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This page deals with those parts of the solar system that are less well known to us, and until recent advances in earth- and satellite-based telescopes were almost a complete mystery. The major components of the inner solar system mostly have their own individual pages which can be accessed from the main solar system page. Also included there are the dwarf planets, because one (Ceres) is in the main asteroid belt and another (Pluto) was classified as a planet until recently.
So this page describes that group and all the other solar system objects beyond Neptune, the “Trans-Neptunian Objects”:
The Kuiper Belt, The Oort Cloud, The Hills Cloud, Plutinos, Cubewanos, Twotinos, Centaurs, Plutoids, the Heliosphere, the Solar Wind, the Edge of the Heliosphere, and the Bow Shock, the Heliosheath, the Heliopause, the Hydrogen Wall, the Interstellar Medium and the Scattered Disc. It also includes so-called Detached Objects.
I’d’ve liked to include pictures of some of these objects, but unfortunately the best we can manage is a tiny pixellated image from Hubble or one of the other excellent telescopes, in space or high in the mountains of Chile or Hawaii, or an artist’s impression of the object, painted using the latest scientific data available. So the only thumbnail pictures that appear here are diagrams or graphs or whatever else best illustrates the matter under consideration.
The most distant observed object in the Solar System is a trans-Neptunian object (TNO) with a radius roughly half that of Pluto or somewhat smaller. V774104 (its present name until its orbital elements are known) is so far from the Sun, with an observation arc of just a few weeks at the time of its discovery announcement (at the November 2015 meeting of the American Astronomical Society’s Division for Planetary Sciences), that its perihelion and aphelion have not been accurately determined. Currently it is approximately 103 AU (15.4 billion km) from the Sun. V774104 was discovered on 13th October 2015 by a team using the Subaru Telescope, a large reflecting telescope at the summit of Mauna Kea with a primary mirror 8 m in diameter. The discovery team was led by Scott Sheppard and Chad Trujillo.
[These three diagrams can be enlarged by clicking]
A trans-Neptunian object (TNO) is any object in the Solar System that orbits the Sun at a greater average distance (semi-major axis of its orbit) than Neptune. The first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second TNO orbiting the Sun directly, (15760) 1992 QB1, with only the discovery of Pluto’s moon Charon in 1978 before that. Now over 1375 TNOs appear on the Minor Planet Center’s List Of Transneptunian Objects. As of 2015, two hundred of these have their orbits well-enough determined that they have been given permanent minor planet designations. Wikipedia has a current List Of Trans-Neptunian Objects which includes their known moons.
In the first two diagrams, the horizontal axis represents the mean distance from the Sun (in Astronomical Units) and the orbital period (in years); the upper diagram also shows resonances with Neptune’s orbital period (e.g. the plutinos at “2│3” which means that the resonance between Neptune and the plutino is such that while the plutino makes two complete orbits of the Sun, Neptune makes three). The vertical axis indicates the inclination of the orbit to the ecliptic (“i” in degrees). “D” indicates the diameter of the object and “H” (with an open circle) its absolute magnitude.
The third diagram is an artistic comparison of the eight brightest TNOs: Eris, Pluto, Makemake, Haumea, Sedna, 2007 OR10, Quaoar and Orcus, and their known moons, with a segment of the Earth for comparison.
Five of these TNOs are known to have one or more moons. 2007 OR10 is estimated to be larger than Sedna. 2002 TC302, 2002 MS4 and Salacia are estimated to be larger than Quaoar and Orcus, but are less bright due to lower albedos. As Quaoar and Orcus have moons, it is known that Quaoar is much more massive than Orcus. The top four are IAU-accepted dwarf planets while the bottom four are dwarf-planet candidates that are accepted as dwarf planets by some astronomers. These eight TNOs have the brightest absolute magnitudes, although several other TNOs have been found to be physically larger than Orcus, and several more may yet be found.
The largest known TNOs are Eris and Pluto, followed by Makemake and Haumea. The Kuiper belt, scattered disc, and Oort cloud are three conventional divisions of this volume of space, though treatments vary and a few objects such as 90377 Sedna do not fit easily into any division, and are termed “detached objects”.
The 1:2 resonance with Neptune appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.
Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU, so this sudden drastic falloff, known as the Kuiper cliff, was completely unexpected, and its cause, to date, is unknown. In 2003 evidence was found that the rapid decline in objects of 100 km or more in radius beyond 50 AU is real, and not due to observational bias. Possible explanations include that material at that distance is too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those which did form. It has been claimed that the gravitational attraction of an unseen large planetary object, perhaps the size of Earth or Mars, might be responsible.
[Above, click to enlarge] Centaurs and other TNOs. This plot displays the known positions of objects in the outer Solar System within 60 astronomical units (AU) of the Sun. Epoch as of 1stJanuary 2015.
Legend: Sun; Jupiter trojans (6,178); Giant planets: Jupiter (J), Saturn (S), Uranus (U) and Neptune (N); Centaurs (44,000); Kuiper belt (>1,000); Scattered disc; Neptune trojans (9)
Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets; 443 have been recorded. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years. There may be 44,000 centaurs with diameters larger than 1 km. Saturn’s moon Phoebe, may be a captured centaur. Three centaurs have been found to display cometary comas and are classified as both asteroids and comets. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
The first centaur-like object to be discovered was (944) Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of (2060) Chiron in 1977. The largest known centaur is (10199) Chariklo, discovered in 1997, which at 260 km in diameter is as big as a medium-sized main-belt asteroid.
No centaur has been photographed up close, although there is evidence that Saturn’s moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of (8405) Asbolus.
Three centaurs have been found to display cometary comas: Chiron, (60558) Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore both asteroids and comets. Other centaurs such as (52872) Okyrhoe are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
Centaurs are believed to be Kuiper Belt bodies that migrated into the inner solar system. Centaurs are found between the asteroid belt and the Kuiper Belt, and are considered a kind of intermediate type of small body, neither an asteroid nor a Kuiper Belt object. If Phoebe is indeed a captured Centaur, images and scientific data of Phoebe taken by the Cassini spacecraft will give scientists the first opportunity to study a Centaur.
Centaurs are of extreme interest to scientists because they are believed to be primordial; that is, they appear to date from the formation of the solar system. These objects are the building blocks of the solar system, the leftovers that never pulled into a planet. And because of its relative small size, Phoebe might never have heated up enough to change its chemical composition – which increases the scientific value of its study.
[Right, click to enlarge] Plot of known Kuiper belt objects, set against the four gas giants
Legend: Sun; Giant planet; Trojan of Jupiter;
Trojan of Neptune; Kuiper belt object;
Scattered disc object or Centaur.
Axes list distances in AU, projected onto the ecliptic, with ecliptic longitude zero being to the right; radial “spokes” of higher density in this image, or gaps in particular directions are due to observational bias, rather than any real physical structure; the gap at the bottom is due to obscuration by the band of the Milky Way
The Kuiper belt (rhymes with “viper”) is a region beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger (twenty times as wide and twenty to two hundred times as massive. Like the asteroid belt, it consists mainly of small bodies, remnants from the Solar System’s formation though the Kuiper objects are composed largely of frozen volatile material (termed ices), such as methane, ammonia and water. The belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, are thought to have come from the region.
The belt was discovered in 1992; the number of known Kuiper belt objects (KBOs) is over a thousand, and more than 70,000 KBOs over 100 km in diameter are believed to exist.
“TNOs are Cool”: A Survey of the Transneptunian Region IV is a technical scientific paper that analyses the size/albedo characterization of 15 scattered disc and detached objects observed with the Herschel Space Observatory’s Photodetector Array Camera and Spectrometer (PACS). [It is a 20-page PDF manuscript for Astronomy & Astrophysics, dated 8th February 2012, © ESO 2012.]
Pluto is the largest known Kuiper belt object, if the scattered disc is excluded. Originally considered a planet, Pluto’s position as part of the Kuiper belt has caused it to be reclassified as a dwarf planet. It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as plutinos. In Pluto’s honour, the four currently accepted dwarf planets beyond Neptune’s orbit are called plutoids.
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs).
The Oort cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and Scattered Disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth of the Oort cloud’s distance. The outer limit of the Oort cloud defines the boundary of the Solar System and the region of the Sun’s gravitational dominance.
The Oort cloud is thought to have two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud (the Hills Cloud – not to be confused with the Hill sphere). Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. Astronomers believe that the matter composing the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System’s evolution.
Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way Galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.
The semi-major axes and inclinations of all known scattered-disc objects (in blue) up to 100 AU together with Kuiper-belt objects (in grey) and resonant objects (in green). The eccentricity of the orbits is represented by segments (extending from the perihelion to the aphelion) with the inclination represented on the Y-axis.
The Scattered Disc is a distant region of the Solar System that is sparsely populated by icy minor planets. SDOs have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 AU (4.5×109 km). These extreme orbits are believed to be the result of gravitational “scattering” by the gas giant planets, and the objects continue to be subject to perturbation by Neptune. While the nearest distance to the Sun approached by scattered objects is about 30 to 35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects among the most distant and cold objects in the Solar System.
The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects, the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper. Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets observed in the Solar System; the centaurs are the intermediate stage in an object’s migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many Oort-cloud objects are also believed to have originated in the scattered disc.
Planet 9: Secret, dark world could be hiding in our solar system including NASA’s most stunning pictures of space; and Evidence suggests huge ninth planet exists past Pluto at solar system’s edge including a 
3-minute video explanation by two Caltech scientists, Konstantin Batygin and Mike Brown.
Between the 2:3 and 1:2 resonances with Neptune, at approximately 42 to 48 AU, the gravitational influence of Neptune is negligible, and objects can exist with their orbits essentially unmolested. This region is known as the “classical Kuiper belt”, and its members comprise roughly two thirds of KBOs observed to date. Unlike Pluto they do not cross Neptune’s orbit, that is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets. There are 896 recorded. Because the first modern KBO discovered, (15760) 1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as cubewanos (“Q-B-1-os“). The guidelines established by the IAU demand that classical KBOs be given names of mythological beings associated with creation. Similar objects found later were often called “QB1-o”s, or cubewanos, after this object, though the term classical is much more frequently used in the scientific literature.
Objects identified as cubewanos include: (15760) 1992 QB1, Makemake (the largest known cubewano and a dwarf planet), (50000) Quaoar and (20000) Varuna, (19521) Chaos, (58534) Logos, (53311) Deucalion, (66652) Borasisi, (88611) Teharonhiawako, (33001) 1997 CU29, (55636) 2002 TX300, (55565) 2002 AW197 and (55637) 2002 UX25, but Haumea no longer is because it has a highly elliptical orbit that is in a fifth-order 7:12 resonance with Neptune with the perihelion distance of 35 AU being near the limit of stability with Neptune. See Quaoar for some cubewano orbits.
The classical Kuiper belt appears to be a composite of two separate populations. The first, known as the “dynamically cold“ population, has orbits much like the planets; nearly circular, with an orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). The second, the “dynamically hot“ population, has orbits much more inclined to the ecliptic, by up to 30°. The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up. The two populations not only possess different orbits, but different colours; the cold population is markedly redder than the hot. If this is a reflection of different compositions, it suggests they formed in different regions. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, has been proposed to have formed more or less in its current position, although it might also have been later swept outwards by Neptune during its migration, particularly if Neptune’s eccentricity was transiently increased. While the Nice model appears to be able to at least partially explain a compositional difference, it has also been suggested the colour difference may reflect differences in surface evolution.
(307261) 2002 MS4 is a large trans-Neptunian object (TNO), the second-largest known object in the Solar System without a name, after 2007 OR10. It was discovered in 2002 by Chad Trujillo and Michael E. Brown. It is classified as a cubewano by the Minor Planet Center.
Mike Brown’s website lists it as nearly certain to be a dwarf planet. The Spitzer Space Telescope estimated it to have a diameter of 726±123 km. The Herschel team estimates it to be 934±47 km, which would make it one of the 10 largest TNOs currently known and large enough to be considered a dwarf planet under the current definition. It is currently 47.2 AU from the Sun and will come to perihelion around 2122.
It has been observed 46 times, with precovery (short for “pre-discovery recovery”) images back to 1954.
The heliosphere is the volume of space around the Sun, dominated by the solar wind. It is a bubble in space “blown” into the Interstellar Medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. The heliosheath area is not smooth but filled with magnetic “bubbles”.
