The
Solar System[a] consists of the
Sun and its
planetary system of eight
planets, their
moons, and
other non-stellar objects. It
formed 4.6 billion years ago from the gravitational collapse of a giant
molecular cloud. The vast majority of the system's
mass is in the Sun, with most of the remaining mass contained in
Jupiter. The four smaller inner planets,
Mercury,
Venus,
Earth and
Mars, also called the
terrestrial planets, are primarily composed of rock and metal. The four outer planets, called the
gas giants, are substantially more massive than the terrestrials. The two largest,
Jupiter and
Saturn, are composed mainly of hydrogen and helium; the two outermost planets,
Uranus and
Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called
ices, such as water, ammonia and
methane, and are often referred to separately as "ice giants". All planets have almost circular orbits that lie within a nearly flat disc called the
ecliptic plane.
The Solar System also contains a number of regions populated by
smaller objects. The
asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it mostly contains objects composed of rock and metal. Beyond Neptune's orbit lie the
Kuiper belt and
scattered disc; linked populations of
trans-Neptunian objects composed mostly of ices. Within these populations, several dozen to more than ten thousand objects may be large enough to have been rounded by their own gravity.
[9] Such objects are referred to as
dwarf planets. Identified dwarf planets include the asteroid
Ceres and the trans-Neptunian objects
Pluto,
Eris,
Haumea, and
Makemake. In addition to these two regions, various other small-body populations including
comets,
centaurs and
interplanetary dust freely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by
natural satellites,
[c] usually termed "moons" after Earth's
Moon. Each of the outer planets is encircled by
planetary rings of dust and other small objects.
Discovery and exploration
Planets of the Solar System to scale. Jupiter and Saturn (top row), Uranus and Neptune (top middle), Earth and Venus (bottom middle), Mars and Mercury
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the centre of the
universeand categorically different from the divine or ethereal objects that moved through the sky. Although the
Greek philosopher
Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos,
[10]Nicolaus Copernicus was the first to develop a mathematically predictive
heliocentric system.
[11] His 17th-century successors,
Galileo Galilei,
Johannes Kepler and
Isaac Newton, developed an understanding of
physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of
unmanned spacecraft have enabled the investigation of geological phenomena such as
mountains and
craters, and seasonal meteorological phenomena such as
clouds,
dust storms and
ice caps on the other planets.
Structure and composition
The
orbits of the bodies in the Solar System to scale (clockwise from top left)
Solar System showing the plane of the Earth's orbit around the Sun in 3D. Mercury, Venus, Earth, and Mars are shown in both panels; the right panel also shows Jupiter making one full revolution with Saturn and Uranus making less than one full revolution.
The principal component of the Solar System is the Sun, a
G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
[12] The Sun's four largest orbiting bodies, the
gas giants, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%.
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the
ecliptic. The planets are very close to the ecliptic while comets and
Kuiper belt objects are frequently at significantly greater angles to it.
[16][17]All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole).
[18] There are
exceptions, such as
Halley's Comet.
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions. The
inner Solar System includes the four terrestrial planets and the asteroid belt. The
outer Solar System is beyond the asteroids, including the four gas giants.
[19] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.
[20]
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called
natural satellites, or moons (two of which are larger than the planet
Mercury), or, in the case of the four
gas giants, by
planetary rings; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in
synchronous rotation, with one face permanently turned toward their parent.
Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an
ellipse with the Sun at one
focus. Objects closer to the Sun (with smaller
semi-major axes) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its
perihelion, while its most distant point from the Sun is called its
aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using
numerical models.
Although the Sun dominates the system by mass, it accounts for only about 2% of the
angular momentum[21] due to the differential rotation within the gaseous Sun.
[22] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.
[21]
Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33
astronomical units (AU)
[e] farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the
Titius–Bode law),
[23] but no such theory has been accepted.
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium.
[24] Jupiter and
Saturn, which comprise nearly all the remaining matter, possess atmospheres composed of roughly 99% of those same elements.
[25][26] A composition gradient exists in the Solar System, created by heat and
light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.
[27] The boundary in the Solar System beyond which those volatile substances could condense is known as the
frost line, and it lies at roughly 5 AU from the Sun.
[4]
The objects of the inner Solar System are composed mostly of
rock,
[28] the collective name for compounds with high melting points, such as
silicates, iron or nickel, that remained solid under almost all conditions in the
protoplanetary nebula.
[29] Jupiter and Saturn are composed mainly of
gases, the astronomical term for materials with extremely low melting points and high
vapor pressure such as
molecular hydrogen,
helium, and
neon, which were always in the gaseous phase in the nebula.
[29] Ices, like
water,
methane,
ammonia,
hydrogen sulfide and
carbon dioxide,
[28] have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.
[29] They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.
[29] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "
ice giants") and the numerous small objects that lie beyond Neptune's orbit.
[28][30] Together, gases and ices are referred to as
volatiles.
