The Solar System[a] is the gravitationally bound system comprising the
Sun and the objects that orbit it, either directly or indirectly.[b]
Of those objects that orbit the
Sun directly, the largest eight are
the planets,[c] with the remainder being smaller objects, such as
dwarf planets and small
Solar System bodies. Of the objects that orbit
Sun indirectly, the moons, two are larger than the smallest
Solar System formed 4.6 billion years ago from the
gravitational collapse of a giant interstellar molecular cloud. The
vast majority of the system's mass is in the Sun, with the majority of
the remaining mass contained in Jupiter. The four smaller inner
planets, Mercury, Venus,
Earth and Mars, are terrestrial planets,
being primarily composed of rock and metal. The four outer planets are
giant planets, being substantially more massive than the terrestrials.
The two largest,
Jupiter and Saturn, are gas giants, being composed
mainly of hydrogen and helium; the two outermost planets,
Neptune, are ice giants, being composed mostly of substances with
relatively high melting points compared with hydrogen and helium,
called volatiles, such as water, ammonia and methane. All eight
planets have almost circular orbits that lie within a nearly flat disc
called the ecliptic.
Solar System also contains smaller objects.[e] The asteroid belt,
which lies between the orbits of
Mars and Jupiter, mostly contains
objects composed, like the terrestrial planets, of rock and metal.
Beyond Neptune's orbit lie the
Kuiper belt and scattered disc, which
are populations of trans-Neptunian objects composed mostly of ices,
and beyond them a newly discovered population of sednoids. Within
these populations are several dozen to possibly tens of thousands of
objects large enough that they have been rounded by their own
gravity. Such objects are categorized as dwarf planets. Identified
dwarf planets include the asteroid Ceres and the trans-Neptunian
Pluto and Eris.[e] In addition to these two regions, various
other small-body populations, including comets, centaurs and
interplanetary dust clouds, freely travel between regions. Six of the
planets, at least four of the dwarf planets, and many of the smaller
bodies are orbited by natural satellites,[f] usually termed "moons"
after the Moon. Each of the outer planets is encircled by planetary
rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from
the Sun, creates a bubble-like region in the interstellar medium known
as the heliosphere. The heliopause is the point at which pressure from
the solar wind is equal to the opposing pressure of the interstellar
medium; it extends out to the edge of the scattered disc. The Oort
cloud, which is thought to be the source for long-period comets, may
also exist at a distance roughly a thousand times further than the
Solar System is located in the Orion Arm, 26,000
light-years from the center of the Milky Way.
1 Discovery and exploration
2 Structure and composition
2.1 Distances and scales
3 Formation and evolution
5 Interplanetary medium
6 Inner Solar System
6.1 Inner planets
7 Outer Solar System
7.1 Outer planets
9 Trans-Neptunian region
9.1 Kuiper belt
Pluto and Charon
Makemake and Haumea
9.2 Scattered disc
10 Farthest regions
10.2 Detached objects
10.3 Oort cloud
11 Galactic context
11.2 Comparison with extrasolar systems
12 Visual summary
13 See also
16 External links
Discovery and exploration
Main article: Discovery and exploration of the Solar System
Andreas Cellarius's illustration of the Copernican system, from the
Harmonia Macrocosmica (1660)
For most of history, humanity did not recognize or understand the
concept of the Solar System. Most people up to the Late Middle
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
Aristarchus of Samos had speculated on a heliocentric reordering of
Nicolaus Copernicus was the first to develop a
mathematically predictive heliocentric system. In the 17th
century, Galileo Galilei, Johannes Kepler, and
Isaac Newton developed
an understanding of physics that led to the gradual acceptance of the
Earth moves around the
Sun and that the planets are governed
by the same physical laws that govern Earth. The invention of the
telescope led to the discovery of further planets and moons.
Improvements in the telescope and the use of unmanned spacecraft have
enabled the investigation of geological phenomena, such as mountains,
craters, seasonal meteorological phenomena, such as clouds, dust
storms and ice caps on the other planets.
Structure and composition
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 giant planets, account for 99% of the remaining mass, with
Saturn together comprising more than 90%. The remaining
objects of the
Solar System (including the four terrestrial planets,
the dwarf planets, moons, asteroids, and comets) together comprise
less than 0.002% of the Solar System's total mass.[g]
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, whereas 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
rotating (counter-clockwise, as viewed from above Earth'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 mostly rocky asteroids, and four giant planets surrounded by
Kuiper belt of mostly 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 giant planets. 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.
