- The Kuiper belt
and the Oort cloud
were both proposed as hypothetical
reservoirs (or population) of
rocky-icy bodies
(i.e., icy planetesimals,
protoplanets)
beyond the
Neptune orbit
mean orbital radius
30.11 AU.
- The Kuiper belt
and Oort cloud were hypothesized
in order resupply, respectively,
short-period comets
and long-period comets.
As we discuss below,
comets of either sort can only exist
for a short period of time compared to the
Solar-System age = 4.6 Gyr.
The Oort cloud
(which more of a swarm than a cloud) was first proposed by
Jan Oort (1900--1992)
in 1950
(see Wikipedia: Jan Oort:
A few of Oort's discoveries).
By the by, my colleague
UNLV astronomy
professor
George Rhee was
a graduate student
of Oort in
the 1980s when
Oort was still
very research active.
- Short-period comets
have orbital periods less than
200
years and have
orbital planes
somewhat concentrated near the ecliptic plane.
- Long-period comets
have orbital periods ranging from
200
years
to thousands
of years and
their orbital planes
have a
spherically symmetric
distribution.
- Both types of comets originate when
astronomical perturbations
perturb members of the appropriate reservoir
into highly eccentric
elliptical orbits.
The astronomical perturbations
can be
gravity assists
by other reservoir members,
collisions (with probably fragmentation) with other reservoir members,
and gravitational perturbations
by a passing star on
a near flyby.
- Comets, as aforesaid,
can only exist for a small fraction of
Solar-System age = 4.6 Gyr.
They lose volatiles with
every trip through the
inner Solar System
(which includes a perihelion necessarily)
and eventually become
extinct comets.
So comets must be resupplied from
reservoirs: i.e., the Kuiper belt
and the Oort cloud.
- What happens to extinct comets?
Extinct comets
are just
Near-Earth asteroids (NEAs)
(see
Wikipedia:
Extinct comet: Distinction between comets and asteroids).
So their fate is the same as for other
NEAs.
A NEA
eventually impacts
some other
Solar System
astro-body
(Sun,
planet,
moon,
etc.)
or, via a gravity assist,
is launched back into a long-lasting orbit probably
entirely beyond the Neptune orbit
or ejected from the Solar System
on an escape trajectory
(AKA escape orbit):
i.e.,
a parabolic trajectory
or hyperbolic trajectory.
- The Kuiper belt
has been confirmed since 1992
by the
discovery
over a 1000
of Kuiper Belt objects (KBOs)
(see Wikipedia: Kuiper Belt).
Though Pluto
(discovery
1930)
and its moon
Charon
(discovery
1978)
are KBOs) their
discoveries are NOT
considered the discovery
of the Kuiper belt.
How many KBOs are there?
In fact, it's hard to do an exact count of
known KBOs since
there is tendency in references to lump all
trans-Neptunian objects (TNOs)
together in lists
(see Wikipedia: List of trans-Neptunian objects).
It is estimated that there over 10**5
KBOs with
diameter over ∼ 100 km
(see Wikipedia: Kuiper Belt).
We now know that the Kuiper belt
extends approximately from 30 AU to 50 AU from the
Sun and has
approximately a torus shape (but all
KBOs
orbit
the Sun, of course).
In some respects it is similar to the
Asteroid Belt,
but it is much larger: ∼ 20
times as wide and with
20 to
200 times more
mass.
Also, its members probably have a much higher fraction of
ices
than asteroids
which are more
carbonaceous and/or rocky and/or metallic
at least on the surface.
- The Oort cloud, on the other hand,
has NOT been confirmed to exist observationally since
NO Oort cloud object (OCO)
has ever been discovered.
OCOs
are very hard to identify: they are very dim, tiny, slow-moving far-off
astronomical objects
and they exist against a background of jillions of dim
stars which to
1st order look
exactly the same as
OCOs.
Note that KBOs, which are similar
astronomical objects
to OCOs,
are only 30 to 50 AU away and they are hard to discover.
The upshot is that the Oort cloud
is likely to remain unobserved system for a long time to come, but
it certainly exists.
- However, the distribution of
long-period comets
allows some inferences about the
Oort cloud.
It is an approximately
spherically-symmetric
reservoir of
rocky-icy bodies
(i.e., icy planetesimals,
protoplanets,
and fragments there of) extending approximately
50,000 to 200,000 astronomical units (AU) = 1.49597870700*10**11 m
(0.8 to 3.2 light-years (ly) = 0.94607304725808*10**16 m = 63241.0770 ... AU = 0.306601393 ... pc ≅ 0.3066 pc;
0.24 to 0.97 parsecs (pc) = 3.08567758 ... *10**16 m = 206264.806 ... AU = 3.26156377 ... ly ≅ 3.26 ly)
from the Sun.
The image shows the Oort cloud
with some structure: probably this is based on some analysis.
It is estimated that the Oort cloud may contain
10**11 to 10**12 OCOs
of at least a few kilometers in size scale that can become
long-period comets
(Se-569;
HI-248).
The OCOs probably did NOT form
in situ.
It is thought that the concentration of dust and gas was probably
too low in situ
in the
primordial solar nebula
to have brought about coalescence
into largish bodies like the
OCOs.
Instead it is thought that the rocky-icy bodies
that formed among the
gas giant planets were kicked
via gravity assists
by the gas giant planets
far out to the Oort cloud region
(Se-573).
- Because the Oort cloud is
approximately
spherically symmetric
long-period comets can come into the
inner Solar System from
any direction.
They are NOT confined to the vicinity of the
ecliptic plane
as are most short-period comets
that originate in the Kuiper belt.
- Keywords:
asteroids,
astronomical objects,
astronomical unit (AU) = 1.49597870700*10**11 m,
Charon,
comets
(extinct comets,
long-period comets,
short-period comets),
eccentricity,
elliptical orbits,
Kuiper belt,
mean orbital radius,
Near-Earth asteroids (NEAs),
orbital inclination
(to the ecliptic plane),
Oort cloud,
orbital period,
planet,
planetesimal,
Pluto,
protoplanet,
rocky-icy bodies,
Solar System
(inner Solar System,
outer Solar System),
Sun.
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