The Solar System
Capitalization
Capitalization (American English) or capitalisation (British English) is writing a word with its first letter as a capital letter (uppercase letter) and the remaining letters in lower case, in writing systems with a case distinction. The term a ...
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 in all aspects, including promoting astronomical research, outreac ...
, 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 first and foundational historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a com ...
'' an
''Merriam-Webster's 11th Collegiate Dictionary''
. is the
gravitationally
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
bound system of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and the objects that orbit it. It
formed 4.6 billion years ago from the
gravitational collapse
Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formatio ...
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 (99.86%) of the system's
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
is in the Sun, with most of the
remaining mass contained in the planet
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 ...
. The four
inner system planets—
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
,
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
—are
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
s, being composed primarily of rock and metal. The four
giant planet
The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...
s of the
outer system are substantially larger and more massive than the terrestrials. The two
largest
Large means of great size.
Large may also refer to:
Mathematics
* Arbitrarily large, a phrase in mathematics
* Large cardinal, a property of certain transfinite numbers
* Large category, a category with a proper class of objects and morphisms (o ...
,
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 ...
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 has only one-eighth the average density of Earth; h ...
, are
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, being composed mainly of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
; the next two,
Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
and
Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
, are
ice giant
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
In astrophysics and planetary science t ...
s, being composed mostly of
volatile substances with relatively high melting points compared with hydrogen and helium, such as
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
,
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
, and
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
. All eight planets have nearly
circular orbit
A circular orbit is an orbit with a fixed distance around the barycenter; that is, in the shape of a circle.
Listed below is a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Here the centripetal force is ...
s that lie near the plane of
Earth's orbit
Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes days (1 sidereal year), during which time Earth ...
, called the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth 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 again ...
.
There are an unknown number of smaller
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, 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 dwarf planets to p ...
s and innumerable
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 fol ...
orbiting the Sun. Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by
natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s, commonly called "moons" after Earth's
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. Two natural satellites, Jupiter's moon
Ganymede and Saturn's moon
Titan, are larger but not more massive than Mercury, the smallest terrestrial planet, and Jupiter's moon
Callisto Callisto most commonly refers to:
*Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*''Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten Kro ...
is nearly as large. Each of the giant planets and some smaller bodies are encircled by
planetary ring
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known a ...
s of ice, dust and moonlets. The
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. Beyond Neptune's orbit lie the
Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
and
scattered disc
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obje ...
, which are populations of objects composed mostly of ice and rock.
In the outer reaches of the Solar System lies a class of minor planets called
detached object
Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun (perihelio ...
s. There is considerable debate as to how many such objects there will prove to be.
Some of these objects are large enough to have rounded under their own gravity and thus to be categorized as dwarf planets. Astronomers generally accept about nine objects as dwarf planets: the asteroid , the Kuiper-belt 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 is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, , , , and , and the scattered-disc objects , , and .
Various small-body populations, including
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 ar ...
s,
centaurs
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
and
interplanetary dust cloud
The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System. ...
s, freely travel between the regions of the Solar System.
The
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region of
interplanetary medium
The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM sto ...
in the
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
known as the
heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
. 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
The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
, 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 are ...
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 and Bank of China Tower. With a height of , it comprises 73 storeys. The center is ...
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 ...
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
in the
Orion Arm
The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...
, which contains most of the visible stars in the
night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky include ...
. The
nearest stars
This list covers all known stars, brown dwarfs, and sub-brown dwarfs within of the Sun. So far, 131 such objects have been found, of which only 22 are bright enough to be visible without a telescope. The visible light needs to reach or exc ...
are within the so-called
Local Bubble
The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the closest of celestial neighbours and among others, the Local Interstellar Cloud (which contains the Sol ...
, with the closest,
Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
, at 4.2441 light-years.
Formation and evolution
![The_Mysterious_Case_of_the_Disappearing_Dust](https://upload.wikimedia.org/wikipedia/commons/2/2d/The_Mysterious_Case_of_the_Disappearing_Dust.jpg)
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
pre-solar nebula, collapsed,
conservation of angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed system ...
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
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
with a diameter of roughly
and a hot, dense
protostar
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 5 ...
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 or ejected, leaving the planets, dwarf planets, and leftover
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, Kuiper belt, and Oort cloud.
The
Nice model
The Nice () model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France.
It proposes the migration of the giant p ...
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.
![NGC7293 (2004)](https://upload.wikimedia.org/wikipedia/commons/b/b1/NGC7293_%282004%29.jpg)
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
Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
.
The temperature,
reaction rate
The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
, pressure, and density increased until
hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a
main-sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
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
Remnant or remnants may refer to:
Religion
* Remnant (Bible), a recurring theme in the Bible
* Remnant (Seventh-day Adventist belief), the remnant theme in the Seventh-day Adventist Church
* ''The Remnant'' (newspaper), a traditional Catholic ne ...
life combined.
Solar wind from the Sun created the
heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
and swept away the remaining gas and dust from the protoplanetary disc into interstellar space. As helium accumulates at its core 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 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
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
. Because of its increased surface area, the surface of the Sun will be cooler ( at its coolest) than it is on the main sequence.
