distance and eliminating further planetary encounters. The stellar encounters, which
could number as many as 50,000 within 1 parsec of the Sun over the lifetime of the
Solar System (Dones et al. 2004; Morbidelli 2010), and the tides thus serve to both
populate and de-populate the Oort cloud.
For the JFCs, the Tisserand parameter already suggests that different pathways
are involved. While, the study of their origin is a little more complicated, significant
progress has been made in the past 25 years and there is a vast amount of literature
related (to a greater or lesser extent) to this topic. The existence of objects crossing
the orbits of the giant planets (a class of object known as Centaurs) was already
known in 1977 following the discovery of the prototype, the unusual asteroid (2060)
Chiron, by Kowal et al. (1977). This object occasionally shows comet-like activity
despite its perihelion distance being more than 8.4 AU (e.g. Luu and Jewitt 1990).
However, the key observation was the confirmation of the predictions by Edgeworth
and Kuiper of the existence of a class of objects beyond the orbit of Neptune by
Jewitt and Luu (1993) in 1992 that were generically referred to as Kuiper Belt
Objects (KBOs). When combined with the Centaurs, nearly 2500 objects are now
known and division into sub-categories is now possible although the nomenclature
associated with these categories is sometimes confusing (Fig. 1.13).
The Centaurs are now defined as orbiting between the orbits of Jupiter and
Neptune and having T J > 3.05. They are a sub-category of the cis-Neptunian objects
(Table 1.2) which includes, in addition, the Neptune Trojans—the objects in a 1:1
resonance with Neptune (of which 22 were known per 31 Dec 2019). TransNeptunian Objects (TNOs) are any minor planet with a semi-major axis greater
than the orbit of Neptune but excluding objects associated with the Oort cloud
(Table 1.2). The TNO category can itself be sub-divided into two main categories.
Objects, with semi-major axes between that of Neptune and about 50 AU, and with
low eccentricities and low inclinations, form a group that have relatively stable
orbits. In Fig. 1.13, these objects (as defined by the Minor Planet Center and marked
as diamonds) all fall within the area delinated by the broken line on the eccentricity
versus semi-major axis graph. Some of these objects, Pluto being the prototype, have
perihelion distances closer than the orbit of Neptune. Pluto is not perturbed by
Fig. 1.13 The semi-major
axes of 2478 TransNeptunian Objects (marked
as diamonds), Scattered
Disc objects and Centaurs
(marked as squares) in the
Minor Planet Center
database (per 31 Dec. 2016)
expressed against their
orbital eccentricity
1.2 Orbits and Origins
17
could number as many as 50,000 within 1 parsec of the Sun over the lifetime of the
Solar System (Dones et al. 2004; Morbidelli 2010), and the tides thus serve to both
populate and de-populate the Oort cloud.
For the JFCs, the Tisserand parameter already suggests that different pathways
are involved. While, the study of their origin is a little more complicated, significant
progress has been made in the past 25 years and there is a vast amount of literature
related (to a greater or lesser extent) to this topic. The existence of objects crossing
the orbits of the giant planets (a class of object known as Centaurs) was already
known in 1977 following the discovery of the prototype, the unusual asteroid (2060)
Chiron, by Kowal et al. (1977). This object occasionally shows comet-like activity
despite its perihelion distance being more than 8.4 AU (e.g. Luu and Jewitt 1990).
However, the key observation was the confirmation of the predictions by Edgeworth
and Kuiper of the existence of a class of objects beyond the orbit of Neptune by
Jewitt and Luu (1993) in 1992 that were generically referred to as Kuiper Belt
Objects (KBOs). When combined with the Centaurs, nearly 2500 objects are now
known and division into sub-categories is now possible although the nomenclature
associated with these categories is sometimes confusing (Fig. 1.13).
The Centaurs are now defined as orbiting between the orbits of Jupiter and
Neptune and having T J > 3.05. They are a sub-category of the cis-Neptunian objects
(Table 1.2) which includes, in addition, the Neptune Trojans—the objects in a 1:1
resonance with Neptune (of which 22 were known per 31 Dec 2019). TransNeptunian Objects (TNOs) are any minor planet with a semi-major axis greater
than the orbit of Neptune but excluding objects associated with the Oort cloud
(Table 1.2). The TNO category can itself be sub-divided into two main categories.
Objects, with semi-major axes between that of Neptune and about 50 AU, and with
low eccentricities and low inclinations, form a group that have relatively stable
orbits. In Fig. 1.13, these objects (as defined by the Minor Planet Center and marked
as diamonds) all fall within the area delinated by the broken line on the eccentricity
versus semi-major axis graph. Some of these objects, Pluto being the prototype, have
perihelion distances closer than the orbit of Neptune. Pluto is not perturbed by
Fig. 1.13 The semi-major
axes of 2478 TransNeptunian Objects (marked
as diamonds), Scattered
Disc objects and Centaurs
(marked as squares) in the
Minor Planet Center
database (per 31 Dec. 2016)
expressed against their
orbital eccentricity
1.2 Orbits and Origins
17