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The Solar System
Capitalization Capitalization (North American English North American English (NAmE, NAE) is the most generalized variety (linguistics), variety of the English language as spoken in the United States and Canada. Because of their related histories and cultur ...
of the name varies. The
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy Astronomy (from el, ἀστρονομία, literally mea ...
, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in thei
naming guidelines document
The name is commonly rendered in lower case ("solar system"), as, for example, in the ''
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary A historical dictionary or dictionary on historical principles is a dictionary which deals not only with the latterday meanings of words but also the historica ...
'' an
''Merriam-Webster's 11th Collegiate Dictionary''
is the
gravitationally Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitationally
bound system of the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
and the objects that orbit it, either directly or indirectly.The
natural satellites A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
(moons) orbiting the Solar System's
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...
are an example of the latter.
Of the objects that orbit the Sun directly, the largest are the
eight planets
eight planets
,Historically, several other bodies were once considered planets, including, from its discovery in 1930 until 2006,
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
. See
Former planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...
.
with the remainder being smaller objects, the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s and
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as foll ...
. Of the objects that orbit the Sun indirectly—the
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s—two are larger than the smallest planet,
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
, and one more almost equals it in size.The two moons larger than Mercury are , which orbits
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
, and
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
, which orbits
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
. Although bigger than Mercury, both moons have less than half its mass. In addition, the radius of Jupiter's moon Callisto is over 98% that of Mercury.
The Solar System formed 4.6 billion years ago from the
gravitational collapse Gravitational collapse is the contraction of an astronomical object In , an astronomical object or celestial object is a naturally occurring , association, or structure that exists in the . In , the terms ''object'' and ''body'' are often ...
of a giant interstellar
molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
. The vast majority of the system's
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
is in the Sun, with the majority of the remaining mass contained in
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. The four smaller
inner system
inner system
planets,
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
,
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
,
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
and
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, are
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Su ...
s, being primarily composed of rock and metal. The four outer system planets are
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s, being substantially more massive than the terrestrials. The two largest planets,
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
, are
gas giant A gas giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilib ...
s, being composed mainly of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
; the two outermost planets,
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
, are
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest el ...
s, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
, such as
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
,
ammonia Ammonia is a compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fort ...

ammonia
and
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
. All eight planets have almost circular orbits that lie within a nearly flat disc called the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

ecliptic
. The Solar System also contains smaller objects. The
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

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
Neptune's
orbit lie the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
and
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or colli ...
, which are populations of
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
s composed mostly of ices, and beyond them a newly discovered population of
sednoid A sednoid is a trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally c ...
s. Within these populations, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be. Such objects are categorized as
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s. Astronomers generally accept at least nine objects as dwarf planets: the asteroid and the trans-Neptunian objects
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
, , , , , , , and . In addition to these two regions, various other small-body populations, including
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s,
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek ...
and
interplanetary dust cloud The interplanetary dust cloud, or zodiacal cloud, consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System The Solar SystemCapitalizati ...
s, freely travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by
natural satellites A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
, usually termed "moons" after the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
. Each of the outer planets is encircled by
planetary ring A ring system is a disc or ring orbiting an astronomical object that is composed of solid material such as cosmic dust, dust and moonlets, and is a common component of satellite system (astronomy), satellite systems around giant planets. A ring ...
s of dust and other small objects. The
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
known as the
heliosphere The heliosphere is the vast, bubble-like region of space that surrounds and is created by the Sun. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuou ...
. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
; it extends out to the edge of the scattered disc. The
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of ...

Oort cloud
, which is thought to be the source for
long-period comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena a ...
s, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located 26,000 light-years from
the center The Center () is the fifth tallest skyscraper in Hong Kong, after International Commerce Centre, Two International Finance Centre (88 storeys), Central Plaza (Hong Kong), Central Plaza and Bank of China Tower (Hong Kong), Bank of China Tower. ...
of the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Milky Way
galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

galaxy
in the
Orion Arm upright=1.75, Observed structure of the Milky Way's spiral arms The Orion Arm is a minor spiral arm of the Milky Way Galaxy The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy's appearance fr ...
, which contains most of the visible stars in the
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

night sky
. The nearest stars are within the so-called
Local Bubble 200px, Model of space near the Sun: (grey) Local Bubble, (magenta) molecular clouds, (cyan) Loop I Bubble. The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. ...
, with the closest,
Proxima Centauri Proxima Centauri is a small, low-mass star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star ...

Proxima Centauri
, at 4.25 light-years.