The eight planets of the
Solar System (by decreasing size) are
Jupiter, Saturn, Uranus, Neptune, Earth, Venus,
Mars and Mercury.
Most of the planets in the
Solar System have secondary systems of
their own, being orbited by planetary objects called natural
satellites, or moons (two of which, Titan and Ganymede, are larger
than the planet Mercury), and, in the case of the four giant planets,
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.
All planets of the
Solar System lie very close to the ecliptic. The
closer they are to the Sun, the faster they travel (inner planets on
the left, all planets except
Neptune on the right).
Kepler's laws of planetary motion
Kepler's laws of planetary motion describe the orbits of objects about
the Sun. Following Kepler's laws, each object travels along an ellipse
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
Sun is called its perihelion, whereas its most distant point from
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.
Sun dominates the system by mass, it accounts for only
about 2% of the angular momentum. 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, are also primarily
composed of hydrogen and helium. 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.
composed mainly of gases, the astronomical term for materials with
extremely low melting points and high vapour pressure, such as
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. They can be found as ices, liquids, or gases in various
places in the Solar System, whereas 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
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
Earth to the
Sun is 1 astronomical unit
(150,000,000 km), or AU. For comparison, the radius of the
0.0047 AU (700,000 km). Thus, the
Sun occupies 0.00001%
(10−5 %) of the volume of a sphere with a radius the size of
Earth's orbit, whereas Earth's volume is roughly one millionth
(10−6) 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), whereas the most distant planet,
Neptune, is 30 AU (4.5×109 km) from the Sun.
With a few exceptions, the farther a planet or belt is from the Sun,
the larger the distance between its orbit and the orbit of the next
nearer object to the Sun. For example,
Venus is approximately 0.33 AU
farther out from the
Sun than Mercury, whereas
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.
Solar System models attempt to convey the relative scales
involved in the
Solar System on human terms. Some are small in scale
(and may be mechanical—called orreries)—whereas others extend
across cities or regional areas. The largest such scale model, the
Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe
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, whereas 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 100 metres, then the Sun
would be about 3 cm in diameter (roughly two-thirds the diameter
of a golf ball), the giant planets would be all smaller than about
3 mm, and Earth's diameter along with that of the other
terrestrial planets would be smaller than a flea (0.3 mm) at this
The Solar System. Distances are to scale, objects are not.
Distances of selected bodies of the
Solar System from the Sun. The
left and right edges of each bar correspond to the perihelion and
aphelion of the body, respectively, hence long bars denote high
orbital eccentricity. The radius of the
Sun is 0.7 million km, and the
Jupiter (the largest planet) is 0.07 million km, both too
small to resolve on this image.
Formation and evolution
Main article: Formation and evolution of the Solar System
Solar System formed 4.568 billion years ago from the
gravitational collapse of a region within a large molecular cloud.[h]
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
Artist's concept of the early Solar System
Due to their higher boiling points, only metals and silicates could
exist in solid form in the warm inner
Solar System close to the Sun,
and these would eventually form the rocky planets of Mercury, Venus,
Earth, and Mars. Because metallic elements only comprised a very small
fraction of the solar nebula, the terrestrial planets could not grow
very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune)
formed further out, beyond the frost line, the point between the
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
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 equalled the force of gravity. At this point, the Sun
became a main-sequence star. The main-sequence phase, from
beginning to end, will last about 10 billion years for the Sun
compared to around two billion years for all other phases of the Sun's
pre-remnant life combined.
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
Sun is growing brighter; early in its
main-sequence life its brightness was 70% that of what it is
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 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 contract with hydrogen fusion occurring along a shell
surrounding the inert helium, and the energy output will be much
greater than at present. The outer layers of the
Sun will expand to
roughly 260 times its current diameter, and the
Sun will become a red
giant. Because of its vastly increased surface area, the surface of
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 render
Earth uninhabitable. 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 the 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 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.
Main article: Sun
Size comparison of the
Sun and the planets
Sun is the Solar System's star and by far its most massive
component. Its large mass (332,900
Earth masses) produces
temperatures and densities in its core high enough to sustain nuclear
fusion of hydrogen into helium, making it a main-sequence star.
This releases an enormous amount of energy, mostly radiated into space
as electromagnetic radiation peaking in visible light.
