Subsellun

Subsellun is a term used in speculative cosmology and science fiction to designate a class of very small, transient satellites that orbit a planet at distances within the planet's Roche limit. In fictional and theoretical discussions, subselluns are typically fragments formed by tidal disruption of a larger moon or by collisional debris that briefly coalesces into a loosely bound body before tidal forces disperse it again.

Etymology and usage: The name combines the prefix sub- with a fictional root sellun, drawn from moon-related terminology in some speculative lexicons. It is not an established scientific term and is largely confined to thought experiments, worldbuilding, and genre fiction.

Physical characteristics: Subselluns are small, usually tens to hundreds of meters in diameter, irregular in shape, and composed of rocky or icy material. They have low to moderate bulk densities and highly variable albedo. Orbits are close to the planet, often with rapid periods of hours to a day, and their existence is inherently ephemeral due to strong tidal stresses.

Formation and dynamics: They arise when material from a moon or ring system is drawn inside the Roche limit or during high-velocity impacts that fragment a body. The objects are typically short-lived, existing from days to years in some models, before disintegrating or rejoining ring material. Their dynamics illustrate tidal fragmentation and near-planet gravitational effects.

Detection and status: No confirmed real-world observations exist; subselluns are used primarily as a speculative concept in fiction and theoretical discussions. In exoplanet scenarios, their detection would be challenging and would rely on precise transit timing variations or subtle light-curve anomalies.

See also: Roche limit, sub-satellite, ring system.