Planets and dwarf planets of the Solar System. Sizes are to scale, but relative distances
from the Sun are not.
The Solar System consists of the Sun and those celestial objects bound to it by gravity,
all of which formed from the collapse of a giant molecular cloud approximately 4.6
billion years ago. The Sun's retinue of objects circle it in a nearly flat disc called the
ecliptic plane, most of the mass of which is contained within eight relatively solitary
planets whose orbits are almost circular. The four smaller inner planets; Mercury, Venus,
Earth and Mars, also called the terrestrial planets, are primarily composed of rock and
metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas
giants, are composed largely of hydrogen and helium and are far more massive than the
The Solar System is also home to two main belts of small bodies. The asteroid belt,
which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed
mainly of rock and metal. The Kuiper belt (and its subpopulation, the scattered disc),
which lies beyond Neptune's orbit, is composed mostly of ices such as water, ammonia
and methane. Within these belts, five individual objects, Ceres, Pluto, Haumea,
Makemake and Eris, are recognised to be large enough to have been rounded by their
own gravity, and are thus termed dwarf planets. The hypothetical Oort cloud, which acts
as the source for long-period comets, may also exist at a distance roughly a thousand
times beyond these regions.
Within the Solar System, various populations of small bodies, such as comets, centaurs
and interplanetary dust, freely travel between these regions, while the solar wind, a flow
of plasma from the Sun, creates a bubble in the interstellar medium known as the
heliosphere, which extends out to the edge of the scattered disc.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually
termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary
rings of dust and other particles.
Discovery and exploration of the Solar System
For many thousands of years, humanity, with a few notable exceptions, did not recognise
the existence of the Solar System. They believed the Earth to be stationary at the centre
of the universe and categorically different from the divine or ethereal objects that moved
through the sky. Although the Indian mathematician-astronomer Aryabhata and the
Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of
Nicolaus Copernicus was the first to develop a mathematically predictive
heliocentric system. His 17th-century successors Galileo Galilei, Johannes Kepler, and
Isaac Newton developed an understanding of physics which led to the gradual acceptance
of the idea that the Earth moves around the Sun and that the planets are governed by the
same physical laws that governed the Earth. In more recent times, improvements in the
telescope and the use of unmanned spacecraft have enabled the investigation of
geological phenomena such as mountains and craters and seasonal meteorological
phenomena such as clouds, dust storms and ice caps on the other planets.
The orbits of the bodies in the Solar System to scale (clockwise from top left)
The principal component of the Solar System is the Sun, a main sequence G2 star that
contains 99.86 percent of the system's known mass and dominates it gravitationally.
Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90
percent of the remaining mass.
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the
ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects
are frequently at significantly greater angles to it.
All of the planets and most other objects also orbit with the Sun's rotation (counter-
clockwise, as viewed from above the Sun's north pole). There are exceptions, such as
The mass of the Sun compared to the
remaining bodies of the Solar System.
Objects smaller than Saturn are not
visible at this scale.
The relative masses of the Solar planets. Jupiter at
71% of the total and Saturn at 21% dominate the
system. Mercury and Mars, which together are less
than 0.1%, are not visible at this scale.
Kepler's laws of planetary motion describe the orbits of objects about the Sun. According
to Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects
closer to the Sun (with smaller semi-major axes) have shorter years. On an elliptical orbit,
a body's distance from the Sun varies over the course of its year. A body's closest
approach to the Sun is called its perihelion, while its most distant point from the Sun is
called its aphelion. Each body moves fastest at its perihelion and slowest at its aphelion.
The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt
objects follow highly elliptical orbits.
To cope with the vast distances involved, many representations of the Solar System show
orbits the same distance apart. In reality, with a few exceptions, the farther a planet or
belt is from the Sun, the larger the distance between it and the previous orbit. For
example, Venus is approximately 0.33 astronomical units (AU) farther out than Mercury,
while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus.
