Interest
The
Solar System comprises 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 are several dozen to
more than ten thousand objects that may be large enough to have been
rounded by their own gravity. 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, usually termed "moons" after Earth's
Moon. Each of the outer planets is encircled by
planetary rings of dust and other small objects.
The
solar wind, a flow of
plasma from the Sun, creates a
bubble in the
interstellar medium known as the
heliosphere, which extends out to the edge of the
scattered disc. The
Oort cloud, which is believed to be the source for
long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The
heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of
interstellar wind. The Solar System is located within one of the outer arms of the
Milky Way galaxy, which contains about 200 billion stars.
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. If the Sun-Neptune distance were
scaled
to the length of a football or soccer field of about 100 meters long,
the Sun would be less than 3 cm in diameter (about two-thirds that of a
golf ball), the gas giants would all be less than 3 mm across (smaller
than a
BB pellet) and Earth's diameter and the other terrestrial planets would be less than 0.3 mm (smaller than a flea).
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
universe and 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,
Nicolaus Copernicus was the first to develop a mathematically predictive
heliocentric system. 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. 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. 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). 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. 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.
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 because 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
due to the differential rotation within the gaseous Sun. 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.
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium.
Jupiter and
Saturn,
which comprise nearly all the remaining matter, possess atmospheres
composed of roughly 99% of those same elements. 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. 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.
The objects of the inner Solar System are composed mostly of
rock, 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. 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.
Ices, like
water,
methane,
ammonia,
hydrogen sulfide and
carbon dioxide,
have melting points up to a few hundred kelvins, while their phase
depends on the ambient pressure and temperature. 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. 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. Together, gases and ices are referred to as
volatiles.
Distances and scales
The distance from the Earth to the Sun is 1
astronomical unit (150,000,000
km). For comparison, the radius of the Sun is .0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10 %) of the volume of a sphere with a radius the size of the Earth's orbit, while the Earth's volume is roughly one million (10)
times smaller than that of the Sun. Jupiter, the largest planet, is 5.2
astronomical units (780,000,000 km) from the Sun and has a radius of
71,000 km (0.00047 AU), while the most distant planet, Neptune, is 30 AU
(4.5
×10 km) from the Sun.
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 AU 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),
but no such theory has been accepted. The images at the beginning of
this section show the orbits of the various constituents of the Solar
System on different scales.
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. The largest such scale model, the
Sweden Solar System, uses the 110-metre (361-ft)
Ericsson Globe in
Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at
Arlanda International Airport, 40 km (25 mi) away, while the farthest current object, Sedna, is a 10-cm (4-in) sphere in
Luleå, 912 km (567 mi) away.
If the Sun–Neptune distance is
scaled
to the length of a football or soccer pitch of about 100 metres, then
the Sun is about 3 cm in diameter (roughly two-thirds the diameter of a
golf ball) at one goal line with the gas giants all smaller than about
3 mm and Neptune found at the opposite goal line. Earth's diameter along
with the other terrestrial planets would be smaller than a flea
(0.3 mm) at this scale.
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.
This initial cloud was likely several light-years across and probably
birthed several stars. 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, 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. As the
contracting nebula rotated faster, it began to flatten into a
protoplanetary disc with a diameter of roughly 200
AU and a hot, dense
protostar at the centre. The planets formed by
accretion
from this disc, 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. 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. 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 (2,600 K at its coolest) than it is on the
main sequence. 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 move 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. 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 the Solar System's
star, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its
core high enough to sustain
nuclear fusion, which releases enormous amounts of
energy, mostly
radiated into
space as
electromagnetic radiation, peaking in the 400–700 nm band of
visible light.
The Sun is a type G2
main-sequence star. Compared to the majority of stars in the
Milky Way, the Sun is rather large and bright. 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, whereas substantially dimmer and cooler
stars, known as
red dwarfs, are common, making up 85% of the stars in the galaxy.
Evidence suggests that the Sun's position on the main sequence puts
it in the "prime of life" for a star, not yet having exhausted its store
of hydrogen for nuclear fusion. The Sun is growing brighter; early in
its history its brightness was 70% that of what it is today.
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 the older population II stars. 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, whereas stars born later have more. This high
metallicity is thought to have been crucial to the Sun's development of a
planetary system because the planets form from the accretion of "metals".
Interplanetary medium
The vast majority of the volume of the Solar System consists of a near-
vacuum known as the
interplanetary medium. However, along with
light, the Sun radiates a continuous stream of charged particles (a
plasma) known as the
solar wind.
This stream of particles spreads outwards at roughly 1.5 million
kilometres (932 thousand miles) per hour, creating a tenuous atmosphere
(the heliosphere) that permeates the interplanetary medium out to at
least 100 AU (see
heliopause). Activity on the Sun's surface, such as
solar flares and
coronal mass ejections, disturb the heliosphere, creating
space weather and causing
geomagnetic storms. The largest structure within the heliosphere is the
heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the 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.
