HTCs (with one now missing) and are plotted in Fig. 1.10. The inclinations of the
orbits appear to be well distributed.
There is also a population of comets with near-parabolic orbits (0.985 < e x < 1.0)
with periods greater than 200 years. These comets also appear to show a random
distribution in inclination. In addition, over 320 comets have been computed to have
hyperbolic orbits. However, these orbits are the osculating orbits—the gravitational
Kepler orbits (i.e. the conics) that the objects would have about the Sun if perturbations were not present computed from observations close to their perihelion passage.
Oort, in his quite remarkable paper from 1950, used the value 1/a s as a proxy for the
orbital energy. If the orbits are integrated backwards taking the perturbations into
account, then the distribution in orbital energy changes significantly to produce a
spike in the distribution corresponding to semi-major axes between 27,000 AU and
36,000 AU (or roughly ½ light year) depending on the selection criteria used (Dones
et al. 2004). The source of these comets is assumed to be an isotropic cloud of comets
surrounding our Sun now referred to as the Oort cloud. In defining the distance to,
and breadth of, the Oort cloud, there remain numerous issues. In particular, gravitational perturbations may be dominated by non-gravitational forces resulting from
outgassing from the surface near perihelion which in turn affects our knowledge of
the original orbit. Furthermore, the gravitational forces arising from the galactic
mass distribution (so-called galactic tides) and passing giant molecular clouds
(GMCs) and stars are also significant as has been recently demonstrated in a series
of papers by Fouchard et al. (see, for example, Fouchard et al. 2017a, b). Given the
uncertainties in the properties of the Oort cloud, definitions of its characteristics such
as that given by Brasser and Schwamb (2015) in which a s > 250 AU and a
perihelion distance of >45 AU, are necessarily broad and generally reflect the idea
that the Oort cloud should not be influenced by planet-related perturbing forces.
Fouchard et al. distinguish between objects that “creep” into the planetary domain
with multiple giant planet perturbations and those that “jump” into the planetary
system with a rapid reduction in perihelion distance. This work has also shown
several important results, including
– that planetary perturbations are primarily responsible for removal of Oort cloud
objects but that Galactic tides or stellar encounters are needed initially to bring the
perihelia into the planetary domain so that gravitational interactions can then
“kick” the objects out of the system,
– that, over the lifetime of the solar system, comet “showers” may have occurred
one or more times that have radically modified the cloud and resulted in major
mass loss from the cloud and re-shuffling of its members (Fouchard et al. 2014b),
– that a slight but significant majority of the modelled comets entering the planetary
domain should be in retrograde orbits, contrary to observation. A clear preference
for retrograde comets when the original semi-major axis is <25,000 AU was
found. However, this will depend strongly on the initial distribution of objects in
the cloud and the fundamental assumption that the cloud is isotropic (Fouchard
et al. 2017b),
– that, in general, the assumed initial properties of the Oort cloud have been shown
in models to have a strong effect on both its long term dynamics and final structure.
14
1 Light Curves, Orbits, and Reservoirs
orbits appear to be well distributed.
There is also a population of comets with near-parabolic orbits (0.985 < e x < 1.0)
with periods greater than 200 years. These comets also appear to show a random
distribution in inclination. In addition, over 320 comets have been computed to have
hyperbolic orbits. However, these orbits are the osculating orbits—the gravitational
Kepler orbits (i.e. the conics) that the objects would have about the Sun if perturbations were not present computed from observations close to their perihelion passage.
Oort, in his quite remarkable paper from 1950, used the value 1/a s as a proxy for the
orbital energy. If the orbits are integrated backwards taking the perturbations into
account, then the distribution in orbital energy changes significantly to produce a
spike in the distribution corresponding to semi-major axes between 27,000 AU and
36,000 AU (or roughly ½ light year) depending on the selection criteria used (Dones
et al. 2004). The source of these comets is assumed to be an isotropic cloud of comets
surrounding our Sun now referred to as the Oort cloud. In defining the distance to,
and breadth of, the Oort cloud, there remain numerous issues. In particular, gravitational perturbations may be dominated by non-gravitational forces resulting from
outgassing from the surface near perihelion which in turn affects our knowledge of
the original orbit. Furthermore, the gravitational forces arising from the galactic
mass distribution (so-called galactic tides) and passing giant molecular clouds
(GMCs) and stars are also significant as has been recently demonstrated in a series
of papers by Fouchard et al. (see, for example, Fouchard et al. 2017a, b). Given the
uncertainties in the properties of the Oort cloud, definitions of its characteristics such
as that given by Brasser and Schwamb (2015) in which a s > 250 AU and a
perihelion distance of >45 AU, are necessarily broad and generally reflect the idea
that the Oort cloud should not be influenced by planet-related perturbing forces.
Fouchard et al. distinguish between objects that “creep” into the planetary domain
with multiple giant planet perturbations and those that “jump” into the planetary
system with a rapid reduction in perihelion distance. This work has also shown
several important results, including
– that planetary perturbations are primarily responsible for removal of Oort cloud
objects but that Galactic tides or stellar encounters are needed initially to bring the
perihelia into the planetary domain so that gravitational interactions can then
“kick” the objects out of the system,
– that, over the lifetime of the solar system, comet “showers” may have occurred
one or more times that have radically modified the cloud and resulted in major
mass loss from the cloud and re-shuffling of its members (Fouchard et al. 2014b),
– that a slight but significant majority of the modelled comets entering the planetary
domain should be in retrograde orbits, contrary to observation. A clear preference
for retrograde comets when the original semi-major axis is <25,000 AU was
found. However, this will depend strongly on the initial distribution of objects in
the cloud and the fundamental assumption that the cloud is isotropic (Fouchard
et al. 2017b),
– that, in general, the assumed initial properties of the Oort cloud have been shown
in models to have a strong effect on both its long term dynamics and final structure.
14
1 Light Curves, Orbits, and Reservoirs