Attempts have been made to determine a correlation between these orbital distances (see
Titius-Bode law), but no such theory has been accepted.
Most of the planets in the Solar System possess secondary systems of their own. Many
are in turn orbited by planetary objects called natural satellites, or moons, some of which
are larger than planets. Most of the largest natural satellites are in synchronous rotation,
with one face permanently turned toward their parent. The four largest planets, the gas
giants, also possess planetary rings, thin bands of tiny particles that orbit them in unison.
Informally, the Solar System is sometimes divided into separate regions. The inner Solar
System includes the four terrestrial planets and the main asteroid belt. The outer Solar
System is beyond the asteroids, including the four gas 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.
Dynamically and physically, objects orbiting the Sun are officially classed into three
categories: planets, dwarf planets and small Solar System bodies. A planet is any body in
orbit around the Sun that has enough mass to form itself into a spherical shape and has
cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar
System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
and Neptune. Pluto does not fit this definition, as it has not cleared its orbit of
surrounding Kuiper belt objects. A dwarf planet is a celestial body orbiting the Sun that is
massive enough to be rounded by its own gravity but which has not cleared its
neighbouring region of planetesimals and is not a satellit By this definition, the Solar
System has five known dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Other
objects may be classified in the future as dwarf planets, such as Sedna, Orcus, and
Quaoar. Dwarf planets that orbit in the trans-Neptunian region are called "plutoids". The
remainder of the objects in orbit around the Sun are small Solar System bodies.
The regions (or zones) of the Solar system: the inner solar system, the asteroid belt, the
giant planets (Jovians) and the Kuiper belt. Sizes and orbits not to scale, view is tilted.
Planetary scientists use the terms gas, ice, and rock to describe the various classes of
substances found throughout the Solar System. Rock is used to describe compounds with
high melting points that remained solid under almost all conditions in the protoplanetary
nebula. Rocky substances typically include silicates and metals such as iron and nickel
They are prevalent in the inner Solar System, forming most of the terrestrial planets and
asteroids. Gases are materials with extremely low melting points and high vapor pressure
such as molecular hydrogen, helium, and neon, which were always in the gaseous phase
in the nebula. They dominate the middle region, comprising most of Jupiter and Saturn.
Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide, have melting
points up to a few hundred kelvins, while their phase depends on the ambient pressure
and temperature. They can be found as ices, liquids, or gases in various places in the
Solar System, while in the nebula they were either in the solid or gaseous phase. Icy
substances comprise the majority of the satellites of the giant planets, as well as most of
Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie
beyond Neptune's orbit. Together, gases and ices are referred to as volatiles.
The Sun as seen in the x-ray region of the electromagnetic spectrum
The Sun is the Solar System's star, and far and away its chief component. Its large mass
(332,900 Earth masses)
gives it an interior density high enough to sustain nuclear
fusion, which releases enormous amounts of energy, mostly radiated into space as
misleading as, compared to majority of stars in our galaxy, the Sun is rather large and
bright. Stars electromagnetic radiation, peaking in the 400–to–700 nm band we call
The Sun is classified as a moderately large yellow dwarf, but this name is are classified
by the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against
their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern
are said to be on the main sequence, and the Sun lies right in the middle of it. However,
stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler
stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy.
It is believed that the Sun's position on the main sequence puts it in the "prime of life" for
a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is
growing brighter; early in its history it was 70 percent as bright as it is today. The Sun is
a population I star; it was born in the later stages of the universe's evolution, and thus
contains more elements heavier than hydrogen and helium ("metals" in astronomical
parlance) than 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, while stars born later have more. This high metallicity is thought to have been
crucial to the Sun's developing a planetary system, because planets form from accretion
The heliospheric current sheet.