[31]
A number of
Solar System models on Earth attempt to convey the relative scales involved in the Solar System on human terms. Some models are mechanical — called
orreries — while others extend across cities or regional areas.
[32] The largest such scale model, the
Sweden Solar System, uses the 110-metre
Ericsson Globe in
Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5 metre sphere at
Arlanda International Airport, 40 km away, while the farthest current object, Sedna, is a 10-cm sphere in
Luleå, 912 km away.
[33][34]
Range of selected bodies of the
Solar System from the middle of the Sun. The left and right edges of each bar correspond to the
perihelion and
aphelion of the body, respectively. Long bars denote high
orbital eccentricity.
Formation and evolution
Artist's concept of the early Solar System
The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large
molecular cloud.
[35] This initial cloud was likely several light-years across and probably birthed several stars.
[36] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the
pre-solar nebula,
[37]collapsed, conservation of
angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.
[36] As the contracting nebula rotated faster, it began to flatten into a
protoplanetary disc with a diameter of roughly 200
AU[36] and a hot, dense
protostar at the centre.
[38][39] The planets formed by
accretion from this disc,
[40] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover
minor bodies.
Due to their higher boiling points, only metals and silicates could exist in the warm inner Solar System close to the Sun, and these would form the rocky planets of Mercury, Venus, Earth, and Mars. Since metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The gas giants (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the
asteroid belt,
Kuiper belt, and
Oort cloud. The
Nice model is an explanation for the creation of these regions, and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.
Within 50 million years, the pressure and density of
hydrogen in the centre of the protostar became great enough for it to begin
thermonuclear fusion.
[41] The temperature, reaction rate, pressure, and density increased until
hydrostatic equilibrium was achieved: the thermal pressure equaled the force of gravity. At this point the Sun became a
main-sequence star.
[42] Solar wind from the Sun created the
heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.
The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5.4 billion years from now. This will mark the end of the Sun's main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly up to 260 times its current diameter and the Sun will become a
red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).
[43] The expanding Sun is expected to vaporize Mercury and Venus and render the Earth uninhabitable, as the
habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will fall away into space, leaving a
white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.
[44] The ejected outer layers will form what is known as a
planetary nebula, returning some of the material that formed the Sun—but now enriched with
heavier elements like carbon—to the interstellar medium.
Sun
The Sun is classified as a type G2
yellow dwarf, but this name is misleading as, compared to the majority of stars in
our galaxy, the Sun is rather large and bright.
[48] Stars are classified by the
Hertzsprung–Russell diagram, a graph that plots the brightness of stars with their surface
temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the
main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as
red dwarfs, are common, making up 85% of the stars in the galaxy.
[48][49]
Evidence suggests that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history its brightness was 70% of that of it today.
[50]
The Sun is a
population I star; it was born in the later stages of the
universe's evolution, and thus contains more elements heavier than hydrogen and helium ("
metals" in astronomical parlance) than older population II stars.
[51]Elements heavier than hydrogen and helium were formed in the
cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a
planetary system, because planets form from accretion of "metals".
[52]
Interplanetary medium
Earth's magnetic field stops
its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space.
[58] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into the Earth's upper atmosphere, where its interactions create
aurorae seen near the
magnetic poles.
Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the
interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.
[59]
The interplanetary medium is home to at least two disc-like regions of
cosmic dust. The first, the
zodiacal dust cloud, lies in the inner Solar System and causes
zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.
[60] The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the
Kuiper belt.
[61][62]
Inner Solar System
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.
[63]Composed mainly of
silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between the orbits of Jupiter and Saturn.
Inner planets
The inner planets. From left to right:
Mercury,
Venus,
Earth, and
Mars in true-colour (sizes to scale, interplanetary distances not)
Mercury
- Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.[64]Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.[65] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.[66][67]
Venus
- Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses), and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount ofgreenhouse gases in the atmosphere.[68] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.[69]
Earth
- Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the Solar System where life is known to exist.[70] Its liquidhydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[71] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.
Mars
- Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of the Earth).[72] Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[73] Its red colour comes from iron oxide (rust) in its soil.[74] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.[75]
Asteroid belt
Main article:
Asteroid belt
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.
[77]
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids except the largest, Ceres, are classified as small Solar System bodies.
[78]
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.
[79]Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth.
[15] The asteroid belt is very sparsely populated;
spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10
−4 m are called
meteoroids.
[80]
Ceres
Ceres (2.77 AU) is the largest asteroid, a
protoplanet, and a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified to asteroid in the 1850s as further observations revealed additional asteroids.
[81] It was classified as a dwarf planet in 2006.
Asteroid groups
Asteroids in the asteroid belt are divided into
asteroid groups and
families based on their orbital characteristics.
Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains
main-belt comets, which may have been the source of Earth's water.
[82]
Jupiter trojans are located in either of Jupiter's
L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point.
Hilda asteroids are in a 2:3
resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.