Discovery and exploration

For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the
Late Middle Ages The Late Middle Ages or Late Medieval Period was the period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical com ...
Renaissance The Renaissance ( , ) , from , with the same meanings. is a period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in ...

Renaissance
believed Earth to be stationary at the centre of the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
and categorically different from the divine or ethereal objects that moved through the sky. Although the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
philosopher
Aristarchus of Samos Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient ...

Aristarchus of Samos
had speculated on a
heliocentric Heliocentrism is the astronomical Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics ...

heliocentric
reordering of the cosmos,
Nicolaus Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic Church, C ...

Nicolaus Copernicus
was the first to develop . In the 17th century,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific qu ...

Galileo
discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.
Christiaan Huygens Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician, physicist, astronomer and inventor, who is regarded as one of the greatest scientists of all time and a major fig ...

Christiaan Huygens
followed on from Galileo's discoveries by discovering Saturn's moon
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
and the shape of the
rings of Saturn The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely ...

rings of Saturn
. Around 1677,
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such ...

Edmond Halley
observed a
transit of Mercury A transit of Mercury across the Sun takes place when the planet Mercury (planet), Mercury passes directly (Transit (astronomy), transits) between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) t ...
across the Sun, leading him to realise that observations of the
solar parallax Parallax () is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Due to perspective (graph ...
of a planet (more ideally using the
transit of Venus A transit of Venus across the Sun takes place when the planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own ...

transit of Venus
) could be used to determine the distances between Earth,
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
, and the Sun. In 1705, Halley realised that repeated sightings of were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun, though this had been theorized about comets in the 1st century by
Seneca Seneca may refer to: People and language *Seneca (name), a list of people with either the given name or surname *Seneca the Elder, a Roman rhetorician, writer and father of the stoic philosopher Seneca the Younger *Seneca the Younger, a Roman Stoi ...
. Around 1704, the term "Solar System" first appeared in English. In 1838,
Friedrich Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astro ...
successfully measured a
stellar parallax Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time interva ...

stellar parallax
, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of
uncrewed spacecraft Uncrewed spacecraft or unmanned spacecraft are spacecraft without people on board, used for robotic spaceflight. Uncrewed spacecraft may have varying levels of autonomy from human input; they may be Remotely operated vehicle, remote controll ...
have since enabled the detailed investigation of other bodies orbiting the Sun.


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 planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Su ...
s, the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s,
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
,
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s, and
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s) together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

ecliptic
. The planets are very close to the ecliptic, whereas comets and
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
objects are frequently at significantly greater angles to it. As a result of the
formation of the Solar System The formation and evolution of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all indi ...
, planets (and most other objects) orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole). There are exceptions, such as
Halley's Comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a List of periodic comets, short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is regularly visible to the naked eye fro ...

Halley's Comet
. Most of the larger moons orbit their planets in this '' prograde'' direction (with
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

Triton
being the largest ''retrograde'' exception) and most larger objects rotate themselves in the same direction (with Venus being a notable ''retrograde'' exception). 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 the 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. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s, or moons (two of which,
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
and , are larger than the planet
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
). The four giant planets have
planetary ring A ring system is a disc or ring orbiting an astronomical object that is composed of solid material such as cosmic dust, dust and moonlets, and is a common component of satellite system (astronomy), satellite systems around giant planets. A ring ...
s, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in
synchronous rotation and Charon are tidally locked to each other. Charon is massive enough that the barycenter of Pluto's system lies outside of Pluto; thus Pluto and Charon are sometimes considered to be a binary system. Tidal locking (also called gravitational loc ...

synchronous rotation
, with one face permanently turned toward their parent.
Kepler's laws of planetary motion In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the Copernican heliocentrism, heliocentric theory of Nicolaus Copernicus, repl ...
describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an
ellipse In , an ellipse is a surrounding two , such that for all points on the curve, the sum of the two distances to the focal points is a constant. As such, it generalizes a , which is the special type of ellipse in which the two focal points are t ...

ellipse
with the Sun at one
focus FOCUS is a fourth-generation programming language (4GL) computer programming programming language, language and development environment that is used to build database queries. Produced by Information Builders Inc., it was originally developed for d ...
. 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 upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...

perihelion
'', whereas its most distant point from the Sun is called its ''
aphelion upright=1.15, The two-body system of interacting primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object consti ...

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. 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 Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

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 (astrophysics), frost line, and it lies at roughly 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, vapour pressure, such as
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
,
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
, and neon, which were always in the gaseous phase in the nebula. Ices, like
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
,
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
,
ammonia Ammonia is a compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fort ...

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 Uranus and Neptune (the so-called "
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest el ...
s") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as ''
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
''.


