Semimassive

Semimassive is an informal, nonstandard descriptor used in astronomy and planetary science to indicate an object's mass lies between well-defined categories such as small rocky bodies and larger gas-rich planets, or between planetary and stellar scales. It is not a formal taxonomic class in major catalogs or surveys.

Because there is no universal mass range assigned to semimassive, its meaning varies by author and context. In exoplanet literature, the term has appeared to describe planets with substantial mass and atmospheric envelopes that are not quite gas giants, roughly in the range between typical terrestrial planets and Neptunian-mass planets. In astrophysical contexts, some authors have used it to refer to intermediate-mass black holes, but this usage is rare and not standard.

In practice, semimassive signals a relative position along a spectrum of masses rather than a precise category. When precise classification is required, researchers prefer explicit descriptors such as Earth masses, Neptune masses, or representative black-hole mass ranges (for example, intermediate-mass black holes in the 10^2–10^5 solar-mass range).

The prefix semi- suggests a halfway or mid-range status, aligning with a sense of mid-range mass. The term is more common in informal or exploratory discussions than in formal taxonomy, and its interpretation should be clarified within each specific scientific context.