Along with light, the Sun radiates a continuous stream of charged particles (a plasma)
known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million
kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the
Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary
medium. Geomagnetic storms on the Sun's surface, such as solar flares and coronal mass
ejections, disturb the heliosphere, creating space weather. The largest structure within the
heliosphere is the heliospheric current sheet, a spiral form created by the actions of the
Sun's rotating magnetic field on the interplanetary medium.
Aurora australis seen from orbit.
Earth's magnetic field stops its atmosphere from being stripped away by the solar wind.
Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their
atmospheres to gradually bleed away into space. The interaction of the solar wind with
Earth's magnetic field funnels charged particles at right angles to the Earth's upper
atmosphere, where its interactions create aurorae seen near the magnetic poles.
Cosmic rays originate outside the Solar System. The heliosphere partially shields the
Solar System, and planetary magnetic fields (for those planets that have them) also
provide some protection. The density of cosmic rays in the interstellar medium and the
strength of the Sun's magnetic field change on very long timescales, so the level of
cosmic radiation in the Solar System varies, though by how much is unknown.
The interplanetary medium is home to at least two disc-like regions of cosmic dust. The
first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It
was likely formed by collisions within the asteroid belt brought on by interactions with
the planets. The second extends from about 10 AU to about 40 AU, and was probably
created by similar collisions within the Kuiper belt.
Inner Solar System
The inner Solar System is the traditional name for the region comprising the terrestrial
planets and asteroids. Composed mainly of silicates and metals, the objects of the inner
Solar System huddle very closely to the Sun; the radius of this entire region is shorter
than the distance between Jupiter and Saturn.
Inner planets (Terrestrial planet)
The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)
The four inner or terrestrial planets have dense, rocky compositions, few or no moons,
and no ring systems. They are composed largely of minerals with high melting points,
such as the silicates which form their crusts and mantles, and metals such as iron and
nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars)
have substantial atmospheres; all have impact craters and tectonic surface features such
as rift valleys and volcanoes. The term inner planet should not be confused with inferior
planet, which designates those planets which are closer to the Sun than Earth is (i.e.
Mercury and Venus).
Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth
masses). Mercury has no natural satellites, and its only known geological features besides
impact craters are lobed ridges or rupes, probably produced by a period of contraction
early in its history. Mercury's almost negligible atmosphere consists of atoms blasted off
its surface by the solar wind. Its relatively large iron core and thin mantle have not yet
been adequately explained. Hypotheses include that its outer layers were stripped off by a
giant impact, and that it was prevented from fully accreting by the young Sun's energy.
Venus (0.7 AU) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick
silicate mantle around an iron core, a substantial atmosphere and evidence of internal
geological activity. However, it is much drier than Earth and its atmosphere is ninety
times as dense. Venus has no natural satellites. It is the hottest planet, with surface
temperatures over 400 °C, most likely due to the amount of greenhouse gases in the
atmosphere. No definitive evidence of current geological activity has been detected on
Venus, but it has no magnetic field that would prevent depletion of its substantial
atmosphere, which suggests that its atmosphere is regularly replenished by volcanic
Earth (1 AU) is the largest and densest of the inner planets, the only one known to have
current geological activity, and is the only place in the universe where life is known to
Its liquid hydrosphere is unique among the terrestrial planets, and it is also the
only planet where plate tectonics has been observed. Earth's atmosphere is radically
different from those of the other planets, having been altered by the presence of life to
contain 21% free oxygen. It has one natural satellite, the Moon, the only large satellite of
a terrestrial planet in the Solar System.
Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a
tenuous atmosphere of mostly carbon dioxide. Its surface, peppered with vast volcanoes
such as Olympus Mons and rift valleys such as Valles Marineris, shows geological
activity that may have persisted until very recently.
Its red colour comes from iron
oxide (rust) in its soil.
Mars has two tiny natural satellites (Deimos and Phobos)
thought to be captured asteroids.
Asteroids are mostly small Solar System bodies composed mainly of rocky and metallic
The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and
3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that
failed to coalesce because of the gravitational interference of Jupiter.
