It was established fairly quickly after their discovery that the cubewanos have
very long dynamical lifetimes and as the resonances prevent close encounters with
Neptune, the resonant objects should also be stable. This stability suggests that these
objects are not the source of JFCs. However, as can be seen in Fig. 1.13, there are a
vast number of other objects that have higher eccentricity over a wide range of semimajor axis. Note that the paucity of objects with low eccentricity above 50 AU may
be attributable to observational selection effects as these objects will have large
perihelion distances and will therefore be relatively faint. Nonetheless there is
speculation that there is a depletion of objects beyond 50 AU—a hypothesis that
should be solved in the near future. In addition, there are probably many objects
beyond 60 AU which do not interact at all with our giant planet system. The physical
difference between these objects and objects which we occasionally see as nearparabolic comets may not be large.
Superimposed upon this low inclination (cold) population, there is a population
with a larger spread of inclinations suggesting that some of the original population
have been excited and their inclinations changed. Note also that the histogram in
Fig. 1.15, which is an updated version of that shown in Morbidelli and Brown
(2004), shows a depletion of objects with zero inclination with respect to the ecliptic.
The maximum is however coincident with the invariable plane of the solar system
which is the plane passing through the barycentre (centre of mass) perpendicular to
the total angular momentum vector.
The high eccentricity, high inclination objects are now classed as Scattered Disc
Objects (SDOs). (Detailed approaches to classification and sub-classification have
been proposed by several authors, e.g. Elliot et al. 2005). These objects are thought
to have been scattered by the giant planets at some stage during Solar System
evolution. The exact mechanism and timing remains uncertain. It was however
pointed out more than a decade ago that dynamically there is little to distinguish
SDOs and Centaurs. Frequent changes in orbital parameters can lead to objects
switching between these two categories and this has led the Minor Planet Center to
group these two categories together. It was shown 20 years ago that the dynamical
lifetimes of Centaurs are short—typically no more than a few million years although
the distribution in lifetime is very broad. This suggests replenishment of Centaurs on
relatively short timescales and the SDOs appear to be the only viable source.
However, getting SDOs into Centaur orbits and from there to JFC orbits seems to
be a sophisticated interplay between the objects and the giant planets. Morbidelli
(2010) describes the probable process succinctly.
An SDO is essentially in a region dominated by gravitational interaction with
Neptune. When interacting, the Tisserand parameter is conserved but the perihelion
distance is reduced. From Eq. (1.17), one can see that a range of solutions exist
fulfilling this requirement (Fig. 1.16). The conservation of the Tisserand parameter
limits the reduction in the periapsis but this would still be sufficient for the object to
come under the influence of Uranus in many cases. Levison and Duncan (1997) gave
the example of an object close to the 2:3 resonance with an eccentricity of 0.25
which, following interaction ended with a perihelion distance inside the orbit of
Uranus. An interaction can then take place with Uranus reducing the perihelion
20
1 Light Curves, Orbits, and Reservoirs
very long dynamical lifetimes and as the resonances prevent close encounters with
Neptune, the resonant objects should also be stable. This stability suggests that these
objects are not the source of JFCs. However, as can be seen in Fig. 1.13, there are a
vast number of other objects that have higher eccentricity over a wide range of semimajor axis. Note that the paucity of objects with low eccentricity above 50 AU may
be attributable to observational selection effects as these objects will have large
perihelion distances and will therefore be relatively faint. Nonetheless there is
speculation that there is a depletion of objects beyond 50 AU—a hypothesis that
should be solved in the near future. In addition, there are probably many objects
beyond 60 AU which do not interact at all with our giant planet system. The physical
difference between these objects and objects which we occasionally see as nearparabolic comets may not be large.
Superimposed upon this low inclination (cold) population, there is a population
with a larger spread of inclinations suggesting that some of the original population
have been excited and their inclinations changed. Note also that the histogram in
Fig. 1.15, which is an updated version of that shown in Morbidelli and Brown
(2004), shows a depletion of objects with zero inclination with respect to the ecliptic.
The maximum is however coincident with the invariable plane of the solar system
which is the plane passing through the barycentre (centre of mass) perpendicular to
the total angular momentum vector.
The high eccentricity, high inclination objects are now classed as Scattered Disc
Objects (SDOs). (Detailed approaches to classification and sub-classification have
been proposed by several authors, e.g. Elliot et al. 2005). These objects are thought
to have been scattered by the giant planets at some stage during Solar System
evolution. The exact mechanism and timing remains uncertain. It was however
pointed out more than a decade ago that dynamically there is little to distinguish
SDOs and Centaurs. Frequent changes in orbital parameters can lead to objects
switching between these two categories and this has led the Minor Planet Center to
group these two categories together. It was shown 20 years ago that the dynamical
lifetimes of Centaurs are short—typically no more than a few million years although
the distribution in lifetime is very broad. This suggests replenishment of Centaurs on
relatively short timescales and the SDOs appear to be the only viable source.
However, getting SDOs into Centaur orbits and from there to JFC orbits seems to
be a sophisticated interplay between the objects and the giant planets. Morbidelli
(2010) describes the probable process succinctly.
An SDO is essentially in a region dominated by gravitational interaction with
Neptune. When interacting, the Tisserand parameter is conserved but the perihelion
distance is reduced. From Eq. (1.17), one can see that a range of solutions exist
fulfilling this requirement (Fig. 1.16). The conservation of the Tisserand parameter
limits the reduction in the periapsis but this would still be sufficient for the object to
come under the influence of Uranus in many cases. Levison and Duncan (1997) gave
the example of an object close to the 2:3 resonance with an eccentricity of 0.25
which, following interaction ended with a perihelion distance inside the orbit of
Uranus. An interaction can then take place with Uranus reducing the perihelion
20
1 Light Curves, Orbits, and Reservoirs