Sun is a G2-type main-sequence star. Hotter main-sequence stars
are more luminous. The Sun's temperature is intermediate between that
of the hottest stars and that of the coolest stars. Stars brighter and
hotter than the
Sun are rare, whereas substantially dimmer and cooler
stars, known as red dwarfs, make up 85% of the stars in the Milky
Sun is a population I star; it has a higher abundance of 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 and Solar wind
The heliospheric current sheet
The vast majority of the
Solar System consists of a near-vacuum known
as the interplanetary medium. 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 per hour, creating a tenuous
atmosphere that permeates the interplanetary medium out to at least
100 AU (see § Heliosphere). 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
Earth's magnetic field
Earth's magnetic field stops its atmosphere from being stripped away
by the solar wind.
Mars do not have magnetic fields, and
as a result the solar wind is causing 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
Earth's magnetic field funnels charged particles into Earth's upper
atmosphere, where its interactions create aurorae seen near the
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
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 gravitational
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
Solar System is the region comprising the terrestrial
planets and the asteroid belt. 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 less than the distance
between the orbits of
Jupiter and Saturn. This region is also within
the frost line, which is a little less than 5 AU (about 700 million
km) from the Sun.
Main article: Terrestrial planet
The inner planets. From left to right: Earth, Mars, Venus, and Mercury
(sizes to scale).
The four terrestrial or inner 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
Earth is (i.e. Mercury and Venus).
Main article: Mercury (planet)
Mercury (0.4 AU from the Sun) is the closest planet to the Sun
and the smallest planet in the
Solar System (0.055
Mercury has no natural satellites; besides impact craters, its only
known geological features are lobed ridges or rupes that were probably
produced by a period of contraction early in its history.
Mercury's very tenuous 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; or, that it was
prevented from fully accreting by the young Sun's energy.
Main article: 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. 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
being replenished by volcanic eruptions.
Main article: 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 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.
Main article: Mars
Mars (1.5 AU from the Sun) is smaller than
Earth masses). It has an atmosphere of mostly carbon dioxide with a
surface pressure of 6.1 millibars (roughly 0.6% of that of 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
The donut-shaped asteroid belt is located between the orbits of Mars
Asteroids except for the largest, Ceres, are classified as small Solar
System bodies[e] and are composed mainly of refractory rocky and
metallic minerals, with some ice. They range from a few metres
to hundreds of kilometres in size. Asteroids smaller than one meter
are usually called meteoroids and micrometeoroids (grain-sized),
depending on different, somewhat arbitrary definitions.
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. 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 Earth. The
asteroid belt is very sparsely populated; spacecraft routinely pass
through without incident.
Main article: Ceres (dwarf planet)
Ceres - map of gravity fields: red is high; blue, low.
Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a
dwarf planet.[e] It has a diameter of slightly under 1,000 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 1801, and was reclassified to asteroid in the 1850s as further
observations revealed additional asteroids. It was classified as a
dwarf planet in 2006 when the definition of a planet was created.
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
Solar System also contains near-
Earth asteroids, many of
which cross the orbits of the inner planets. Some of them are
potentially hazardous objects.
Outer Solar System
The outer region of the
Solar System is home to the giant planets and
their large moons. The centaurs and many short-period comets 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 those of the inner
Solar System because the lower temperatures allow these compounds to
Outer planets and Giant planet
From top to bottom: Neptune, Uranus, Saturn, and
Jupiter (Montage with
approximate colour and relative size)
The four outer planets, or giant planets (sometimes called Jovian
planets), collectively make up 99% of the mass known to orbit the
Saturn are together more than 400 times the mass
Earth and consist overwhelmingly of hydrogen and helium;
Neptune are far less massive (<20
Earth masses each) and are
composed primarily of ices. For these reasons, some astronomers
suggest they belong in their own category, "ice giants". All four
giant planets have rings, although only Saturn's ring system is easily
observed from Earth. The term superior planet designates planets
outside Earth's orbit and thus includes both the outer planets and
Main article: 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 semi-permanent
features in its atmosphere, such as cloud bands and the Great Red
Jupiter has 69 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.
Main article: 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
Saturn is the only
planet of the
Solar System that is less dense than water. The
Saturn are made up of small ice and rock particles. Saturn
has 62 confirmed satellites composed largely of ice. Two of these,
Titan and Enceladus, show signs of geological activity. 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.
Main article: Uranus
Uranus (19.2 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 giant planets and radiates very little
heat into space.
Uranus has 27 known satellites, the largest ones
being Titania, Oberon, Umbriel, Ariel, and Miranda.