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids
save the largest, Ceres, are classified as small Solar System bodies, but some asteroids
such as Vesta and Hygieia may be reclassed as dwarf planets if they are shown to have
achieved hydrostatic equilibrium.
The asteroid belt contains tens of thousands, possibly millions, of objects over one
kilometre in diameter. Despite this, the total mass of the main belt is unlikely to be more
than a thousandth of that of the Earth. The main belt is very sparsely populated;
spacecraft routinely pass through without incident. Asteroids with diameters between 10
m are called meteoroids.
Image of the main asteroid belt and the Trojan asteroids Ceres
Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet.
It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it
into a spherical shape. Ceres was considered a planet when it was discovered in the 19th
century, but was reclassified as an asteroid in the 1850s as further observation revealed
additional asteroids. It was again reclassified in 2006 as a dwarf planet.
Asteroids in the main belt are divided into asteroid groups and families based on their
orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are
not as clearly distinguished as planetary moons, sometimes being almost as large as their
partners. The asteroid belt also contains main-belt comets which may have been the
source of Earth's water.
Trojan asteroids are located in either of Jupiter's L4 or L5 points (gravitationally stable
regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small
bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3
resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter
The inner Solar System is also dusted with rogue asteroids, many of which cross the
orbits of the inner planets.
Outer Solar System
The outer region of the Solar System is home to the gas giants and their planet-sized
satellites. Many short period comets, including the centaurs, also orbit in this region. Due
to their greater distance from the Sun, the solid objects in the outer Solar System are
composed of a higher proportion of volatiles (such as water, ammonia, methane, often
called ices in planetary science) than the rocky denizens of the inner Solar System, as the
colder temperatures allow these compounds to remain solid.
Outer planets (Gas giant)
From top to bottom: Neptune, Uranus, Saturn, and Jupiter (not to scale)
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make
up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist
overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater
proportion of ices in their makeup. Some astronomers suggest they belong in their own
category, “ice giants.” All four gas giants have rings, although only Saturn's ring system
is easily observed from Earth. The term outer planet should not be confused with
superior planet, which designates planets outside Earth's orbit (the outer planets and
Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times all the mass of all the other planets put
together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat
creates a number of semi-permanent features in its atmosphere, such as cloud bands and
the Great Red Spot. Jupiter has sixty-three known satellites. The four largest, Ganymede,
Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and
Ganymede, the largest satellite in the Solar System, is larger than
Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to
Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has
60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it
the least dense planet in the Solar System. Saturn has sixty known satellites (and three
unconfirmed); two of which, Titan and Enceladus, show signs of geological activity,
though they are largely made of ice.
Titan is larger than Mercury and the only satellite
in the Solar System with a substantial atmosphere.
Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely
among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the
ecliptic. It has a much colder core than the other gas giants, and radiates very little heat
Uranus has twenty-seven known satellites, the largest ones being Titania,
Oberon, Umbriel, Ariel and Miranda.
Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17
Earths) and therefore more dense. It radiates more internal heat, but not as much as
Jupiter or Saturn.
Neptune has thirteen known satellites. The largest, Triton, is
geologically active, with geysers of liquid nitrogen.
Triton is the only large satellite
with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets,
termed Neptune Trojans, that are in 1:1 resonance with it.
Comets are small Solar System bodies, usually only a few kilometres across, composed
largely of volatile ices. They have highly eccentric orbits, generally a perihelion within
the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the
inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise,
creating a coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets
have orbits lasting thousands of years. Short-period comets are believed to originate in
the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in
the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the
breakup of a single parent. Some comets with hyperbolic orbits may originate outside the
Solar System, but determining their precise orbits is difficult. Old comets that have had
most of their volatiles driven out by solar warming are often categorised as asteroids.
