Splanets

Splanets is a term used in speculative astronomy and in fiction to denote a class of small planetary bodies smaller than Earth. It is not an officially recognized category in major astronomical taxonomies. The word combines the sense of “small” or “sub‑Earth” with the word “planets” and is used as a convenient label in thought experiments and narrative worlds.

In educational contexts and some debates about planet formation, splanets are discussed as a placeholder category to compare small rocky, icy, or mixed-composition bodies that do not fit neatly into Earth-like planets or larger gas-dominated giants. The concept helps isolate questions about how small planets form, migrate, and evolve, without implying a fixed, formal classification.

There is no fixed specification for splanets; rather, they are defined by size, typically smaller than Earth and ranging up to near-Earth scale. They may be rocky, metallic, or icy, and can host thin atmospheres or be airless. Orbits can vary from close-in to widely spaced, around different star types, and can occur in a variety of planetary system architectures.

In fiction, splanets are common settings with varied environments, from barren deserts to oceans or complex megastructures. In serious discourse, they serve to explore questions about formation, composition, and habitability at small planetary scales, without committing to a universal, formal definition.

As a label, splanets remain informal and discursive rather than a formal taxonomic class, contrasting with terms such as terrestrial planets, dwarf planets, and exoplanets.