A small solar system body (SSSB) is a collective term for the wide range of minor objects that orbit the Sun but are not classified as planets or dwarf planets. The name was adopted by the International Astronomical Union in 2006 to group together the many asteroids, comets, and smaller trans‑Neptunian objects that populate the Solar System. These objects vary greatly in size, shape, composition and origin, and they are important both for understanding planetary formation and for assessing impact risks to Earth.

Types and main characteristics

SSSBs include several familiar categories, which overlap in some respects but are useful for organization:

  • Asteroids: rocky or metallic bodies mostly concentrated in the main asteroid belt between Mars and Jupiter; most known minor planets that are not dwarf planets fall in this group. Examples include Vesta and many smaller bodies (note that 1 Ceres is classified as a dwarf planet and is excluded from the SSSB list).
  • Comets: icy bodies that develop comae and tails when heated by the Sun; they originate from distant reservoirs and are included among SSSBs when not reclassified.
  • Trans‑Neptunian Objects (TNOs): objects beyond Neptune; many small TNOs are SSSBs while the larger, nearly round ones are designated dwarf planets.
  • Centaurs and Trojans: bodies with orbits between the giant planets or co-orbiting with a planet (for example, Neptune Trojans) are usually classed as SSSBs unless they meet dwarf‑planet criteria.

These categories emphasize orbit and physical properties. Typical SSSBs range from kilometer-scale objects down to meter-scale meteoroids; the exact lower size limit of the SSSB designation has not been formally fixed and is treated differently in various studies.

Definition and historical context

In 2006 the IAU resolution formalized a taxonomy that separates planets, dwarf planets and small solar system bodies. The decision, made by the International Astronomical Union, placed most previously catalogued minor planets and comets under the SSSB label while reserving the terms planet and dwarf planet for objects meeting specific orbital and structural criteria. That reform reflected improved observational coverage of the outer Solar System and the discovery of many bodies that blur older categories.

Scientific importance and practical relevance

SSSBs are scientifically valuable because they preserve records of conditions in the early Solar System. Studying their composition, cratering, and orbits helps reconstruct the processes that led to planet formation. Space missions have visited several SSSBs to collect imagery, spectroscopy and in some cases samples; such missions inform models of material distribution and migration. In practical terms, some SSSBs pose an impact hazard to Earth, and others are considered potential sources of resources for future exploration.

Classification challenges and future changes

The SSSB category is intentionally broad and includes objects that may be reassigned if their shapes and internal structures are better determined. Some larger members might be reclassified as dwarf planets if observations show they are in hydrostatic equilibrium. Additionally, the boundaries between comets, asteroids and TNOs are not always sharp: bodies can display both cometary activity and asteroid‑like orbits. Specialized groups such as classical asteroids, Centaurs, and distant icy bodies continue to be the focus of ongoing surveys and targeted missions.

For authoritative definitions and ongoing updates, readers can consult the IAU statements and summaries of recent surveys and missions. General overviews and educational resources often provide accessible introductions to the many populations grouped under the small solar system body label, while technical literature addresses classification criteria and dynamical evolution in greater detail.

Relevant resources: official IAU materials (resolution text), the IAU website, introductory pages on planets, the category of dwarf planets, materials on asteroids, overviews of Centaurs, and general comet information (comets).