenuous atmospheres or transient volatiles in colder regions.

They would orbit stars and may reside in regions where gravity prevents easy orbital clearing by larger bodies but doesn't require proximity to a planet. Their orbits could be stable for long periods but subject to perturbations by planets and collisional evolution.

No confirmed planetoidien have been identified to date; the term remains hypothetical. Researchers studying the continuum of small solar-system bodies may reference planetoidien when discussing the transition between asteroids, comets, and dwarf planets, and future measurements could refine the definition.

See also: Planetoid, Dwarf planet, Asteroid, Kuiper belt object, Trans-Neptunian object.