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.
The heliosphere and planetary magnetic fields (for those planets that
have them) partially shield the Solar System from high-energy
interstellar particles called
cosmic rays. 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-ray penetration in the Solar System
varies, though by how much is unknown.
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 the
zodiacal light.
It was likely formed by collisions within the asteroid belt brought on
by interactions with the planets. The second dust cloud extends from
about 10 AU to about 40 AU, and was probably created by similar
collisions within the
Kuiper belt.
Inner Solar System
The inner Solar System is the traditional name for the region
comprising the terrestrial planets and asteroids. 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)
The four inner or terrestrial planets have dense,
rocky compositions, few or no
moons, and no
ring systems. They are composed largely of
refractory minerals, such as the
silicates, which form their
crusts and
mantles, and metals such as
iron and
nickel, which form their
cores. Three of the four inner planets (Venus, Earth and Mars) have
atmospheres substantial enough to generate
weather; all have
impact craters and
tectonic surface features such as
rift valleys and
volcanoes. The term
inner planet should not be confused with
inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
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.
Mercury's almost negligible atmosphere consists of atoms blasted off its
surface by the solar wind. 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.
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 (752°F), most likely due to the amount of greenhouse gases
in the atmosphere. 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.
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 the only
place where life is known to exist. Its liquid hydrosphere 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. 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). 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. Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.
Asteroid belt
Main article:
Asteroid belt
are
small Solar System bodies composed mainly of
refractory rocky and metallic
minerals, with some ice.
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.
Asteroids range in size from hundreds of kilometres across to
microscopic. All asteroids except the largest, Ceres, are classified as
small Solar System bodies.
The asteroid belt contains tens of thousands, possibly millions, of
objects over one kilometre in diameter. Despite this, the total mass of
the asteroid belt is unlikely to be more than a thousandth of that of
the Earth. The asteroid belt is very sparsely populated;
spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10 m are called
meteoroids.
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. 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.
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.
The inner Solar System is also dusted with
rogue asteroids, many of which cross the orbits of the inner planets.
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 because 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". 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 hydrogen and 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. 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.
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.
- 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.
- Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor 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. 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.
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. Some comets with
hyperbolic
orbits may originate outside the Solar System, but determining their
precise orbits is difficult. Old comets that have had most of their
volatiles driven out by solar warming are often categorised as
asteroids.
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. 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. Many Kuiper belt
objects have multiple satellites, and most have orbits that take them
outside the plane of the ecliptic.
The Kuiper belt can be roughly divided into the "
classical" belt and the
resonances.
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. 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.
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,
Styx,
Nix,
Kerberos, 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.
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. Its
orbit is far more inclined than Pluto's, at 29°.
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. 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.
Scattered disc
Main article:
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". Some astronomers also classify centaurs as inward-scattered
Kuiper belt objects along with the outward-scattered residents of the
scattered disc.
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 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. However, the Sun's
Hill sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.
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. Here the wind slows
dramatically, condenses, and becomes more turbulent, 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; however, evidence from the
Cassini and
Interstellar Boundary Explorer
spacecraft has suggested that it is in fact forced into a bubble shape
by the constraining action of the interstellar magnetic field. 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. 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.
The shape and form of the outer edge of the heliosphere is likely affected by the
fluid dynamics
of interactions with the interstellar medium 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.
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. 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.
Detached objects
90377 Sedna
(520 AU average) is a large, reddish object with a gigantic, highly
elliptical orbit that takes it from about 76 AU at perihelion to 940 AU
at aphelion and takes 11,400 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. Brown terms this population the "inner Oort
cloud" because it may have formed through a similar process, although it
is far closer to the Sun, whereas Jewitt believes these bodies were
scattered outward and then had their perihelia lifted through
interaction with planets besides Neptune. Sedna is very likely a dwarf
planet, though its shape has yet to be determined.
Oort cloud
An artist's rendering of the Oort cloud, the Hills cloud, and the Kuiper belt (inset)
The Oort cloud is a hypothetical spherical cloud of up to a trillion
icy objects that is believed to be the source for all long-period comets
and to surround the Solar System at roughly 50,000 AU (around 1
light-year
(ly)), and possibly to as far as 100,000 AU (1.87 ly). It is believed
to be composed of comets that were ejected from the inner Solar System
by gravitational interactions with the outer planets. Oort cloud objects
move very slowly, and can be perturbed by infrequent events such as
collisions, the gravitational effects of a passing star, or the
galactic tide, the
tidal force exerted by the
Milky Way.
