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 some of the dwarf planets, the Kuiper Belt, or scattered disk objects, trans-Neptunian objects (aka 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.
The Solar Wind, having passed though the inner regions of the Solar System undergoes its most radical transformation as it encounters the Interstellar Medium. A complex structure no doubt exists. Among the implications of the Heliosphere is the 22 yr 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. Among the functions of the heliopause is to generally deflect cosmic rays, possibly originate some cosmic rays, and generally divert the charged, and to a lesser extent the neutral, components of the interstellar medium.
The orbiting objects, the diffuse environment they move through, and temporary influences from beyond, do interact.
The Voyager and Pioneer space probes are traveling through this region over these decades. Only one object of the Solar System so far is probable to travel back and forth over the heliopause - 90377 Sedna. Several objects the size of Pluto are known and it is possible larger bodies exist in this outer region. Other objects may well exist that cross the heliopause as well, and already known objects might be identified to cross it as well once its structure is more certain (there should be a large asymmetry in the shape of the heliopause - a tail should point down-wind with the flow of the Interstellar Medium for a very long distance but probably still less than 1 LY but the specific geometry hasn't been identified yet.) 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
More traditional, the graph on the left represents polar and ecliptic views of the (aligned) orbits of the scattered disk objects (in black) on the background of cubewanos (in blue) and resonant (2:5) objects (in green). As yet unclassified objects in 50-100AU region are plotted in grey 1.
Sedna has a highly elliptical orbit, with its aphelion estimated at 975 AU and its perihelion at about 76.16 AU. At its discovery it was approaching perihelion at about 90 AU from the Sun. It was the farthest from the Sun that any solar system object had up to then been observed, although some objects like long-period comets originally observed at closer distances were most likely further from the Sun than Sedna but too dim to be observed. Eris was later detected at 97 AU.
Sedna's orbit takes about 12000 years. It will reach perihelion in 2075 or 2076.
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 (http://www.planetary.brown.edu/~noble/Myresearch.html) and Beth E. Clark (http://www.agu.org/sci_soc/EISclark.html and http://baritone.tn.cornell.edu/~beth/) 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 occuring 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].