Distances and scales

The distance from Earth to the Sun is . For comparison, the radius of the Sun is . 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 from the Sun and has a radius of , whereas the most distant planet, Neptune, is 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. Some 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 Orrery, 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 in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, 90377 Sedna, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away. If the Sun–Neptune distance is Solar System model, scaled to , then the Sun would be about in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about , and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea () at this scale.


Formation and evolution

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large
molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
. 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 solar nebula, 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 and a hot, dense protostar at the centre. The planets formed by accretion (astrophysics), 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 small Solar System body, minor bodies. 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 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 The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
,
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
, and
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of ...

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 Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
in the centre of the protostar became great enough for it to begin nuclear fusion, 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, 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-stellar remnant, remnant life combined. Solar wind from the Sun created the
heliosphere The heliosphere is the vast, bubble-like region of space that surrounds and is created by the Sun. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuou ...
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 today. The Solar System will remain roughly as it is known 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 that 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 the Sun will be considerably cooler ( 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 metallicity, heavier elements like carbon—to the interstellar medium.


Sun

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses), which comprises 99.86% of all the mass in the Solar System, produces temperatures and densities in its solar core, core high enough to sustain nuclear fusion of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
into
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
, making it a main sequence, main-sequence star. This releases an enormous amount of energy, mostly radiant energy, radiated into outer space, space as electromagnetic radiation peaking in visible light. The Sun is a G-type main-sequence star, G2-type main-sequence star. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the O-type main-sequence star, 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 Way. The Sun is a Population I stars, population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metallicity, 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 Solar System consists of a near-vacuum known as the interplanetary medium. Along with Sunlight, light, the Sun radiates a continuous stream of charged particles (a plasma (physics), plasma) known as the
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
. This stream of particles spreads outwards at roughly , creating a tenuous atmosphere that permeates the interplanetary medium out to at least ''(see )''. Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs 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 Earth's atmosphere, 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 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 funnels charged particles into Earth's upper atmosphere, where its interactions create Aurora (astronomy), aurorae seen near the Earth's magnetic field#Magnetic poles, 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 In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
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 interplanetary dust cloud, 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 to about , and was probably created by similar collisions within the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
.


Inner Solar System

The inner Solar System is the region comprising the #Terrestrial planets, terrestrial planets and the #Asteroid belt, 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 (astrophysics), frost line, which is a little less than from the Sun.


Inner planets

The four terrestrial or inner planets have dense, rocky compositions, few or no natural satellite, moons, and no planetary ring, ring systems. They are composed largely of Refractory (planetary science), refractory minerals such as the silicateswhich form their crust (geology), crusts and mantle (geology), mantlesand metals such as iron and nickel which form their planetary core, cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonics, 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 ( from the Sun) is the closest planet to the Sun. The smallest planet in the Solar System (), 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.


Venus

Venus ( from the Sun) is close in size to Earth () 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 , 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.


Earth

Earth ( 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 Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, the only large satellite of a terrestrial planet in the Solar System.


Mars

Mars ( from the Sun) is smaller than Earth and Venus (). It has an atmosphere of mostly carbon dioxide with a surface pressure of (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(III) oxide, iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos (moon), Deimos and Phobos (moon), Phobos) thought to be either captured
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s, or ejected debris from a massive impact early in Mars's history.


Asteroid belt

Asteroids except for the largest, Ceres, are classified as
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as foll ...
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 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.


Ceres

Ceres () is the largest asteroid, a protoplanet, and a dwarf planet. It has a diameter of slightly under , 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 then reclassified again as a dwarf planet in 2006 when the IAU definition of planet was created.


Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and Asteroid family, 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 L5 point, L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term is also used for small bodies in any other planetary or satellite Lagrange point. Hilda family, Hilda asteroids are in a 2:3 Orbital resonance, resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits. The inner Solar System also contains near-Earth object, 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 planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s and their large moons. The
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek ...
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 remain solid.


Outer planets

The four outer planets, also called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together more than 400 times the Earth mass, mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as
gas giant A gas giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilib ...
s. Uranus and Neptune are far less massiveless than 20 Earth masses () eachand are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants. All four giant planets have Planetary ring, 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 Mars.


Jupiter

Jupiter (), at , is 2.5 times the mass of all the other planets put together. It is composed largely of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has Moons of Jupiter, 80 known satellites. The four largest, , Callisto, Io (moon), Io, and Europa (moon), Europa, are called the Galilean moons: they show similarities to the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury; Callisto is almost as large.