The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter
(5.5 AU) and less than Neptune (30 AU). The largest known centaur, 10199 Chariklo, has
a diameter of about 250 km. The first centaur discovered, 2060 Chiron, has also been
classified as comet (95P) since it develops a coma just as comets do when they approach
The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It
appears to consist overwhelmingly of small worlds (the largest having a diameter only a
fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of
rock and ice. This region is sometimes known as the "outer Solar System", though others
use that term to mean the region beyond the asteroid belt.
Plot of all known Kuiper belt objects, set against the four outer planets
The Kuiper belt, the region's first formation, is a great ring of debris similar to the
asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the
Sun. It is composed mainly of small Solar System bodies, but many of the largest Kuiper
belt objects, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets.
There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than
50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a
hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites, and
most have orbits that take them outside the plane of the ecliptic.
Diagram showing the 3:2 resonant and classical Kuiper belt divisions
The Kuiper belt can be roughly divided into the "classical" belt and the resonances.
Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits,
or once for every two). The first resonance actually begins within the orbit of Neptune
itself. The classical belt consists of objects having no resonance with Neptune, and
extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are
classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB1,
and are still near primordial, low-eccentricity orbits.
Pluto and Charon
Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt.
When discovered in 1930, it was considered to be the ninth planet; this changed in 2006
with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit
inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at
perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
Pluto and its three known moons
It is unclear whether Charon, Pluto's largest moon, will continue to be classified as such
or as a dwarf planet itself. Both Pluto and Charon orbit a barycenter of gravity above
their surfaces, making Pluto-Charon a binary system. Two much smaller moons, Nix and
Hydra, orbit Pluto and Charon.
Pluto lies in the resonant belt and has a 3:2 resonance with Neptune, meaning that Pluto
orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose
orbits share this resonance are called plutinos.
Haumea and Makemake
Haumea (43.34 AU average), and Makemake (45.79 AU average) are the largest known
objects in the classical Kuiper belt. Haumea is an egg-shaped object with two moons.
Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated
2003 EL61 and 2005 FY9 respectively, they were granted names (and the status of dwarf
planet) in 2008.
Their orbits are far more inclined than Pluto's (28° and 29°)
unlike Pluto are not affected by Neptune, being part of the classical KBO population.
Projection of the aligned orbits of the scattered, classical and resonant objects. Black:
scattered disc objects; blue: classical Kuiper belt objects; green: 5:2 resonant objects
The scattered disc overlaps the Kuiper belt but extends much further outwards. This
region is thought to be the source of short-period comets. Scattered disc objects are
believed to have been ejected into erratic orbits by the gravitational influence of
Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia
within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also
highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some
astronomers consider the scattered disc to be merely another region of the Kuiper belt,
and describe scattered disc objects as "scattered Kuiper belt objects." Some astronomers
classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered
residents of the scattered disc.
Eris and its moon Dysnomia
Eris (68 AU average) is the largest known scattered disc object, and caused a debate
about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated
diameter of 2400 km (1500 mi). It is the largest of the known dwarf planets. It has one
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.
The point at which the Solar System ends and interstellar space begins is not precisely
defined, since its outer boundaries are shaped by two separate forces: the solar wind and
the Sun's gravity. The outer limit of the solar wind's influence is roughly four times
Pluto's distance from the Sun; this heliopause is considered the beginning of the
interstellar medium. However, the Sun's Roche sphere, the effective range of its
gravitational influence, is believed to extend up to a thousand times farther.
The Voyagers entering the heliosheath
The heliosphere is divided into two separate regions. The solar wind travels at roughly
400 km/s until it collides with flows of plasma in the interstellar medium. The collision
occurs at the termination shock, which is roughly 80–100 AU from the Sun in the upwind
direction and roughly 200 AU from the Sun downwind. Here the wind slows
dramatically, condenses and becomes more turbulent, forming a great oval structure
known as the heliosheath that looks and behaves very much like a comet's tail, extending
outward for a further 40 AU on the upwind side but tailing many times that distance in
the opposite direction. Both Voyager 1 and Voyager 2 are reported to have passed the
termination shock and entered the heliosheath, at 94 and 84 AU from the Sun,
respectively. The outer boundary of the heliosphere, the heliopause, is the point at which
the solar wind finally terminates and is the beginning of interstellar space.