The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well). 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 be ejected into space, leaving behind a dense
white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
, 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
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelate ...
, returning some of the material that formed the Sun—but now enriched with
heavier elements like carbon—to the interstellar medium.
Structure and composition
The word ''solar'' means "pertaining to the Sun", which is derived from the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
word ''sol'', meaning Sun. The Sun is the dominant gravitational member of the Solar System, and its planetary system is maintained in a relatively stable, slowly evolving state by following isolated,
gravitationally bound
The gravitational binding energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. A gravitationally bound system has a lower (''i.e.'', more negative) gravitati ...
orbits around the Sun.
Orbits
The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.
Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. Many of the largest natural satellites are in
synchronous rotation
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...
, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison.
As a result of the
formation of the Solar System
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened int ...
, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, 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 short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the o ...
. Most of the larger moons orbit their planets in
prograde direction, matching the planetary rotation; Neptune's moon
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' ...
is the largest to orbit in the opposite,
retrograde manner. Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.
To a good first approximation,
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 heliocentric theory of Nicolaus Copernicus, replacing its circular orbits ...
describe the orbits of objects about the Sun.
These laws stipulate that each object travels along an
ellipse
In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...
with the Sun at one
focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
, which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its ''
perihelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
'', whereas its most distant point from the Sun is called its ''
aphelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ell ...
''.
The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these additional perturbations can be accounted for using
numerical models,
but the planetary system can change chaotically over billions of years.
The
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of the Solar System is a measure of the total amount of orbital and
rotational momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syste ...
possessed by all its moving components. 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.
Composition
The overall structure of the charted regions of the Solar System consists of the Sun, four smaller 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.
The principal component of the Solar System is the Sun, a
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 Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons,
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s, and comets) together comprise less than 0.002% of the Solar System's total mass.
The Sun is composed of roughly 98% hydrogen and helium, as are Jupiter and Saturn.
A composition gradient exists in the Solar System, created by heat and
light pressure
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is ...
from the early 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 coalesce is known as the
frost line
The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...
, and it lies at roughly five times the Earth's distance from the Sun.
The objects of the inner Solar System are composed mostly of rocky materials,
such as
silicates
In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name i ...
, iron or nickel.
Jupiter and Saturn are composed mainly of gases with extremely low melting points and high
vapour pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
, such as hydrogen, helium, and
neon
Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...
.
Ices, like water, methane, ammonia,
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
, and
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
,
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.
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 and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
In astrophysics and planetary science t ...
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 vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.
On planet Earth, the term ' ...
''.
Distances and scales
![Solar System distance to scale](https://upload.wikimedia.org/wikipedia/commons/d/df/Solar_System_distance_to_scale.svg)
The
astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
U() would be the distance from the Earth to the Sun if the planet's orbit were perfectly circular. 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, like the
Titius–Bode law
The Titius–Bode law (sometimes termed just Bode's law) is a formulaic prediction of spacing between planets in any given solar system. The formula suggests that, extending outward, each planet should be approximately twice as far from the Sun as ...
and
Johannes Kepler's model based on the
Platonic solid
In geometry, a Platonic solid is a convex, regular polyhedron in three-dimensional Euclidean space. Being a regular polyhedron means that the faces are congruent (identical in shape and size) regular polygons (all angles congruent and all edges c ...
s, but ongoing discoveries have invalidated these hypotheses.
Some
Solar System model
Solar System models, especially mechanical models, called '' orreries'', that illustrate the relative positions and motions of the planets and moons in the Solar System have been built for centuries. While they often showed relative sizes, these ...
s 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)
Avicii Arena in
Stockholm
Stockholm () is the Capital city, capital and List of urban areas in Sweden by population, largest city of Sweden as well as the List of urban areas in the Nordic countries, largest urban area in Scandinavia. Approximately 980,000 people liv ...
as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at
Stockholm Arlanda Airport
Stockholm Arlanda Airport is an international airport located in the Sigtuna Municipality of Sweden, near the town of Märsta, north of Stockholm and nearly south-east of Uppsala. The airport is located within Stockholm County and the prov ...
, 40 km (25 mi) away, whereas the farthest current object,
Sedna, is a 10 cm (4 in) sphere in
Luleå
Luleå ( , , locally ; smj, Luleju; fi, Luulaja) is a city on the coast of northern Sweden, and the capital of Norrbotten County, the northernmost county in Sweden. Luleå has 48,728 inhabitants in its urban core (2018) and is the seat of Lu ...
, 912 km (567 mi) away.
If the Sun–Neptune distance is 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
Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly (equatorial radius, deno ...
along with that of the other terrestrial planets would be smaller than a flea () at this scale.
Sun
![The Sun in white light](https://upload.wikimedia.org/wikipedia/commons/8/83/The_Sun_in_white_light.jpg)
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
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
high enough to sustain nuclear fusion of hydrogen into helium. This releases an enormous amount of
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
, mostly
radiated into
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
as
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
peaking in
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
.