Main article: Neptune
Neptune (30.1 AU), though slightly smaller than Uranus, is more
massive (equivalent to 17 Earths) and hence 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 several minor planets, termed
Neptune trojans, that are in 1:1
resonance with it.
Main article: Centaur (minor planet)
The centaurs are icy comet-like bodies whose orbits have semi-major
axes 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.
Hale–Bopp seen in 1997
Main article: Comet
Comets are small
Solar System bodies,[e] 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 thought to originate in the Kuiper belt,
whereas long-period comets, such as Hale–Bopp, are thought 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.
Beyond the orbit of
Neptune lies the area of the "trans-Neptunian
region", with the doughnut-shaped Kuiper belt, home of
several other dwarf planets, and an overlapping disc of scattered
objects, which is tilted toward the plane of the
Solar System and
reaches much further out than the Kuiper belt. The entire region is
still largely unexplored. It appears to consist overwhelmingly of many
thousands of small worlds—the largest having a diameter only a fifth
Earth and a mass far smaller than that of the Moon—composed
mainly of rock and ice. This region is sometimes described as the
"third zone of the Solar System", enclosing the inner and the outer
Main article: Kuiper belt
Known objects in the Kuiper belt
Size comparison of some large TNOs with Earth:
Pluto and its moons,
Eris, Makemake, Haumea, Sedna, 2007 OR10, Quaoar, and Orcus.
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
estimated to contain anything from dozens to thousands of dwarf
planets, it is composed mainly of small
Solar System bodies. Many of
Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may
prove to be dwarf planets with further data. There are estimated to be
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 Earth. Many Kuiper
belt objects have multiple satellites, and most have orbits that
take them outside the plane of the ecliptic.
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
Kuiper belt are classified as cubewanos, after the first
of their kind to be discovered,
15760 Albion (which previously had the
provisional designation 1992 QB1), and are still in near primordial,
Pluto and Charon
Pluto and Charon (moon)
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.
Pluto has a 3:2 resonance with Neptune,
Pluto orbits twice round the
Sun for every three
Kuiper belt objects whose orbits share this
resonance are called plutinos.
Charon, the largest of Pluto's moons, 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, orbit within the system.
Makemake and Haumea
Makemake and Haumea
Makemake (45.79 AU average), although smaller than Pluto, is the
largest known object in the classical
Kuiper belt (that is, a Kuiper
belt object not in a confirmed resonance with Neptune).
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 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.
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 thought 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
Kuiper belt but aphelia far beyond it (some more 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
Kuiper belt objects along with the outward-scattered
residents of the scattered disc.
Main article: Eris (dwarf planet)
Eris (68 AU average) is the largest known scattered disc object,
and caused a debate about what constitutes a planet, because 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.
Sun to the nearest star: The
Solar System on a logarithmic
scale in astronomical units (AU)
The point at which the
Solar System ends and interstellar space begins
is not precisely defined because its outer boundaries are shaped by
two separate forces: the solar wind and the Sun's gravity. The limit
of the solar wind's influence is roughly four times Pluto's distance
from the Sun; this heliopause, the outer boundary of the heliosphere,
is considered the beginning of the interstellar medium. The Sun's
Hill sphere, the effective range of its gravitational dominance, is
thought to extend up to a thousand times farther and encompasses the
theorized Oort cloud.
Main article: Heliosphere
The bubble-like heliosphere with its various transitional regions
moving through the interstellar medium
The heliosphere is a stellar-wind bubble, a region of space dominated
by the Sun, which radiates at roughly 400 km/s its solar wind, a
stream of charged particles, until it collides with the wind of the
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
thought 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; evidence from Cassini and Interstellar
Boundary Explorer spacecraft has suggested that it is forced into a
bubble shape by the constraining action of the interstellar magnetic
The outer boundary of the heliosphere, the heliopause, is the point at
which the solar wind finally terminates and is the beginning of
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.
Voyager 1 is reported
to have crossed the heliopause in August 2012.
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
it travels through the Milky Way.
Zooming out the Solar System:
Solar System and Jupiter
Solar System and Pluto
orbit of Sedna (detached object)
inner part of the Oort Cloud
Due to a lack of data, conditions in local interstellar space are not
known for certain. It is expected that NASA's Voyager spacecraft, as
they pass the heliopause, will transmit valuable data on radiation
levels and solar wind to Earth. How well the heliosphere shields
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 object and Sednoid
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 because 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, sometimes termed "distant detached objects"
(DDOs), 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. Sedna is very likely a
dwarf planet, though its shape has yet to be determined. The second
unequivocally detached object, with a perihelion farther than Sedna's
at roughly 81 AU, is 2012 VP113, discovered in 2012. Its aphelion is
only half that of Sedna's, at 400–500 AU.