Boundaries
Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of
surrounding stars
out to about two light years (125,000 AU). Lower estimates for the
radius of the Oort cloud, by contrast, do not place it farther than
50,000 AU. Despite discoveries such as Sedna, the region between the
Kuiper belt and the Oort cloud, an area tens of thousands of AU in
radius, is still virtually unmapped. There are also ongoing studies of
the region between Mercury and the Sun. Objects may yet be discovered in
the Solar System's uncharted regions.
In November 2012 NASA announced that as
Voyager I
approached the transition zone to the outer limit of the Solar System
its instruments detected a sharp intensification of the magnetic field.
No change in the direction of the magnetic field had occurred, which
NASA scientists then interpreted to indicate that Voyager I had not yet
left the Solar System.
Galactic context
Location of the Solar System within our
galaxy
The Solar System is located in the
Milky Way galaxy, a
barred spiral galaxy with a diameter of about 100,000
light-years containing about 200 billion stars. The Sun resides in one of the Milky Way's outer spiral arms, known as the
Orion–Cygnus Arm or Local Spur. The Sun lies between 25,000 and 28,000 light years from the
Galactic Centre, and its speed within the galaxy is about 220
kilometres per second (140 mi/s), so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's
galactic year. The
solar apex, the direction of the Sun's path through interstellar space, is near the constellation of
Hercules in the direction of the current location of the bright star
Vega. The plane of the ecliptic lies at an angle of about 60° to the
galactic plane.
The Solar System's location in the galaxy is a factor in the
evolution of
life
on Earth. Its orbit is close to circular, and orbits near the Sun are
at roughly the same speed as that of the spiral arms. Therefore, the Sun
passes through arms only rarely. Since spiral arms are home to a far
larger concentration of
supernovae,
gravitational instabilities, and radiation which could disrupt the
Solar System, this has given Earth long periods of stability for life to
evolve. The Solar System also lies well outside the star-crowded
environs of the galactic centre. Near the centre, gravitational tugs
from nearby stars could perturb bodies in the Oort Cloud and send many
comets into the inner Solar System, producing collisions with
potentially catastrophic implications for life on Earth. The intense
radiation of the galactic centre could also interfere with the
development of complex life. Even at the Solar System's current
location, some scientists have hypothesised that recent supernovae may
have adversely affected life in the last 35,000 years by flinging pieces
of expelled stellar core towards the Sun as radioactive dust grains and
larger, comet-like bodies.
Neighbourhood
The Solar System is currently located in the
Local Interstellar Cloud or Local Fluff. It is thought to be near the
G-Cloud,
but it is unknown if the Solar System is embedded in the Local
Interstellar Cloud, or if it is in the region where the Local
Interstellar Cloud and G-Cloud are interacting. The Local Interstellar
Cloud is an area of denser cloud in an otherwise sparse region known as
the
Local Bubble, an hourglass-shaped cavity in the
interstellar medium
roughly 300 light years across. The bubble is suffused with
high-temperature plasma that suggests it is the product of several
recent supernovae.
There are relatively few
stars within ten light years (95 trillion km, or 60 trillion mi) of the Sun. The closest is the triple star system
Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small
red dwarf Alpha Centauri C (also known as
Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs
Barnard's Star (at 5.9 light years),
Wolf 359 (7.8 light years), and
Lalande 21185 (8.3 light years). The largest star within ten light years is
Sirius, a bright
main-sequence star roughly twice the Sun's mass and orbited by a
white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red-dwarf system
Luyten 726-8 (8.7 light years) and the solitary red dwarf
Ross 154 (9.7 light years). The Solar System's closest solitary sun-like star is
Tau Ceti, which lies 11.9 light years away. It has roughly 80% of the Sun's mass but only 60% of its luminosity. The closest known
extrasolar planet to the Sun lies around Alpha Centauri B. Its one confirmed planet,
Alpha Centauri Bb, is at least 1.1 times Earth's mass and orbits its star every 3.236 days.
Visual summary
This section is a sampling of Solar System bodies, selected for size
and quality of imagery, and sorted by volume. A number of omitted
objects are larger than the ones included here, notably
Pluto, because these have not been imaged in high quality.
See also
Notes
References
-
^ Mike Brown (August 23, 2011). "Free the dwarf planets!". "Mike Brown's Planets (self-published)".
-
^ Sheppard, Scott S. "The Giant Planet Satellite and Moon Page". Departament of Terrestrial Magnetism at Carniege Institution for science. Retrieved 2013-07-23.
-
^ Wm. Robert Johnston (2013-07-13). "Asteroids with Satellites". Johnston's Archive. Retrieved 2013-07-23.
-
^ "How Many Solar System Bodies". NASA/JPL Solar System Dynamics. Retrieved 2013-07-23.
,
27° 07′ 42.01″ and 
12h 51m 26.282 are the declination and right ascension of the north galactic pole, while 
66° 33′ 38.6″ and 
18h 0m 00 are those for the north pole of the ecliptic. (Both pairs of coordinates are for