Because the Sun fuses hydrogen into helium at its
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
, it is a main-sequence star. More specifically, it is a
G2-type main-sequence star, where the type designation refers to its
effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
. 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 dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s, make up about 75% of the stars in the Milky Way.
The Sun is a
population I star
During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.
In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926:
Baade noticed th ...
; it has a higher abundance of elements heavier than hydrogen and helium ("
metals
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
" 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 higher metallicity is thought to have been crucial to the Sun's development of a
planetary system
A planetary system is a set of gravitationally
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interacti ...
because the planets form from the accretion of "metals".
Environment and habitability
![Submillimeter Array Night](https://upload.wikimedia.org/wikipedia/commons/9/9a/Submillimeter_Array_Night.jpg)
Outside of the main part of the Sun's atmosphere extends the
heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
and dominates the Solar
planetary system
A planetary system is a set of gravitationally
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interacti ...
. The vast majority of the heliosphere is occupied by a near-
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
known as the
interplanetary medium
The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM sto ...
. Along with
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
, the Sun radiates a continuous stream of charged particles (a
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
) called the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
. This stream of particles spreads outwards at speeds from to , creating a tenuous atmosphere that permeates the interplanetary medium out to at least .
Activity on the Sun's surface, such as
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s and
coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accept ...
s, disturbs the heliosphere, creating
space weather
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the ma ...
and causing
geomagnetic storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
s.
The largest structure within the heliosphere is the
heliospheric current sheet
The heliospheric current sheet, or interplanetary current sheet, is a surface separating regions of the heliosphere where the interplanetary magnetic field points toward and away from the Sun. A small electrical current with a current density of ...
, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
stops
its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result the solar wind 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
aurorae
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in polar regions of Earth, high-latitude regions (around the Arctic and Antarctic). Auroras display ...
seen near the
magnetic poles.
The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called
cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. The density of cosmic rays in the
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
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
Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...
. The first, the
zodiacal dust cloud
The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System ...
, lies in the inner Solar System and causes the
zodiacal light
The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in ...
. It may have been formed by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is the planet Mars. The second dust cloud extends from about to about , and was probably created by collisions within the Kuiper belt.
The
zone of hability of the Solar System is located in the Inner Solar System. Beside the Solar conditions for hability on Solar System objects such as Earth, habitability might be possibly in
subsurface ocean
Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of undergr ...
s of various Outer Solar System moons.
Inner Solar System
![Inner solar system objects top view for wiki](https://upload.wikimedia.org/wikipedia/commons/1/11/Inner_solar_system_objects_top_view_for_wiki.png)
The inner Solar System is the region comprising the
terrestrial planets
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ven ...
and the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
. Composed mainly of
silicate
In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
s 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
The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...
, which is a little less than from the Sun.
Inner planets
![Terrestrial planet sizes2](https://upload.wikimedia.org/wikipedia/commons/7/7c/Terrestrial_planet_sizes2.jpg)
The four terrestrial or inner planets have dense, rocky compositions, few or no
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
, and no
ring systems. They are composed largely of
refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
minerals such as the silicateswhich form their
crusts and
mantlesand metals such as iron and nickel which form their
cores. Three of the four inner planets (Venus, Earth and Mars) have
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
s substantial enough to generate weather; all have impact craters and
tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
surface features, such as
rift valley
A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear dep ...
s and volcanoes. The term ''inner planet'' should not be confused with ''
inferior planet
In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun. In this situation, the latter planet is said to be superior to the former. In the ref ...
'', 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. The dominant geological features are impact craters or basins with ejecta blankets, the remains of early volcanic activity including magma flows, and lobed ridges or
rupes
Rupes (plural ) is the Latin word for 'cliff'. It is used in planetary geology to refer to escarpments on other worlds. , the IAU has named 62 such features in the Solar System, on Mercury (17), Venus (7), the Moon (8), Mars (23), the asteroids ...
that were probably produced by a period of contraction early in the planet's history. Mercury's very tenuous atmosphere consists of solar-wind particles trapped by Mercury's magnetic field, as well as 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.
There have been searches for "
Vulcanoid
The vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, which was proposed on the basis of irregulari ...
s", asteroids in stable orbits between Mercury and the Sun, but none have been discovered.
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 , mainly due to the amount of
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es in the atmosphere. The planet has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions. A relatively young planetary surface displays extensive evidence of volcanic activity, but is devoid of
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
. It may undergo resurfacing episodes on a time scale of 700 million years.
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
The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
is unique among the terrestrial planets, and it is the only planet where
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
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
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
.
The planetary
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
shields the surface from solar and cosmic radiation, limiting
atmospheric stripping
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
and maintaining habitability. It has one natural satellite, the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, 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 but sufficient to support weather phenomena. Its surface, peppered with volcanoes, such as
Olympus Mons
Olympus Mons (; Latin for Mount Olympus) is a large shield volcano on Mars. The volcano has a height of over 21.9 km (13.6 mi or 72,000 ft) as measured by the Mars Orbiter Laser Altimeter (MOLA). Olympus Mons is about two and a h ...