Main article: Oort cloud
Schematic of the hypothetical Oort cloud, with a spherical outer cloud
and a disc-shaped inner cloud
Oort cloud is a hypothetical spherical cloud of up to a trillion
icy objects that is thought 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 thought 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.
See also: Vulcanoid, Planets beyond Neptune, and
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
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.
Currently, the furthest known objects, such as
Comet West, have
aphelia around 70,000 AU from the Sun, but as the
Oort cloud becomes
better known, this may change.
Diagram of the
Milky Way with the position of the
Solar System marked
by a yellow arrow
Solar System is located in the Milky Way, a barred spiral galaxy
with a diameter of about 100,000 light-years containing about
100 billion stars. The
Sun resides in one of the Milky Way's
outer spiral arms, known as the
Orion–Cygnus Arm or Local Spur.
Sun lies between 25,000 and 28,000 light-years from the Galactic
Centre, and its speed within the
Milky Way is about
220 km/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
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.[i]
The Solar System's location in the
Milky Way is a factor in the
evolutionary history 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. Because spiral arms are home to a far larger concentration of
supernovae, gravitational instabilities, and radiation that 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
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 speculated 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.
Beyond the heliosphere is the interstellar medium, consisting of
various clouds of gases. The
Solar System currently moves through the
Local Interstellar Cloud.
Solar System is in the
Local Interstellar Cloud
Local Interstellar Cloud or Local Fluff. It
is thought to be near the neighbouring
G-Cloud but it is not known if
Solar System is embedded in the Local Interstellar Cloud, or if it
is in the region where the
Local Interstellar Cloud
Local Interstellar Cloud and
Local Interstellar Cloud
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 (ly) across. The bubble is suffused with high-temperature
plasma, that suggests it is the product of several recent
There are relatively few stars within ten light-years of the Sun. The
closest is the triple star system Alpha Centauri, which is about 4.4
Alpha Centauri A and B are a closely tied pair of
Sun-like stars, whereas the small red dwarf, Proxima Centauri, orbits
the pair at a distance of 0.2 light-year. In 2016, a potentially
habitable exoplanet was confirmed to be orbiting Proxima Centauri,
Proxima Centauri b, the closest confirmed exoplanet to the
Sun. The stars next closest to the
Sun are the red dwarfs
Star (at 5.9 ly),
Wolf 359 (7.8 ly), and Lalande
21185 (8.3 ly).
The largest nearby star is Sirius, a bright main-sequence star roughly
8.6 light-years away and roughly twice the Sun's mass and that is
orbited by a white dwarf,
Sirius B. The nearest brown dwarfs are the
Luhman 16 system at 6.6 light-years. Other systems within ten
light-years are the binary red-dwarf system
Luyten 726-8 (8.7 ly)
and the solitary red dwarf
Ross 154 (9.7 ly). The closest
solitary Sun-like star to the
Solar System is
Tau Ceti at 11.9
light-years. It has roughly 80% of the Sun's mass but only 60% of its
luminosity. The closest known free-floating planetary-mass object
Sun is WISE 0855−0714, an object with a mass less than
Jupiter masses roughly 7 light-years away.
A diagram of Earth's location in the observable Universe. (Click here
for an alternate image.)
Comparison with extrasolar systems
Compared to other planetary systems the
Solar System stands out in
lacking planets interior to the orbit of Mercury. The known
Solar System also lacks super-Earths (
Planet Nine could be a
Earth beyond the known Solar System). Uncommonly, it has
only small rocky planets and large gas giants; elsewhere planets of
intermediate size are typical—both rocky and gas—so there is no
"gap" as seen between the size of
Earth and of
Neptune (with a radius
3.8 times as large). Also, these super-Earths have closer orbits than
Mercury. This led to hypothesis that all planetary systems start
with many close-in planets, and that typically a sequence of their
collisions causes consolidation of mass into few larger planets, but
in case of the
Solar System the collisions caused their destruction
The orbits of
Solar System planets are nearly circular. Compared to
other systems, they have smaller orbital eccentricity. Although
there are attempts to explain it partly with a bias in the
radial-velocity detection method and partly with long interactions of
a quite high number of planets, the exact causes remain
This section is a sampling of
Solar System bodies, selected for size
and quality of imagery, and sorted by volume. Some omitted objects are
larger than the ones included here, notably Eris, because these have
not been imaged in high quality.