The shape and form of the outer edge of the heliosphere is likely affected by the fluid
dynamics of interactions with the interstellar medium as well as solar magnetic fields
prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9
AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the
heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it
travels through the Milky Way.
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for
certain the conditions in local interstellar space. It is expected that NASA's Voyager
spacecraft will pass the heliopause some time in the next decade and transmit valuable
data on radiation levels and solar wind back to the Earth. How well the heliosphere
shields the Solar System from cosmic rays is poorly understood. A NASA-funded team
has developed a concept of a "Vision Mission" dedicated to sending a probe to the
3d diagram model of the Oort cloud
The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed
to be the source for all long-period comets and to surround the Solar System at roughly
50,000 AU (around 1 light-year (LY)), and possibly to as far as 100,000 AU (1.87 LY). It
is believed to be composed of comets which were ejected from the inner Solar System by
gravitational interactions with the outer planets. Oort cloud objects move very slowly,
and can be perturbed by infrequent events such as collisions, the gravitational effects of a
passing star, or the galactic tide, the tidal force exerted by the Milky Way.
Telescopic image of Sedna
90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic,
highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion
and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003,
asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too
distant to have been affected by Neptune's migration. He and other astronomers consider
it to be the first in an entirely new population, which also may include the object 2000
CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of
3,420 years. Brown terms this population the "Inner Oort cloud," as it may have formed
through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf
planet, though its shape has yet to be determined with certainty.
Much of our Solar System is still unknown. The Sun's gravitational field is estimated to
dominate the gravitational forces of surrounding stars out to about two light years
(125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place
it farther than 50,000 AU.]
Despite discoveries such as Sedna, the region between the
Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually
unmapped. There are also ongoing studies of the region between Mercury and the Sun.
Objects may yet be discovered in the Solar System's uncharted regions.
Location of the Solar System within our galaxy
The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a
diameter of about 100,000 light-years containing about 200 billion stars Our Sun resides
in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.
The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its
speed within the galaxy is about 220 kilometres per second, so that it completes one
revolution every 225–250 million years. This revolution is known as the Solar System's
galactic year The solar apex, the direction of the Sun's path through interstellar space, is
near the constellation of Hercules in the direction of the current location of the bright star
The Solar System's location in the galaxy is very likely a factor in the evolution of life on
Earth. Its orbit is close to being circular and is at roughly the same speed as that of the
spiral arms, which means it passes through them only rarely. Since spiral arms are home
to a far larger concentration of potentially dangerous supernovae, this has given Earth
long periods of interstellar stability for life to evolve. The Solar System also lies well
outside the star-crowded environs of the galactic centre. Near the centre, gravitational
tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into
the inner Solar System, producing collisions with potentially catastrophic implications for
life on Earth. The intense radiation of the galactic centre could also interfere with the
development of complex life. Even at the Solar System's current location, some scientists
have hypothesised that recent supernovae may have adversely affected life in the last
35,000 years by flinging pieces of expelled stellar core towards the Sun in the form of
radioactive dust grains and larger, comet-like bodies.
Artist's conception of the Local Bubble
The immediate galactic neighbourhood of the Solar System is known as the Local
Interstellar Cloud or Local Fluff, an area of dense cloud in an otherwise sparse region
known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium
roughly 300 light years across. The bubble is suffused with high-temperature plasma that
suggests it is the product of several recent supernovae.
There are relatively few stars within ten light years (95 trillion km) of the Sun. The
closest is the triple star system Alpha Centauri, which is about 4.4 light years away.
Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red
dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of
0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9
light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest
star within ten light years is Sirius, a bright main sequence star roughly twice the Sun's
mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The
remaining systems within ten light years are the binary red dwarf system Luyten 726-8
(8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).
solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80
percent the Sun's mass, but only 60 percent its luminosity The closest known extrasolar
planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder
than the Sun, which lies 10.5 light years away. Its one confirmed planet, Epsilon Eridani
b, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years
Formation and evolution of the Solar System
Solar System's Most
Hubble image of protoplanetary disks in the Orion Nebula, a light-years-wide "stellar
nursery" likely very similar to the primordial nebula from which our Sun formed.
The Solar System formed from the gravitational collapse of a giant molecular cloud 4.6
billion years ago. This initial cloud was likely several light-years across and probably
birthed several stars.
As the region that would become the Solar System, known as the pre-solar nebula,
collapsed, conservation of angular momentum made it rotate faster. The centre, where
most of the mass collected, became increasingly hotter than the surrounding disc. As the
contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a
diameter of roughly 200 AU and a hot, dense protostar at the centre. At this point in its
evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri stars show
that they are often accompanied by discs of pre-planetary matter with masses of 0.001–
0.1 solar masses, with the vast majority of the mass of the nebula in the star itself. The
planets formed by accretion from this disk.
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.
reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved,
with the thermal energy countering the force of gravitational contraction. At this point the
Sun became a full-fledged main sequence star.
Artist's conception of the future evolution of our Sun. Left: main sequence; middle: red
giant; right: white dwarf
The Solar System as we know it today will last until the Sun begins its evolution off of
the main sequence of the Hertzsprung-Russell diagram. As the Sun burns through its
supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing
it to collapse in on itself. This increase in pressure heats the core, so it burns even faster.
As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion
Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been
entirely converted to helium, ending the main sequence phase. At this time, the outer
layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will
become a red giant. Because of its vastly increased surface area, the surface of the Sun
will be considerably cooler than it is on the main sequence (2600 K at the coolest).
Eventually, the Sun's outer layers will fall away, leaving a white dwarf, an extraordinarily
dense object, half the original mass of the Sun but only the size of the Earth. The ejected
outer layers will form what is known as a planetary nebula, returning some of the
material that formed the Sun to the interstellar medium
The Solar System
For I dipped into the Future, far as human eye could see; saw the vision of the world, and
all the wonder that would be. -Alfred Lord Tennyson, 1842
Our solar system consists of an average star we call the Sun, the planets Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the
planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The
Sun is the richest source of electromagnetic energy (mostly in the form of heat and light)
in the solar system. The Sun's nearest known stellar neighbor is a red dwarf star called
Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together
with the local stars visible on a clear night, orbits the center of our home
galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two
small galaxies orbiting it nearby, which are visible from the southern hemisphere. They
are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large
galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times
as massive and is 2 million light years away. Our galaxy, one of billions of galaxies
known, is traveling through intergalactic space.
The planets, most of the satellites of the planets and the asteroids revolve around the Sun
in the same direction, in nearly circular orbits. When looking down from above the Sun's
north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in
or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the
most highly inclined (18 degrees) and the most highly elliptical of all the planets.
Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of
rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are
Uranus and Pluto, which are tipped on their sides.
Composition Of The Solar System
The Sun contains 99.85% of all the matter in the Solar System. The planets, which
condensed out of the same disk of material that formed the Sun, contain only 0.135% of
the mass of the solar system. Jupiter contains more than twice the matter of all the other
planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the
interplanetary medium constitute the remaining 0.015%. The following table is a list of
the mass distribution within our Solar System.
• Sun: 99.85%
• Planets: 0.135%
• Comets: 0.01% ?
• Satellites: 0.00005%
• Minor Planets: 0.0000002% ?
• Meteoroids: 0.0000001% ?
• Interplanetary Medium: 0.0000001% ?