, and rift valleys, such as
Valles Marineris
Valles Marineris (; Latin for ''Mariner program, Mariner Valleys'', named after the ''Mariner 9'' Mars orbiter of 1971–72 which discovered it) is a system of canyons that runs along the Mars, Martian surface east of the Tharsis region. At more ...
, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
(rust) in its soil. Mars has two tiny natural satellites (
Deimos and
Phobos) thought to be either captured asteroids, or ejected debris from a massive impact early in Mars's history.
Asteroid belt
![Inner_solar_system_linear_map](https://upload.wikimedia.org/wikipedia/commons/a/ae/Inner_solar_system_linear_map.png)
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 fol ...
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
meteoroid
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...
s and
micrometeoroids
A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.
The term "micrometeor ...
(grain-sized), with the exact division between the two categories being debated over the years. As of 2017, the IAU designates asteroids having diameter between about 30
micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
s and 1 metre as micrometeroids, and terms smaller particles "dust".
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 ( from the Sun) is the largest asteroid, a
protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...
, 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, but as further observations revealed additional asteroids, it became common to consider it as one of the minor rather than major planets. It was then reclassified again as a dwarf planet in 2006 when the
IAU definition of planet
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a ''planet'' is a celestial body that:
# is in orbit around the Sun,
# has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), an ...
was established.
Pallas and Vesta
Pallas
Pallas may refer to:
Astronomy
* 2 Pallas asteroid
** Pallas family, a group of asteroids that includes 2 Pallas
* Pallas (crater), a crater on Earth's moon
Mythology
* Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena
* Pa ...
(2.77 AU from the Sun) and
Vesta (2.36 AU from the Sun) are the largest asteroids in the asteroid belt, after Ceres. They are the other two protoplanets that survive more or less intact. At about in diameter, they were large enough to have developed planetary geology in the past, but both have suffered large impacts and been battered out of being round.
Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt, as the
Pallas family The Pallas family (''adj. Palladian''; ) is a small asteroid family of B-type asteroids at very high inclinations in the intermediate asteroid belt.
The namesake of the family is 2 Pallas, an extremely large asteroid with a mean diameter of about 5 ...
and
Vesta family
The Vesta family (adj. ''Vestian''; ) is a family of asteroids. The cratering family is located in the inner asteroid belt in the vicinity of its namesake and principal body, 4 Vesta. It is one of the largest asteroid families with more than ...
. Both were considered planets upon their discoveries in 1802 and 1807 respectively, and then like Ceres generally considered as minor planets with the discovery of more asteroids. Some authors today have begun to consider Pallas and Vesta as planets again, along with Ceres, under geophysical definitions of the term.
Asteroid groups
Asteroids in the asteroid belt are divided into
asteroid group
A minor-planet group is a population of minor planets that share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid. It is customary to n ...
s and
families
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...
based on their orbital characteristics.
Kirkwood gap
A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter.
For example, there ar ...
s are sharp dips in the distribution of asteroid orbits that correspond to
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s with Jupiter.
Asteroid moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
s are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners (e.g. that of
90 Antiope
Antiope ( minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
). The asteroid belt includes
main-belt comet
Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jup ...
s, which may have been the source of Earth's water.
Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s are located in either of Jupiter's
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 asteroids
The Hilda asteroids (adj. ''Hildian'') are a dynamical group of more than 5,000 asteroids located beyond the asteroid belt but within Jupiter's orbit, in a 3:2 orbital resonance with Jupiter. The namesake is the asteroid 153 Hilda.
Hildas mov ...
are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.
The inner Solar System contains
near-Earth asteroids
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). ...
, many of which cross the orbits of the inner planets.
Some of them are
potentially hazardous object
A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and is large enough to cause significant regional damage in the event of impact. They ar ...
s.
Outer Solar System
![Kuiper_belt_plot_objects_of_outer_solar_system](https://upload.wikimedia.org/wikipedia/commons/5/5b/Kuiper_belt_plot_objects_of_outer_solar_system.png)
The outer region of the Solar System is home to the
giant planet
The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...
s and their large moons. The
centaurs
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
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
![Planet collage to scale](https://upload.wikimedia.org/wikipedia/commons/c/cf/Planet_collage_to_scale.jpg)
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 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 composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
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
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
In astrophysics and planetary science th ...
. 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 Mars.
The ring–moon systems of Jupiter, Saturn, and Uranus are like miniature versions of the Solar System; that of Neptune is significantly different, having been disrupted by the capture of its largest moon Triton.
Jupiter
Jupiter ( from the Sun
), 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 H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the
Great Red Spot. The planet possesses a 4.2– strength
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
that spans 22–29 million km, making it, in certain respects, the largest object in the Solar System. Jupiter has
80 known satellites. The four largest,
Ganymede,
Callisto Callisto most commonly refers to:
*Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*''Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten Kro ...