(moon of Jupiter)
(moon of Saturn)
(moon of Jupiter)
(moon of Jupiter)
(moon of Earth)
(moon of Jupiter)
(moon of Neptune)
Kuiper belt object)
(moon of Uranus)
(moon of Saturn)
(moon of Uranus)
(moon of Saturn)
(moon of Pluto)
(moon of Uranus)
(moon of Uranus)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Uranus)
(moon of Neptune)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Saturn)
(moon of Mars)
(moon of Mars)
Voyager 1 views the
Solar System from over 6 billion km from
Earth ("Pale Blue Dot"), Jupiter, Saturn, Uranus,
Book: Solar System
Outline of the Solar System
HIP 11915 (a solar analog whose planetary system contains a Jupiter
Lists of geological features of the Solar System
List of gravitationally rounded objects of the Solar System
Solar System extremes
Solar System in fiction
Capitalization of the name varies. The International Astronomical
Union, the authoritative body regarding astronomical nomenclature,
specifies capitalizing the names of all individual astronomical
objects, but uses mixed "Solar System" and "solar system" in their
naming guidelines document. The name is commonly rendered in lower
case ("solar system"), as, for example, in the Oxford English
Dictionary and Merriam-Webster's 11th Collegiate Dictionary.
^ The natural satellites (moons) orbiting the Solar System's planets
are an example of the latter.
^ Historically, several other bodies were once considered planets,
including, from its discovery in 1930 until 2006, Pluto. See Former
^ The two moons larger than Mercury are Ganymede, which orbits
Jupiter, and Titan, which orbits Saturn. Although bigger than Mercury,
both moons have less than half the mass of Mercury.
^ a b c d e According to IAU definitions, objects orbiting the
classified dynamically and physically into three categories: planets,
dwarf planets, and small
Solar System bodies.
A planet is any body orbiting the
Sun whose mass is sufficient for
gravity to have pulled it into a (near-)spherical shape and that has
cleared its immediate neighbourhood of all smaller objects. By this
Solar System has eight planets: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and Neptune. Because it has not cleared
its neighbourhood of other
Kuiper belt objects,
Pluto does not fit
A dwarf planet is a body orbiting the
Sun that is massive enough to be
made near-spherical by its own gravity but that has not cleared
planetesimals from its neighbourhood and is also not a satellite.
Pluto is a dwarf planet and the IAU has recognized four other dwarf
planets in the Solar System: Ceres, Haumea, Makemake, and Eris.
Other objects commonly (but not officially) treated as dwarf planets
include 2007 OR10, Sedna, Orcus, and Quaoar. In a reference to
Pluto, other dwarf planets orbiting in the trans-Neptunian region are
sometimes called "plutoids".
The remaining objects orbiting the
Sun are known as small Solar System
List of natural satellites
List of natural satellites of the
Solar System for the full list
of natural satellites of the eight planets and first five dwarf
^ a b The mass of the
Solar System excluding the Sun,
Saturn can be determined by adding together all the calculated masses
for its largest objects and using rough calculations for the masses of
Oort cloud (estimated at roughly 3
Earth masses), the Kuiper
belt (estimated at roughly 0.1
Earth mass) and the asteroid belt
(estimated to be 0.0005
Earth mass) for a total, rounded upwards,
Earth masses, or 8.1% of the mass in orbit around the Sun. With
the combined masses of
subtracted, the remaining ~6
Earth masses of material comprise 1.3% of
the total orbiting mass.
^ The date is based on the oldest inclusions found to date in
−0.4 million years, and is thought to be the date of the formation
of the first solid material in the collapsing nebula.
is the angle between the north pole of the ecliptic and the north
galactic pole then:
displaystyle cos psi =cos(beta _ g )cos(beta _ e )cos(alpha _ g
-alpha _ e )+sin(beta _ g )sin(beta _ e )
displaystyle beta _ g
= 27° 07′ 42.01″ and
displaystyle alpha _ g
= 12h 51m 26.282 are the declination and right ascension of the north
galactic pole, whereas
displaystyle beta _ e
= 66° 33′ 38.6″ and
displaystyle alpha _ e
= 18h 0m 00 are those for the north pole of the ecliptic. (Both pairs
of coordinates are for
J2000 epoch.) The result of the calculation is
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