The solar wind consists of particles (ionized atoms from the solar corona) and fields (in particular, magnetic fields). As the Sun rotates once in approximately 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun’s magnetic field are carried outward by the solar wind and can produce magnetic storms in the Earth’s own magnetosphere with spectacular aurorae in polar regions.
The heliosheath is the region of the heliosphere beyond the termination shock. Here the wind is slowed, compressed and made turbulent by its interaction with the interstellar medium. Its distance from the Sun is approximately 80 to 100 astronomical units (AU) at its closest point.
The heliopause is the theoretical boundary where the Sun’s solar wind is stopped by the Interstellar Medium, where the solar wind’s strength is no longer great enough to push back the stellar winds of the surrounding stars. The crossing by a spacecraft of the heliopause should be signalled by a sharp drop in the temperature of charged particles, a change in the direction of the magnetic field, and an increase in the quantity of galactic cosmic rays. In May 2012, Voyager 1 detected a rapid increase in such cosmic rays (a 9% increase in a month, following a more gradual increase of 25% from January 2009 to January 2012), suggesting it was approaching the heliopause.
According to one hypothesis, there is a region of hot hydrogen, the hydrogen wall between the bow shock and the heliopause. The wall is interstellar material interacting with the edge of the heliosphere.
As of June 2011, the heliosheath area is thought to be filled with “magnetic bubbles” (each about 1 AU wide), creating a “foamy zone”. The theory helps explain in situ heliosphere measurements by the two Voyager probes.
Just before its conference on 11thJune 2008, the IAU announced in a press release that the term plutoid would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be of near-spherical shape.
An interesting paper, in English, that is not too technical is Physical and dynamical characteristics of icy “dwarf planets” (plutoids) by Gonzalo Tancredi of the Departamento Astronomía, Facultad de Ciencias, Montevideo, Uruguay and the Observatorio Astronómico Los Molinos, Ministerio de Educación y Cultura, Uruguay.
A plutino, of which there are 244 recorded, is a trans-Neptunian object in 2:3 mean motion resonance with Neptune, with a characteristic semi-major axis of about 39.4 AU. For every two orbits that a plutino makes, Neptune orbits three times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix “-ino”. The name refers only to the orbital resonance and does not imply common physical characteristics; it was invented to describe those bodies smaller than Pluto (hence the diminutive) following similar orbits. The class includes Pluto itself and its moons.
Many plutinos, including Pluto, have orbits which cross that of Neptune, though their resonance means they can never collide. Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune. IAU guidelines dictate that all plutinos must, like Pluto, be named after underworld deities.
Plutinos form the inner part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects (KBOs). Plutinos are the largest class of the resonant trans-Neptunian objects (i.e. bodies in orbital resonances with Neptune). Apart from Pluto itself (and Charon), the first plutino, 1993 RO, was discovered on 16th September 1993.
One interesting plutino I stumbled upon is (47171) 1999 TC36 which is a triple system. It was discovered in 1999 and a moon was found in 2002. Combined observations by the infrared Spitzer Space Telescope, Herschel Space Telescope and the Hubble Space Telescope in 2009 revealed that the primary is itself composed of two similar-sized components. This central pair has a semi-major axis of around 867 km and a period of about 1.9 days. Assuming equal albedos of about 0.079, the primary components are approximately A1=272+17
−19 km and A2=251+16
−17 km in diameter, giving an effective system diameter of 393.1+25.2
−26.8 km. The component B orbits the barycenter of the A1+A2 system. The system mass estimated from the motion of the component B is (12.75±0.06)×1018 kg. The orbital motion of the A1 and A2 components gives somewhat a higher estimated mass of (14.20±0.05)×1018 kg. The discrepancy is probably related to unaccounted gravitational interactions of the components in a complex triple system. [S D Benecchi, K S Noll, W M Grundy, and H F Levison, accepted to Icarus, 9th December 2009]
The 1:2 resonance (whose objects complete half an orbit for each of Neptune’s) corresponds to semi-major axes of about 47.7 AU is sparsely populated. Its residents are sometimes referred to as twotinos. Other resonances also exist at 3:4, 3:5, 4:7 and 2:5. Neptune possesses a number of Trojan objects, which occupy its L4 and L5 points; gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are often described as being in a 1:1 resonance with Neptune and typically have very stable orbits.