,
Io, and
Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
, are called the
Galilean moons
The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
: 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 ( from the Sun
), distinguished by its extensive
ring system
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
, has several similarities to Jupiter, such as its atmospheric composition and
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. 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
83 confirmed satellites composed largely of ice. Two of these,
Titan and
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
, show signs of geological activity; they, as well as five other Saturnian moons (
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
,
Rhea,
Dione,
Tethys, and
Mimas), are large enough to be round. Titan, the second-largest moon in the Solar System, is bigger than Mercury and the only satellite in the Solar System to have a substantial atmosphere.
Uranus
Uranus ( from the Sun
), at , has the lowest mass of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its
axial tilt
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...
is over ninety degrees to the ecliptic. This gives the planet extreme seasonal variation as each pole points toward and then away from the Sun. It has a much colder core than the other giant planets and radiates very little heat into space. As a consequence, it has the coldest planetary atmosphere in the Solar System. Uranus has
27 known satellites, the largest ones being
Titania,
Oberon
Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fair ...
,
Umbriel,
Ariel
Ariel may refer to:
Film and television
*Ariel Award, a Mexican Academy of Film award
* ''Ariel'' (film), a 1988 Finnish film by Aki Kaurismäki
* ''ARIEL Visual'' and ''ARIEL Deluxe'', 1989 and 1991 anime video series based on the novel series ...
, and
Miranda. Like the other giant planets, it possesses a ring system and magnetosphere.
Neptune
Neptune ( from the Sun
), though slightly smaller than Uranus, is more massive () and hence more
dense
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Neptune has
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' ...
, is geologically active, with
geyser
A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist only in ...
s of
liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
.
Triton is the only large satellite with a
retrograde orbit
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or ...
, which indicates that it did not form with Neptune, but was probably captured from the Kuiper belt. Neptune is accompanied in its orbit by several
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s, termed
Neptune trojan
Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, similar to the Trojan (astronomy), trojans of other planets. They therefore have approximately the same orbital period as Neptune and follow rough ...
s, that either lead or trail the planet by about one-sixth of the way around the Sun, positions known as
Lagrange points.
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
10199 Chariklo is the largest confirmed centaur (small body of the outer Solar System). It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings (nicknamed ...
, 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
![Comet_Hale-Bopp_1995O1](https://upload.wikimedia.org/wikipedia/commons/b/ba/Comet_Hale-Bopp_1995O1.jpg)
Comets are small Solar System bodies,
typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to
sublimate and
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
ise, creating a
coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
: 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 sungrazer
The Kreutz sungrazers ( ) are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for Ge ...
s, formed from the breakup of a single parent. Some comets with
hyperbolic
Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry.
The following phenomena are described as ''hyperbolic'' because they ...
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
![EightTNOs](https://upload.wikimedia.org/wikipedia/commons/9/91/EightTNOs.png)
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 region
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
Typically, ...
", with the doughnut-shaped Kuiper belt, home of Pluto and 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 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" belt and the
resonant trans-Neptunian object
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object (TNO) in mean-motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune, e.g. 1:2, 2 ...
s.
[ The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly . Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated 1992 QB1; they are still in near primordial, low-eccentricity orbits.
]
Pluto and Charon
The dwarf planet Pluto (with an average orbit of from the Sun) 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
The definition of ''planet'', since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term (), 'wandering stars', for star-like objects which apparently m ...
. 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 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 plutino
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...
s.
Charon, the largest of Pluto's moons, is sometimes described as part of a binary system
A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...
with Pluto, as the two bodies orbit a barycentre
In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important co ...
of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix
Nix or NIX may refer to:
Places
* Nix, Alabama, an unincorporated community, United States
* Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States
* Nix (moon), a moon of Pluto
People
* Nix (surname), listing people with ...
, Kerberos, and Hydra, orbit Pluto.
Others
Besides Pluto, astronomers generally agree that at least four other Kuiper belt objects are dwarf planets,[ and additional bodies have also been proposed:]
* Makemake
Makemake (minor-planet designation 136472 Makemake) is a dwarf planet and – depending on how they are defined – the second-largest Kuiper belt object in the classical population, with a diameter approximately 60% that of Pluto. It ...
(45.79 AU average from the Sun), 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). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009. Its orbit is far more inclined than Pluto's, at 29°. It has one known moon.
* Haumea
Haumea (minor-planet designation 136108 Haumea) is a dwarf planet located trans-Neptunian object, beyond Neptune's orbit. It was discovered in 2004 by a team headed by Michael E. Brown, Mike Brown of Caltech at the Palomar Observatory in the United ...
(43.13 AU average from the Sun) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune. Like Makemake, it was discovered in 2005. It has two known moons, Hiʻiaka
In Hawaiian religion, Hiʻiaka is a daughter of Haumea and Kāne.
Attributes and history
Hiiaka is the patron goddess of hula dancers, chant, sorcery, and medicine. Owls are her messengers and are sacred to her. Conceived in Tahiti, Hiiaka was c ...
and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an ellipsoid
An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a surface that may be defined as the ...
.[
]
* (43.69 AU average from the Sun) 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
Weywot, officially (50000) Quaoar I Weywot, is the only known moon of the trans-Neptunian planetoid 50000 Quaoar. Discovered by Michael Brown and T.A. Suer using images acquired by the Hubble Space Telescope on 14 February 2006, its existence wa ...