Saturn

Saturn (), distinguished by its extensive Rings of Saturn, ring system, has several similarities to Jupiter, such as its atmospheric composition and Magnetosphere, magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at . Saturn is the only planet of the Solar System that is less dense than water. The rings of Saturn are made up of small ice and rock particles. Saturn has Moons of Saturn, 83 confirmed satellites composed largely of ice. Two of these,
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
and Enceladus, show signs of geological activity: they, as well as five other Saturnian moons (Iapetus (moon), Iapetus, Rhea (moon), Rhea, Dione (moon), Dione, Tethys (moon), Tethys, and Mimas (moon), Mimas), are large enough to be round. 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 (), at , 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 Moons of Uranus, 27 known satellites, the largest ones being Titania (moon), Titania, Oberon (moon), Oberon, Umbriel (moon), Umbriel, Ariel (moon), Ariel, and Miranda (moon), Miranda.


Neptune

Neptune (), though slightly smaller than Uranus, is more massive () and hence more Density, dense. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Neptune has Moons of Neptune, 14 known satellites. The largest,
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

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 Orbital resonance, resonance with it.


Centaurs

The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's () and less than Neptune's (). The largest known centaur, 10199 Chariklo, has a diameter of about . The first centaur discovered, 2060 Chiron, has also been classified as a 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 sublimation (chemistry), sublimate and ionise, creating a coma (cometary), 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 Comet Hale–Bopp, 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 trajectory, hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.


Trans-Neptunian region

Inside the orbit of Neptune is the planetary region of the Solar System. Beyond the orbit of Neptune lies the area of the "trans-Neptunian object, trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is Orbital inclination, tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still Timeline of Solar System exploration, largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of 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 Solar System.


Kuiper belt

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 from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than , 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. The Kuiper belt can be roughly divided into the "Classical Kuiper belt object, classical" belt and the Resonant trans-Neptunian object, 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 . Members of the classical Kuiper belt are classified as Classical Kuiper belt object, 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, low-eccentricity orbits.


Pluto and Charon

The dwarf planet Pluto (with an average orbit of ) 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 from the Sun at perihelion (within the orbit of Neptune) to at aphelion. Pluto has a 2:3 orbital resonance, 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. Charon, the largest of Moons of Pluto, Pluto's moons, is sometimes described as part of a binary system (astronomy), 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 (moon), Styx, Nix (moon), Nix, Kerberos (moon), Kerberos, and Hydra (moon), Hydra, orbit within the system.


Makemake, Haumea, Quaoar, and Orcus

Besides Pluto, astronomers generally agree that at least four other Kuiper belt objects are dwarf planets. Additional bodies have also been proposed. Makemake (45.79 AU average), although smaller than Pluto, is the largest known object in the Classical Kuiper belt object, ''classical'' Kuiper belt (that is, a Kuiper belt object not in a confirmed Resonant trans-Neptunian object, resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was assigned a naming committee under the expectation that it would prove to be a dwarf planet in 2008. Its orbit is far more inclined than Pluto's, at 29°. It has one known moon. Haumea (43.13 AU average) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune. It was named under the same expectation that it would prove to be a dwarf planet. It has two known moons, Hiʻiaka (moon), Hiʻiaka and Namaka (moon), Namaka. (43.69 AU average) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea. It has one known moon, Weywot (moon), Weywot. (39.40 AU average) is in the same 2:3 orbital resonance with Neptune that Pluto is in, and is the largest such object after Pluto itself. Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the Phase (waves)#Phase difference, phase of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa. For this reason, it has been called the ''anti-Pluto''. It has one known moon, Vanth (moon), Vanth.


Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends out to about 200 AU, 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 Formation and evolution of the Solar System#Planetary migration, Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the 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 inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.


Eris and Gonggong

(67.78 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 (moon), Dysnomia. Like Pluto, its orbit is highly eccentric, with a
perihelion upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...

perihelion
of 38.2 AU (roughly Pluto's distance from the Sun) and an
aphelion upright=1.15, The two-body system of interacting primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object consti ...

aphelion
of 97.6 AU, and steeply inclined to the ecliptic plane. (67.38 AU average) is in an orbit similar to Eris, except that it is in a 3:10 resonance with Neptune. It has one known moon, Xiangliu (moon), Xiangliu.


Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two 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, heliosphere, is considered the beginning of the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
. 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 hypothetical #Oort cloud, Oort cloud.