[83]
The inner Solar System is also dusted with
rogue asteroids, many of which cross the orbits of the inner planets.
[84]
Outer Solar System
The outer region of the Solar System is home to the gas giants and their large moons. Many short-period comets, including the
centaurs, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.
Outer planets
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, "ice giants".
[85] All four gas giants have
rings, although only Saturn's ring system is easily observed from Earth. The term
outer planet should not be confused with
superior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.
Jupiter
- Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogenand helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot.
- Jupiter has 67 known satellites. The four largest, Ganymede,Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.[86] Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn
- Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. The rings of Saturn are made up of small ice and rock particles.
- Saturn has 62 confirmed satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.[87] Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
Uranus
- Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.[88]
- Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.
Neptune
- Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.[89]
- Neptune has 13 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[90]Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number ofminor planets, termed Neptune trojans, that are in 1:1 resonance with it.
Centaurs
The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). The largest known centaur,
10199 Chariklo, has a diameter of about 250 km.
[91] The first centaur discovered,
2060 Chiron, has also been classified as comet (95P) because it develops a coma just as comets do when they approach the Sun.
[92]
Comets
Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to
sublimate and
ionise, creating a
coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as
Hale–Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the
Kreutz Sungrazers, formed from the breakup of a single parent.
[93] Some comets with
hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.
[94] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.
[95]
Trans-Neptunian region
The area beyond Neptune, or the "
trans-Neptunian region", is still
largely unexplored. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.
Kuiper belt
Main article:
Kuiper belt
Plot of all Kuiper belt objects known in 2007, set against the four outer planets
The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.
[96] It extends between 30 and 50 AU from the Sun. Though it is thought to contain dozens of dwarf planets, it is composed mainly of small Solar System bodies. Many of the larger Kuiper belt objects, such as
Quaoar,
Varuna, and
Orcus, may be recognized as dwarf planets with further data. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.
[14] Many Kuiper belt objects have multiple satellites,
[97] and most have orbits that take them outside the plane of the ecliptic.
[98]
The Kuiper belt can be roughly divided into the "
classical" belt and the
resonances.
[96] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU.
[99]Members of the classical Kuiper belt are classified as
cubewanos, after the first of their kind to be discovered,
(15760) 1992 QB1, and are still in near primordial, low-eccentricity orbits.
[100]
Pluto and Charon
The dwarf planet
Pluto (39 AU average) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal
definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
Charon, Pluto's largest moon, is sometimes described as part of a
binary system with Pluto, as the two bodies orbit a
barycentre of gravity above their surfaces (i.e., they appear to "orbit each other"). Beyond Charon, four much smaller moons,
P5,
Nix,
P4, and
Hydra are known to orbit within the system.
Pluto has a 3:2
resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called
plutinos.
[101]
Makemake and Haumea
Makemake (45.79 AU average), while smaller than Pluto, is the largest known object in the
classical Kuiper belt(that is, it is not in a confirmed
resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was named and designated a dwarf planet in 2008.
[6] Its orbit is far more inclined than Pluto's, at 29°.
[102]
Haumea (43.13 AU average) is in an orbit similar to Makemake except that it is caught in a 7:12 orbital resonance with Neptune.
[103] It is about the same size as Makemake and has two natural satellites. A rapid, 3.9-hour rotation gives it a flattened and elongated shape. It was named and designated a dwarf planet in 2008.
[104]
Scattered disc
The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of
Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some have aphelia farther than 150 AU from the Sun). SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects".
[105] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.
[106]
Eris
Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is 25% more massive than Pluto
[107] and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon,
Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.
Farthest regions
The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this
heliopause is considered the beginning of the
interstellar medium.
[54] However, the Sun's
Hill sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.
[108]
Heliopause
Energetic neutral atoms map of heliosheath and heliopause by
IBEX. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.
The heliosphere is divided into two separate regions. The solar wind travels at roughly 400 km/s until it collides with the
interstellar wind; the flow of plasma in the
interstellar medium. The collision occurs at the
termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.
[109] Here the wind slows dramatically, condenses and becomes more turbulent,
[109] forming a great oval structure known as the
heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; but evidence from the Cassini and
Interstellar Boundary Explorerspacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field.
[110] Both
Voyager 1 and
Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.
[111][112] The outer boundary of the heliosphere, the
heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.
[54]
The shape and form of the outer edge of the heliosphere is likely affected by the
fluid dynamics of interactions with the interstellar medium
[109] as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the
bow shock, a plasma "wake" left by the Sun as it travels through the
Milky Way.
[113]
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that
NASA's
Voyager spacecraft will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.
[114] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.
[115][116]
Sedna
90377 Sedna (525.86 AU average) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete.
Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the
scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object
2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.
[117] Brown terms this population the "inner Oort cloud", as it may have formed through a similar process, although it is far closer to the Sun.
[118] Sedna is very likely a dwarf planet, though its shape has yet to be determined.