.
* (39.40 AU average from the Sun) is in the same 2:3 orbital resonance with Neptune as Pluto, 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
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
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
Vanth is a chthonic figure in Etruscan mythology shown in a variety of forms of funerary art, such as in tomb paintings and on sarcophagi.
Vanth is a female demon in the Etruscan underworld that is often accompanied either by additional Vanth fi ...
.
Scattered disc
The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can also be inclined up to 46.8° from the ecliptic plane. 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 from the Sun) 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 at an angle of 44°.
(67.38 AU average from the Sun) is in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune. It has one known moon, Xiangliu
Xiangliu (), known in the ''Classic of Mountains and Seas'' as Xiangyao (), is a venomous nine-headed snake monster that brings floods and destruction in Chinese mythology.
Xiangliu may be depicted with his body coiled on itself. The nine heads ...
.
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
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
, is considered the beginning of the interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
. The Sun's Hill sphere
The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...
, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud
The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
.
Edge of the heliosphere
The Sun's stellar-wind bubble, the heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
, a region of space dominated by the Sun, has its boundary 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 solar wind collides with the interstellar medium and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the ''heliosheath''. This structure has been theorized 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'' and Interstellar Boundary Explorer
Interstellar Boundary Explorer (IBEX or Explorer 91 or SMEX-10) is a NASA satellite in Earth orbit that uses energetic neutral atoms (ENAs) to image the interaction region between the Solar System and interstellar space. The mission is part o ...
spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field, but the actual shape remains unknown.
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
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
'' and ''Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
'' passed the termination shock and entered the heliosheath at 94 and 84 AU from the Sun, respectively. ''Voyager 1'' was reported to have crossed the heliopause in August 2012, and ''Voyager 2'' in December 2018.
The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
of interactions with the interstellar medium as well as solar magnetic field
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
s 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
In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of ...
, 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 ...
.
Detached objects
(with an average orbit of 520 AU from the Sun) 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
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obje ...
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 about half that of Sedna's, at 458 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
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
(ly)) from the Sun, 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
A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satelli ...
, the tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
exerted by the Milky Way.
Boundaries
Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. Most of the mass is orbiting in the region between 3,000 and 100,000 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. Learning about this region of space is difficult, because it depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets. Objects may yet be discovered in the Solar System's uncharted regions. The furthest known objects, such as Comet West
Comet West, formally designated C/1975 V1, 1976 VI, and 1975n, was a comet described as one of the brightest objects to pass through the inner Solar System in 1976. It is often described as a "great comet."
History
It was discovered photograp ...
, have aphelia around 70,000 AU from the Sun.
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 ...
, a barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies, and generally affect both the motions of stars and interstellar gas within spiral galaxi ...
with a diameter of about 100,000 light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s 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
The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...
or Local Spur. The Sun lies about 26,660 light-years from the Galactic Center
The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact ra ...
, and its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.[ This revolution is known as the Solar System's ]galactic year
The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy. . The solar apex
The solar apex, or the apex of the Sun's way, refers to the direction that the Sun travels with respect to the local standard of rest. This is not to be confused with the Sun's apparent motion through all constellations of the zodiac, which is ...
, the direction of the Sun's path through interstellar space, is near the constellation Hercules
Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures.
The Romans adapted the Gr ...
in the direction of the current location of the bright star Vega
Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, an ...
. The plane of the ecliptic lies at an angle of about 60° to the galactic plane
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usual ...
.
The Solar System's location in the Milky Way is a factor in the evolutionary history of life
The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest #Origins of life on Earth, emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', fo ...
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 supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e, 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
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It ...
on Earth, according to the Shiva hypothesis
The Shiva hypothesis, also known as coherent catastrophism, is the idea that global natural catastrophes on Earth, such as extinction events, happen at regular intervals because of the periodic motion of the Sun in relation to the Milky Way galax ...
or related theories, but this remains controversial.
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
The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
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. Stellar flybys that pass within of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz's Star
Scholz's Star ( WISE designation WISE 0720−0846 or fully WISE J072003.20−084651.2) is a dim binary stellar system from the Sun in the constellation Monoceros near the galactic plane. It was discovered in 2013 by astronomer Ralf- ...
, which approached to of the Sun some , likely passing through the outer Oort cloud.
Celestial neighbourhood
The Solar System is surrounded by the Local Interstellar Cloud
The Local Interstellar Cloud (LIC), also known as the Local Fluff, is an interstellar cloud roughly across, through which the Solar System is moving. This feature overlaps a region around the Sun referred to as the solar neighborhood. It is unk ...
, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge. Multiple other interstellar cloud
An interstellar cloud is generally an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in t ...
s also exist in the region within 300 light-years of the Sun, known as the Local Bubble
The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the closest of celestial neighbours and among others, the Local Interstellar Cloud (which contains the Sol ...
. The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.
