In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers.
Besides the most dominant 2:3 resonance of the plutinos, other resonances exist (information at 2011):
1. 1:1 resonance (Neptune trojans, 30 AU, period ~ 165 years)
A population of 8 objects currently known.
2. 4:5 resonance (34.9 AU, period ~ 205 years)
A population of 5 objects currently known.
3. 3:4 resonance (36.4 AU, period ~ 220 years)
A population of 9 objects currently known.
4. 3:5 resonance (42.3 AU, period ~275 years)
A population of 10 objects currently known.
5. 4:7 resonance (43.7 AU, period ~290 years)
A population of 24 objects currently known.
6. 5:9 resonance (44.5 AU, period ~295 years)
A population of 3 objects currently known.
7. 1:2 resonance ("twotinos", 47.7 AU, period ~330 years)
A population of 15 objects currently known.
8. 4:9 resonance (52 AU, period ~370 years)
A population of 2 objects currently known.
9. 3:7 resonance (52.9 AU, period ~385 years)
A population of 5 objects currently known.
10. 5:12 resonance (55 AU, period ~395 years)
A population of 2 objects currently known.
11. 2:5 resonance (55.4 AU, period ~410 years)
A population of 15 objects currently known.
12. 3:8 resonance (57 AU, period ~440 years)
1 object currently known.
13. 1:3 resonance ("threetinos", 62.6 AU, period ~495 years)
A population of 2 objects currently known.
14. 2:7 resonance (70 AU, period ~580 years)
1 object currently known.
15. 1:4 resonance ("fourtinos", 75.8 AU, period ~660 years)
1 object currently known.
16. 1:5 resonance (87.9 AU, period ~820 years)
1 object currently known.
Twotinos
The resonance at 47.8 AU is often considered as the outer "edge" of the Kuiper belt and the objects in this resonance are sometimes referred to as twotinos.
There are far fewer objects in this resonance (a total of 15 current known) than plutinos. Long-term orbital integration shows that the 1:2 resonance is less stable than 2:3 resonance; only 15% of the objects in 1:2 resonance were found to survive 4 Gyr as compared with 28% of the plutinos.
Consequently it might be that twotinos were originally as numerous as plutinos, but their population has dropped significantly below that of plutinos since.
Coincidental vs true resonances
One of the concerns is that weak resonances may exist and would be difficult to prove due to the current lack of accuracy in the orbits of these distant objects. Many objects have orbital periods of more than 300 years and most have only been observed over a short observation arc of a couple years.
Due to their great distance and slow movement against background stars, it may be decades before many of these distant orbits are determined well enough to confidently confirm whether a resonance is true or merely coincidental.
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