Heliosphere

The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun, in which it radiates its
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
at approximately 400 km/s, a stream of charged particles, until it collides with the wind of the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
. 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 the ''Cassini (spacecraft), Cassini'' and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field. 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. ''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. See Figures 1 and 2. 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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Milky Way
. Due to a lack of data, conditions in local interstellar space are not known for certain. It is expected that NASA's Voyager program, Voyager spacecraft, as they pass the heliopause, will transmit valuable data on radiation levels and solar wind to 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

(with an average orbit of 520 AU) 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. Michael E. Brown, Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or colli ...
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 , 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 , discovered in 2012. Its aphelion is only half that of Sedna's, at 400–500 AU.


Oort cloud

The 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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Milky Way
.


Boundaries

Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of List of nearest stars, 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. 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.


Galactic context

The Solar System is located in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Milky Way
, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 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. The Sun lies about 26,660 light-years from the Galactic Centre,GRAVITY Collaboration: ''A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty.'' Astronomie & Astrophysics 625, 2019, doi:10.1051/0004-6361/201935656. and its speed around the center of the Milky Way is about 247 km/s, so that it completes one revolution every 210 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 (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. 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. However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the Shiva hypothesis or related theories. The Solar System 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, 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.


Celestial neighbourhood

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it is in the region where the cloud interacts with the neighbouring G-Cloud. Both spaces are interstellar clouds in a region known as the 300 light-years wide
Local Bubble 200px, Model of space near the Sun: (grey) Local Bubble, (magenta) molecular clouds, (cyan) Loop I Bubble. The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. ...
. list of nearest stars, Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and in the G-Cloud. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest to Earth, the small red dwarf
Proxima Centauri Proxima Centauri is a small, low-mass star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star ...

Proxima Centauri
, orbits the pair closer at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun. The next closest known fusor (astronomy), fusors and rogue planets to the Sun are the red dwarf Barnard's Star (at 5.9 ly), the nearest brown dwarfs of the binary Luhman 16 system (6.6 ly), the closest known rogue or free-floating planetary-mass object at less than 10 Jupiter masses the sub-brown dwarf WISE 0855−0714, (7 ly), as well as the red dwarfs Wolf 359 (7.8 ly) and Lalande 21185 (8.3 ly). The next closest at 8.6 ly is Sirius, the list of brightest stars, brightest star in Earth's
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

night sky
, with roughly twice the Sun's mass, orbited by the closest white dwarf to Earth, Sirius B. 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 List of nearby stellar associations and moving groups, nearest and unaided-visible group of stars beyond the immediate celestial neighbourhood is the Ursa Major Moving Group at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible star cluster the Hyades (star cluster), Hyades, which lie at its edge. The Local Bubble is an hourglass-shaped cavity or superbubble in the
interstellar medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
roughly 300 light-years across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae. The Local Bubble is a small superbubble compared to the neighbouring wider Gould Belt and Radcliffe wave each of some thousands of light-years in length, all of which are part of the
Orion Arm upright=1.75, Observed structure of the Milky Way's spiral arms The Orion Arm is a minor spiral arm of the Milky Way Galaxy The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy's appearance fr ...
, that contains most unaided-visible stars, of the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Milky Way
. The closest star forming regions are the Corona Australis Molecular Cloud, Rho Ophiuchi cloud complex and the Taurus Molecular Cloud, the latter lies just beyond the Local Bubble and is part of the Radcliffe wave. Unaided-visible objects within these regions of a thousand light-years towards the 26 thousand light-years far away Galactic Center are objects like Shaula and galactic anticenter, outward in the galactic plane such as Elnath.


Comparison with extrasolar systems

Compared to many 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 super-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 the 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 and ejection. 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 undetermined.


Visual summary

This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some large objects are omitted here (notably the seven large TNOs Eris (dwarf planet), Eris, Haumea, Makemake, , , , and ) because they have not been imaged in high quality.


See also

* Astronomical symbols * Earth phase * Ephemeris is a compilation of positions of naturally occurring astronomical objects as well as artificial satellites in the sky at a given time or times. * HIP 11915 (a solar analog whose planets contains a Jupiter analog) * ''Journey Through the Solar System'' * Lists of geological features of the Solar System * List of gravitationally rounded objects of the Solar System * List of Solar System extremes * List of Solar System objects by size * Outline of the Solar System * Pajamäki Solar System Scale Model, model built in a park in Helsinki, Finland * Planetary mnemonic * Solar System models


Notes


External links

*
A Cosmic History of the Solar System

A Tediously Accurate Map of the Solar System (web based scroll map scaled to the Moon being 1 pixel)

NASA's Solar System ExplorationArchive
*
NASA's Solar System Profile

NASA's Solar System Simulator

NASA Eyes-on-the-Solar-System

NASA/JPL Solar System main page


Further reading

*


References

{{Authority control Solar System, Planetary science Space science Planetary systems with eight confirmed planets