The Local Bubble is a small superbubble compared to the neighbouring wider Radcliffe Wave and ''Split'' linear structures (formerly Gould Belt
The Gould Belt is a local, partial ring of stars in the Milky Way, about 3,000 light-years long, tilted away from the galactic plane by about 16–20 degrees. It contains many O- and B-type stars, amounting to the nearest star-forming regi ...
), each of which are some thousands of light-years in length. All these structures are part of the Orion Arm
The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...
, which contains most of the stars in the Milky Way that are visible to the unaided eye. The density of all matter in the local neighborhood is ·pc−3.
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 may be in the Local Bubble's G-Cloud
The G-Cloud (or G-Cloud complex) is an interstellar cloud located next to the Local Interstellar Cloud, within the Local Bubble. It is unknown whether the Solar System is embedded in the Local Interstellar Cloud or in the region where the two c ...
. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to Earth, the small red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
was found to be orbiting Proxima Centauri, called Proxima Centauri b
Proxima Centauri b (or Proxima b), sometimes referred to as Alpha Centauri Cb, is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system ...
, the closest confirmed exoplanet to the Sun.
The next closest known fusors to the Sun are the red dwarfs Barnard's Star
Barnard's Star is a red dwarf about six light-years from Earth in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and the closest star in the ...
(at 5.9 ly), Wolf 359
Wolf 359 is a red dwarf star located in the constellation Leo, near the ecliptic. At a distance of approximately 7.9 light years from Earth, it has an apparent magnitude of 13.54 and can only be seen with a large telescope. Wolf 359 i ...
(7.8 ly), and Lalande 21185
Lalande 21185 (also known as BD+36 2147, Gliese 411, and HD 95735) is a star in the south of Ursa Major. It is the apparent brightest red dwarf in the northern hemisphere.Only AX Microscopii and Lacaille 9352, in the southern hemisphere ...
(8.3 ly). The nearest brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s belong to the binary Luhman 16
Luhman 16 (WISE 1049−5319, WISE J104915.57−531906.1) is a binary brown-dwarf system in the southern constellation Vela at a distance of approximately from the Sun. These are the closest-known brown dwarfs and the closest syst ...
system (6.6 ly), and the closest known rogue or free-floating planetary-mass object
A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...
at less than 10 Jupiter masses is the sub-brown dwarf WISE 0855−0714
WISE 0855−0714 (full designation WISE J085510.83−071442.5, or W0855 for short) is a sub-brown dwarf () from Earth, therefore the fourth- closest star or (sub-) brown dwarf system to the Sun, the discovery of which was announced i ...
(7.4 ly).
Just beyond at 8.6 ly lies Sirius
Sirius is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinisation ...
, the brightest star in Earth's night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky include ...
, with roughly twice the Sun's mass, orbited by the closest white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
to Earth, Sirius B. Other stars within ten light-years are the binary red-dwarf system Luyten 726-8
Luyten 726-8, also known as Gliese 65, is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus. The two component stars are both flare stars with the variable s ...
(8.7 ly) and the solitary red dwarf Ross 154
Ross 154 (V1216 Sgr) is a star in the southern zodiac constellation of Sagittarius. It has an apparent visual magnitude of 10.44, making it much too faint to be seen with the naked eye. At a minimum, viewing Ross 154 requires a telescop ...
(9.7 ly). The closest solitary Sun-like star to the Solar System is Tau Ceti
Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under from the Solar System, it is a relativ ...
at 11.9 light-years. It has roughly 80% of the Sun's mass but only about half of its luminosity.
The 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
Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...
the Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
, which lie at its edge. The closest star-forming regions are the Corona Australis Molecular Cloud
Corona Australis is a constellation in the Southern Celestial Hemisphere. Its Latin name means "southern crown", and it is the southern counterpart of Corona Borealis, the northern crown. It is one of the 48 constellations listed by the 2nd-centu ...
, the Rho Ophiuchi cloud complex
The Rho Ophiuchi cloud complex is a complex of interstellar clouds with different nebulae, particularly dark nebulae which is centered 1° south of the star ρ Ophiuchi, which it among others extends to, of the constellation Ophiuchus. At an es ...
and the Taurus Molecular Cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.
Comparison with extrasolar systems
Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. The known Solar System also lacks super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively.
The term "super-Earth" refers only to ...
s, planets between one and ten times as massive as the Earth, although the hypothetical Planet Nine
Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orb ...
, if it does exist, could be a super-Earth beyond the Solar System as we understand it today. 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). As many of these super-Earths are closer to their respective stars than Mercury is to the sun, a hypothesis has arisen 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
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
.[ 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.][
]
Humanity's perspective
Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle Ages
The Late Middle Ages or Late Medieval Period was the Periodization, period of European history lasting from AD 1300 to 1500. The Late Middle Ages followed the High Middle Ages and preceded the onset of the early modern period (and in much of Eur ...
–Renaissance
The Renaissance ( , ) , from , with the same meanings. is a period in European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an effort to revive and surpass ideas ...
