A plutino is a small Solar System body in the outer regions beyond Neptune that occupies a particular resonant orbit. The name derives from Pluto, because Pluto itself is the largest and best known member of this dynamical class. Plutinos belong to the broader population of trans‑Neptunian objects in the Kuiper belt and are studied to learn about planetary migration and the early evolution of the Solar System. Outer Solar System surveys revealed many such bodies after Pluto's discovery.
Orbital characteristics
What defines a plutino is its orbital relationship with Neptune. Plutinos are in a 2:3 mean‑motion resonance: they complete two orbits around the Sun for every three orbits by Neptune. Their semimajor axes cluster near the same distance as Pluto, roughly 39–40 astronomical units, though eccentricities and inclinations vary considerably. This characteristic orbit gives them a recognizable pattern of motion.
Dynamics and stability
The 2:3 resonance helps protect plutinos from close encounters with Neptune because the resonance synchronizes their positions. Resonant behavior—often described by the libration of a resonant argument—keeps many plutinos on long‑lived, stable paths. Pluto exemplifies these dynamics: despite an eccentric, inclined path it avoids Neptune encounters due to resonant phasing. See the entry on Pluto for a familiar example.
Population and examples
Since extensive Kuiper belt surveys began in the late 20th century, astronomers have identified many plutinos. The group includes objects with a range of sizes, from sizable bodies visible in telescopes to much smaller, fainter objects. Typical distances from the Sun remain near 39–40 AU, though individual orbits can bring a plutino somewhat closer or farther at different points in its orbit. Well‑known examples are often cited in literature and observational catalogs.
Importance and distinctions
Plutinos are important tracers of Neptune’s past migration and the dynamical sculpting of the Kuiper belt. They are distinct from classical (nonresonant) Kuiper belt objects and from other resonant classes that occupy different commensurabilities with Neptune. Studying the distribution of plutinos helps constrain models of early Solar System history and the processes that placed small bodies into resonant orbits.