The outer Solar System (as opposed to the outer planets)
is that part of the
Solar System which begins at roughly the orbit of
Neptune and terminates at maximum orbit distance, approximately
one
light year from the sun in terms of orbital measurements. The
environment of the whole Solar System is also composed of the
Solar Wind which, in this outermost region of the solar system,
is the
heliopause where the
Solar Wind meets the
Interstellar Medium- especially the Local Interstellar Medium,
aka LISM.
Contents and Structure
The main contents of the outer Solar System are probably very numerous small
bodies akin to comets from one or another population, and the place where the
solar wind
meets the
interstellar medium.
Of the bodies that orbit the
Sun, major
constituents include the
dwarf
planets Pluto
and
Eris, various populations of objects like the
Kuiper
Belt,
scattered disk objects,
trans-Neptunian objects (TNOs) and the unconfirmed
Oort Cloud.
Work by
Michael E. Brown's team has brought many recent discoveries in this area.
From whichever population source,
Comets
frequently originate within the area, their potential orbits being disturbed by
either
Nemesis or system-wide oscillations across the
galactic plane, according to two competing theories. Each population has
some quality to distinguish them. Specific to Comets are some comet families
(comets that originated in the breakup of a parent body) like the
Kreutz Sungrazers among others which will also have some relationship with
the Outer Solar System.
There may also objects which do not properly orbit the sun. So called
hyperbolic comets are projected to be observable from Earth about 4 times
per century, give or take one and perhaps two orders of magnitude. These would
be comets ejected from other
star
systems that pass "close" to the solar system but usually ultimately pass
back out of the solar system. The outer solar system would have many more
continuously passing through. Study of such comets is complicated by the
out-gassing qualities of comets.[1]
However
Comet SWAN(C/2006 M4) from the fall of 2006 is believed to be hyperbolic.
Satellites
Pioneer 10,
Pioneer 11,
Voyager 1,
and Voyager 2
are leaving the solar system on hyperbolic orbits.
The
solar wind, having passed though the inner regions of the
Solar
System undergoes its most radical transformation as it encounters the
interstellar medium. The current mission of the
Voyager 1
and 2
spacecraft is to find and study the termination shock, heliosheath, and
heliopause. Thus far, both Voyager 1 and Voyager 2 have reached the termination
shock according to NASA
Voyager Enters Solar System's Final Frontier at approximately 93 AU from the
Sun.
Among the phenomena of the heliosphere is the 22 year
sunspot cycle of the Sun, which represents high and low flow periods (in
addition to the smaller oscillations caused by
coronal mass ejections which would continue to propagate out as part of the
solar wind). Another strong factor is the condition, movement and density of the
interstellar medium which could radically change the extent and functions of the
heliosphere. The resulting extent and orientation of the plasma environment of
the outer solar system is still largely unknown. The
Voyager and
Pioneer space probes are travelling through this region and will be followed
by New
Horizons.
Two natural objects of the
Solar
System so far are probable to travel back and forth over the
heliopause
- 90377
Sedna(aphelion at 975AU) and
2000 CR105(aphelion
at 219 AU) though many more are universally expected. Neutral components of the
interstellar medium are largely unaffected by the encounter with the solar wind
until they reach the Inner Solar System, roughly equivalent to the
inner
planets aka
terrestrial planets and
Sun outward to the
asteroid belt where the
Sun's light is
sufficient to cause
ionization.
Thus in a sense there are two kinds of "winds" in the outer solar system - a
charged and magnetized particle and
EM field emanating from
the sun and undergoing changes in flow as it reaches the outer solar system, and
a neutral expanse of gas and dust from the interstellar medium blowing across
the whole solar system. Other members of this region are micrometeorites and
cosmic
rays. Some
cosmic
rays may originate with the bow shockwave.
The outer solar system measures on a scale straddling that of interstellar
and interplanetary distances.
Orbit plots
Influence and Evolution
It has been proposed that the
Gas giant
planets evolved in direct relation to the outer solar system; that the former's
orbits significantly changed by interactions with
planetesimals and
comets from the
latter (many of these objects being scattered back into the outer solar system.)
Concordantly, the evolution of the outer solar system appears to have been
influenced by nearby
supernovae
and possibly also passage through
interstellar dust clouds. The surfaces of bodies in the outer solar system,
would experience
space weathering from the
Solar Wind,
micrometeorites, as well as the neutral components of the
interstellar medium, and more momentary influences like
supernovae
and magnetar
eruptions (also called starquakes). Sarah K. Noble
research and Beth E. Clark
here and
here are among those doing research in space weathering or space erosion
though specific implications for the outer solar system aren't yet quantified.
Some lines of evidence now assert that the solar system formed in the
vicinity of nearby supernovae - perhaps more than one
[1],
[2] and
[3] and overall of the formation of the solar system occurring in an
open
cluster so that close passes of stars were more likely and could explain
some qualities of the outer solar system. Some of the issues that have yet to be
reconciled are the highly eccentric orbit of
Sedna vs the
highly circular orbit of Buffy, aka
2004 XR190.[4]
and
[5] and
[6]. Discrepancies to be resolved include the
Pioneer anomaly, and sub-galactic effects of
Dark
Matter, and
Dark
Energy.
There are also questions of more recent nearby supernovae affecting the solar
system.[7].
The Stardust
sample return from
Comet Wild-2 has also revealed some evidence that materials from the early
formation of the solar system migrated to the region of the Kuiper Belt as well
as some of the dust that existed before the solar system formed.[8]
References
-
^
A study of the original orbits of "hyperbolic" comets
Wiki Source