, astronomers from Europe to India believed Earth to be stationary at the centre of the Universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
philosopher Aristarchus of Samos
Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or ...
had speculated on a heliocentric
Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at ...
reordering of the cosmos, Nicolaus Copernicus
Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic Church, Catholic cano ...
was the first person known to have developed a mathematically predictive heliocentric system. Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical as well as circular, and the precise observational data of Tycho Brahe
Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was k ...
, Kepler produced the ''Rudolphine Tables
The ''Rudolphine Tables'' ( la, Tabulae Rudolphinae) consist of a star catalogue and planetary tables published by Johannes Kepler in 1627, using observational data collected by Tycho Brahe (1546–1601). The tables are named in memory of Rudolf ...
'', which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi
Pierre Gassendi (; also Pierre Gassend, Petrus Gassendi; 22 January 1592 – 24 October 1655) was a French philosopher, Catholic priest, astronomer, and mathematician. While he held a church position in south-east France, he also spent much t ...
used them to predict a transit of Mercury
frameless, upright=0.5
A transit of Mercury across the Sun takes place when the planet Mercury passes directly between the Sun and a superior planet. During a transit, Mercury appears as a tiny black dot moving across the Sun as the planet obs ...
in 1631, and Jeremiah Horrocks
Jeremiah Horrocks (16183 January 1641), sometimes given as Jeremiah Horrox (the Latinised version that he used on the Emmanuel College register and in his Latin manuscripts), – See footnote 1 was an English astronomer. He was the first person ...
did the same for a transit of Venus
frameless, upright=0.5
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) the solar disk. During a trans ...
in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.
In the 17th century, Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
publicized the use of the telescope in astronomy; he and Simon Marius
Simon Marius ( latinized form of Simon Mayr; 10 January 1573 – 5 January 1625) was a German astronomer. He was born in Gunzenhausen, near Nuremberg, but spent most of his life in the city of Ansbach. He is most known for being among the first ...
independently discovered that Jupiter had four satellites in orbit around it. Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
followed on from these observations by discovering Saturn's moon Titan 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 entirel ...
. In 1677, Edmond Halley
Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.
From an observatory he constructed on Saint Helena in 1676–77, H ...
observed a transit of Mercury 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 lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, and the Sun. Halley's friend Isaac Newton
Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
, in his magisterial ''Principia Mathematica
The ''Principia Mathematica'' (often abbreviated ''PM'') is a three-volume work on the foundations of mathematics written by mathematician–philosophers Alfred North Whitehead and Bertrand Russell and published in 1910, 1912, and 1913. ...
'' of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
apply on Earth and in the skies.
The term "Solar System" entered the English language by 1704, when John Locke
John Locke (; 29 August 1632 – 28 October 1704) was an English philosopher and physician, widely regarded as one of the most influential of Age of Enlightenment, Enlightenment thinkers and commonly known as the "father of liberalism ...
used it to refer to the Sun, planets, and comets. In 1705, Halley realised that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun, though Seneca
Seneca may refer to:
People and language
* Seneca (name), a list of people with either the given name or surname
* Seneca people, one of the six Iroquois tribes of North America
** Seneca language, the language of the Seneca people
Places Extrat ...
had theorized this about comets in the 1st century. Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as , only 0.8% greater than the modern value. Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
, having occasionally been observed since antiquity, was recognized to be a planet orbiting beyond Saturn by 1783. In 1838, Friedrich Bessel
Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method ...
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, and a basis for determining (through trigonometry) the distance of the object. Created by the different orbital p ...
, 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. Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.
In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space. Since then, humans have landed on the Moon during the Apollo program; the Apollo 13
Apollo 13 (April 1117, 1970) was the seventh crewed mission in the Apollo space program and the third meant to land on the Moon. The craft was launched from Kennedy Space Center on April 11, 1970, but the lunar landing was aborted aft ...
mission marked the furthest any human has been away from Earth at . All eight planets have been visited by space probes; the outer planets were first visited by the ''Voyager'' spacecraft, one of which (''Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
'') is the furthest object made by humankind and the first in interstellar space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...
. In addition, probes have also returned samples from comets and asteroids, as well as flown through the Sun's corona
A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma.
The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total so ...
and made fly-bys of Kuiper belt objects. Six of the planets (all but Uranus and Neptune) have or had a dedicated orbiter.
See also
* List of gravitationally rounded objects of the Solar System
This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (but are not necessarily in hydrostatic equilibrium). Apart from the Sun itself, ...
* List of Solar System objects by size
This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius. These lists can be sorted according to an object's radius and mass and, for the most massive objects, ...
* Lists of geological features of the Solar System
This is a directory of lists of geological features on planets excepting Earth, moons and asteroids ordered by increasing distance from the Sun.
Mercury
* List of craters on Mercury
* List of geological features on Mercury
Venus
* List ...
* List of Solar System extremes
* Outline of the Solar System
The following outline is provided as an overview of and topical guide to the Solar System:
Solar System – gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that ...
Notes
References
External links
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A Tediously Accurate Map of the Solar System (web based scroll map scaled to the Moon being 1 pixel)
NASA's Eyes-on-the-Solar-System
NASA's Solar System Exploration
NASA's Solar System Simulator
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Planetary science
Space science
Planetary systems with eight confirmed planets