In astronomy, a plutino is a Pluto-like object, insofar as it has the same relative orbit as Pluto. These orbits are stabilized by an orbital resonance with Neptune, similar to Pluto's 3:2 orbital resonance. This means that plutinos complete 2 orbits around the sun in the time it takes Neptune to complete 3 orbits. Plutinos form the inner part of the Kuiper belt. About 1/4 of known Kuiper Belt objects are Plutinos.
Since Pluto fits the 3:2 orbital resonance, it is sometimes convenient to include Pluto itself when referring to that orbital category of objects as plutinos. However, others argue that because "plutino" means "little Pluto", Pluto itself can't be a "little Pluto".
External link
David Jewitt (Univ. of Hawaii) on Plutinos (http://www.ifa.hawaii.edu/faculty/jewitt/kb/plutino.html)
By extrapolating from the limited area of the sky so far examined, we have estimated that the number of Plutinos larger than 100 km diameter is 1400, to within a factor of a few, corresponding to a few % of the total.
First, the Plutinos are observationally over-assessed due to their being closer (brighter), on average, than the Classical KBOs giving rise to an observational bias in favor of the Plutinos.
Pluto is distinguished from the Plutinos by its size: it is the largest object identified to date in the 3:2 resonance.
Despite their often extreme proximity to Neptune's orbit, the plutinos do not in fact have the possibility of encountering Neptune itself, because the periods of revolution about the sun of the plutinos and Neptune are precisely in a ratio of three to two.
This means that, after three revolutions of Neptune and two of a plutino (about 500 years), the relative positions of the objects in their orbits repeat, and this cycle does not give the bodies an opportunity to pass within 10 or more astronomical units of each other.
And whereas the plutinos orbit the sun at an average distance of 39 astronomical units, cubewanos have average distances over the range 42-46 astronomical units.
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