Additionally, there is a relative absence of objects with semi-major axes below 39 AU which cannot apparently be explained by the present resonances. The currently accepted hypothesis for the cause of this is that as Neptune migrated outward, unstable orbital resonances moved gradually through this region, and thus any objects within it were swept up, or gravitationally ejected from it.
For the first ten billion kilometres, the solar wind travels at over a million km/h. As it begins to interact with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind slows down is the termination shock, the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the Sun) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. The termination shock is believed to be 75 to 90 AU from the Sun. Voyager 1 reached the termination shock on 23rd to 24th May 2005, and Voyager 2 reached it on 30th August 2007 says NASA.
The termination shock arises because solar wind particles are emitted from stars at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. (The exact speed depends on the density, which fluctuates considerably.) The interstellar medium, although very low in density, nonetheless has a constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the star. As one moves far enough away from the star, the pressure from the interstellar medium becomes sufficient to slow the solar wind down to below its speed of sound; this causes a shock wave.
The point where the interstellar medium, travelling in the opposite direction, slows down as it collides with the heliosphere is the bow shock. However, by 2012, it was found the Sun has no bow shock. Previously it was supposed that the Sun had one produced in its travels within the interstellar medium, resembling the wake left by a ship’s bow and is formed for similar reasons, though of plasma instead of water. Bow shocks occur if the interstellar medium and the solar wind are moving supersonically “toward” and “away”, with respective to the sun. When the interstellar wind hits the heliosphere it slows and creates a region of turbulence. This phenomenon has been observed outside our solar system by the Galaxy Evolution Explorer satellite (GALEX, 2003-017A), an Earth-orbiting ultraviolet space telescope launched in 2003. [In May 2012, GALEX operations were transferred to Caltech; an effort is trying to complete its All-Sky UV Survey; its unique ultraviolet observations shed new light on special studies of galaxies, black-holes, supernova, stars, and beyond.] The red giant star Mira in the constellation Cetus has been shown to have both a comet-like debris tail of ejecta from the star and a distinct bow shock preceding it in the direction of its movement through space (at over 130 km/sec).
The Detached objects are a dynamical class of bodies in the outer Solar System beyond the orbit of Neptune. These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are only moderately affected by it and the other planets; this makes them appear to be “detached” from the Solar System.
In this way, they differ substantially from the majority of the known trans-Neptunian objects (TNOs), which form a loosely defined set of objects that have been perturbed to varying degrees onto their current orbit by gravitational encounters with the gas giants, predominantly Neptune. Detached objects have larger perihelia than these other TNO populations, including the objects in orbital resonance with Neptune, such as Pluto, the classical Kuiper belt objects in non-resonant orbits such as Makemake, and the scattered disc objects like Eris.
At least nine such bodies have been positively identified, of which the largest, most distant, and best known is Sedna. Those with perihelia greater than 75 AU (in what has been called the ‘inner’ Oort cloud) are termed sednoids. As of 2014, there are two known sednoids, Sedna and 2012 VP113.
The interstellar medium is the matter that exists between star systems in a galaxy. It includes gas as ions, atoms, and molecules, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. Space is not “empty” nor a “vacuum”. It is composed of multiple phases, distinguished by whether matter is in the form of ions, atoms, or molecules, and its temperature and density. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure, and are typically more important dynamically than the thermal pressure.
In all phases, the interstellar medium is extremely dilute by terrestrial standards. In cool, dense regions, matter is primarily in molecular form, and reaches densities of 106 molecules/cm3. In hot, diffuse regions, it is primarily ionized, and the density may be as low as 10−4 ions/cm3. By mass, 99% is gas, and 1% is dust. Of the gas, 89% of atoms are hydrogen and 9% are helium, with 2% of atoms being heavier elements. The hydrogen and helium are from primordial nucleosynthesis (after the Big Bang), while the heavier elements are due to enrichment in the process of stellar evolution.
We’ve taken a good look at the solar system, from the Sun, by way of the inner rocky planets, the asteroids, and the gas giant planets. We have seen the Kuiper belt objects and its newly found dwarf planets, the Oort cloud and the Hills Cloud. Now we have reached the end of our solar system, and it’s time to take a look at some of our neighbours.