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Outer solar system starts at the orbit of Neptune.

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The outer Solar System is that part of the Solar System which begins at roughly the orbit of Neptune. The outer Solar System 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 of the outer Solar System.

Outer solar system.
Artist's rendering of the Outer solar system, Kuiper Belt and more distant Oort cloud.
solar system.
The heliopause is the boundary between the heliosphere and the interstellar medium outside the Solar System. As the solar wind approaches the heliopause, it slows suddenly, forming a shock wave.
outer Solar System ecliptic views.
More traditional graph 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.

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, various populations of objects like the Kuiper belt, 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. 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 93AU distance from the Sun. Among the phenomena 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. 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 over these decades. 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 of the outer Solar System.

outer Solar System orbits.
This graph represents polar and ecliptic views of the (aligned) orbits of the classical objects - that is, cubewanos - in blue, together with the plutinos in red, and Neptune (yellow).
outer Solar System astronomical objects.
Panels showing the location of Sedna in relation to other astronomical objects. Sedna has a highly elliptical orbit, with its aphelion estimated at 975 AU and its perihelion at about 76.16 AU. Sedna's orbit takes about 12000 years. It will reach perihelion in 2075 or 2076. Image courtesy of NASA / JPL-Caltech / R. Hurt.

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 and 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, and 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. 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.

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.

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