|
|
|
|
Resonant trans-Neptunian object
|
| |
|
| |
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object (TNO) in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc.†. Resonant TNOs can be either part of the main Kuiper belt population, or the more distant scattered disc population.
diagram illustrates the distribution of the known trans-Neptunian objects (up to 70 AU) in relation to the orbits of the planets together with Centaurs for reference.
Resonant objects are plotted in red.
Orbital resonances with Neptune are marked with vertical bars; 1:1 marks the position of Neptune’s orbit (and its Neptune Trojans), 2:3 marks the orbit of Pluto and plutinos, 1:2, 2:5 etc.
Discussion
Ask a question about 'Resonant trans-Neptunian object' Start a new discussion about 'Resonant trans-Neptunian object' Answer questions from other users |
Encyclopedia
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object (TNO) in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc.†. Resonant TNOs can be either part of the main Kuiper belt population, or the more distant scattered disc population.
Distribution
The diagram illustrates the distribution of the known trans-Neptunian objects (up to 70 AU) in relation to the orbits of the planets together with Centaurs for reference.
Resonant objects are plotted in red.
Orbital resonances with Neptune are marked with vertical bars; 1:1 marks the position of Neptune’s orbit (and its Neptune Trojans), 2:3 marks the orbit of Pluto and plutinos, 1:2, 2:5 etc. a number of smaller families).
†The designation 2:3 or 3:2 refer both to the same resonance for TNOs. There’s no confusion possible as TNO, by definition, have periods longer than Neptune. The usage depends on the author and the field of research. The statement "Pluto is in 2:3 resonance to Neptune" appears to better capture the meaning: Pluto completes 2 orbits for every 3 orbits of Neptune.
Origin
Detailed analytical and numerical studies
of Neptune’s resonances have shown that they are quite narrow i.e. the objects must have a relatively precise range of energy (i.e. semi-major axes). If the object semi-major axis is outside these narrow ranges, the orbit becomes chaotic (widely changing orbital elements).
Curiously, substantial numbers† of TNO's being discovered appear to be in 2:3 resonances, a proportion far from a random distribution.
It is now believed that the objects have been collected from wider distances by sweeping resonances during the migration of Neptune.
Well before the discovery of the first TNO, it was suggested that interaction between giant planets and a massive disk of small particles would, via momentum transfer, make Jupiter migrate inwards and while Saturn, Uranus and especially Neptune would migrate outwards. During this relatively short period of time, Neptune’s resonances, would be sweeping the space, trapping objects on initially varying heliocentric orbits into resonance.
†More than 10% are classified or suspected plutinos
Known populations
2:3 resonance ("plutinos", period ~250 years)
The 2:3 resonance at 39.4 AU is by far the dominant category among the resonant objects, with 92 confirmed and 104 possible member bodies. The objects following orbits in this resonance are named plutinos, after Pluto which has the first known orbit of this type. Large, numbered plutinos include:
3:5 resonance (period ~275 years)
A population of 10 objects at 42.3 AU as of October, 2008, including:
4:7 resonance (period ~290 years)
Another important population of objects (20 identified as of October 2008) is orbiting the Sun at 43.7 AU (in the midst of the classical objects). The objects are rather small (with a single exception, H>6) and most of them follow orbits close to the ecliptic.
Objects with well established orbits include :
, the largest
1:2 resonance ("twotinos", period ~330 years)
This 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. An unknown number of the 2:1 resonants likely did not not originate in a planetimal disk that was swept by the resonance during Neptune's migration.
There are far fewer objects in this resonance (a total of 14 as of October, 2008) than plutinos.
Objects with well established orbits include (in order of the absolute magnitude):
2:5 resonance (period ~410 years)
Objects with well established orbits at 55.4 AU include:
- , large dwarf planet candidate**
In total, the orbits of 11 objects are classified as 2:5 as of October, 2008.
Other resonances
So called higher-order resonances are known for a limited number of objects, including the following numbered objects
- 4:5 (35 AU, ~205 years)
- 3:4 (36.5 AU, ~220 years) ,
- 5:9 (44.5 AU, ~295 years)
- 4:9 (52 AU, ~370 years) ,
- 3:7 (53 AU, ~385 years) , , ,
- 5:12 (55 AU, ~395 years) , (84% probability according to Emel’yanenko)
- 3:8 (57 AU, ~440 years) (84% probability according to Emel’yanenko)
- 2:7 (70 AU, ~580 years) (The preliminary orbit suggests a weak 2:7 resonance. Further observations will be required.)
A few objects are known on simple, distant resonances
- 1:3 (62.5 AU, ~500 years)
- 1:4 (76 AU, ~650 years)
- 1:5 (88 AU, ~820 years) (likely coincidental)
Some notable unproven (they could be coincidental) dwarf planet resonances include:
- 12:7 (43 AU, ~283 years) Haumea (nominal orbit very likely in resonance)
- 11:6 (45 AU, ~302 years) Makemake ( appears to be in the 11:6 resonance)
- 17:5 (67 AU, ~560 years) Eris (2007 OR10 has a similar orbit)
Neptune trojans
A few objects have been discovered following orbits with semi-major axes similar to that of Neptune, near Lagrangian points L4 and L5. These Neptune Trojans, named by analogy to the Trojan asteroids, are in 1:1 resonance with Neptune. Six are known as of December 2007:***
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. A true resonance will smoothly oscillate while a coincidental near resonance will circulate. (See Toward a formal definition)
Simulations by Emel’yanenko and Kiseleva in 2007 show that is librating in a 7:3 resonance with Neptune. This libration can be stable for less than 100 million to billions of years.
Emel’yanenko and Kiseleva also show that appears to have less than a 1% probability of being in a 7:3 resonance with Neptune, but it does execute circulations near this resonance.
Toward a formal definition
The classes of TNO have no universally agreed precise definitions, the boundaries are often unclear and the notion of resonance is not defined precisely. The Deep Ecliptic Survey introduced formally defined dynamical classes based on long-term forward integration of orbits under the combined perturbations from all four giant planets. (see also formal definition of classical KBO)
It should be noted that in general, the mean motion resonance can involve not only orbital periods of the form
where p and q are small integers, ? and ?N are respectively the mean longitudes of the object and Neptune but can also involve the longitude of the perihelion and the longitudes of the nodes (see orbital resonance, for elementary examples)
An object is Resonant† if for some small integers p,q,n,m,r,s, the argument (angle) defined below is librating (i.e. is bounded)
where the are the longitudes of perihelia and the are the longitudes of the ascending nodes, for Neptune (with subscripts "N") and the resonant object (no subscripts).
The term libration denotes here periodic oscillation of the angle around some value and is opposed to circulation where the angle can take all values from 0 to 360°. For example, in the case of Pluto, the resonant angle librates around 180° with an amplitude of around 82° degrees, ie. the angle changes periodically from 180°-82° to 180°+82°.
All new plutinos discovered during the Deep Ecliptic Survey proved to be of the type
similar or Pluto's mean motion resonance.
More generally, this 2:3 resonance is an example of the resonances p (example 1:2, 2:3, 3:4 etc.) that have proved to lead to stable orbits. Their resonant angle is
In this case, the importance of the resonant angle can be understood by noting that when the object is at perihelion i.e. then
i.e. gives a measure of the distance of the object's perihelion from Neptune.
The object is protected from the perturbation by keeping its perihelion far from Neptune provided librates around an angle far from 0°.
†Capital R is used to refer to this formally defined class as opposed to common meaning of resonant
See Also
Jupiter Trojan
Further reading ()
|
| |
|
|