Nearly all the solar system by volume appears to be an empty void. Far from being
nothingness, this vacuum of "space" comprises the interplanetary medium. It includes
various forms of energy and at least two material components: interplanetary dust and
interplanetary gas. Interplanetary dust consists of microscopic solid particles.
Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and
electrons -- plasma -- which stream from the Sun, called the solar wind.
The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It
also has a measurable effect on the motion of spacecraft. The speed of the solar wind is
about 400 kilometers (250 miles) per second in the vicinity of Earth's orbit. The point at
which the solar wind meets the interstellar medium, which is the "solar" wind from other
stars, is called the heliopause. It is a boundary theorized to be roughly circular or
teardrop-shaped, marking the edge of the Sun's influence perhaps 100 AU from the Sun.
The space within the boundary of the heliopause, containing the Sun and solar system, is
referred to as the heliosphere.
The solar magnetic field extends outward into interplanetary space; it can be measured on
Earth and by spacecraft. The solar magnetic field is the dominating magnetic field
throughout the interplanetary regions of the solar system, except in the immediate
environment of planets which have their own magnetic fields.
The Terrestrial Planets
The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus,
Earth and Mars. They are called terrestrial because they have a compact, rocky surface
like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while
Mercury has almost none. The following diagram shows the approximate distance of the
terrestrial planets to the Sun.
The Jovian Planets
Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets,
because they are all gigantic compared with Earth, and they have a gaseous nature like
Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of
them might have small solid cores. The following diagram shows the approximate
distance of the Jovian planets to the Sun.
Solar System Animation
• Formation of the Solar System.
Views of the Solar System
Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA's Cosmic Background
Explorer's (COBE) Diffuse Infrared Background Experiment (DIRBE). This never-
before-seen view shows the Milky Way from an edge-on perspective with the galactic
north pole at the top, the south pole at the bottom and the galactic center at the center.
The picture combines images obtained at several near-infrared wavelengths. Stars within
our galaxy are the dominant source of light at these wavelengths. Even though our solar
system is part of the Milky Way, the view looks distant because most of the light comes
from the population of stars that are closer to the galactic center than our own Sun.
Our Milky Way Gets a Makeover
Like early explorers mapping the continents of our globe, astronomers are busy charting
the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's
Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral
structure is dominated by just two arms wrapping off the ends of a central bar of stars.
Previously, our galaxy was thought to possess four major arms.
This artist's concept illustrates the new view of the Milky Way, along with other findings
presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The
galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the
ends of a thick central bar, while the two now-demoted minor arms (Norma and
Sagittarius) are less distinct and located between the major arms. The major arms consist
of the highest densities of both young and old stars; the minor arms are primarily filled
with gas and pockets of star-forming activity.
The artist's concept also includes a new spiral arm, called the "Far-3 kiloparsec arm,"
discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than
the two major arms and lies along the bar of the galaxy.
Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located
between the Sagittarius and Perseus arms. (Courtesy NASA/JPL-Caltech)
Spiral Galaxy, NGC 4414
The majestic galaxy, NGC 4414, is located 60 million light-years away. Like the Milky
Way, NGC 4414 is a giant spiral-shaped disk of stars, with a bulbous central hub of older
yellow and red stars. The outer spiral arms are considerably bluer due to ongoing
formation of young, blue stars, the brightest of which can be seen individually at the high
resolution provided by the Hubble camera. The arms are also very rich in clouds of
interstellar dust, seen as dark patches and streaks silhouetted against the starlight.
Obliquity of the Eight
This illustration shows the obliquity of the eight planets. Obliquity is the angle between a
planet's equatorial plane and its orbital plane. By International Astronomical Union
(IAU) convention, a planet's north pole lies above the ecliptic plane. By this convention,
Venus, Uranus, and Pluto have a retrograde rotation, or a rotation that is in the opposite
direction from the other planets. (Copyright 2008 by Calvin J. Hamilton)
The Solar System
During the past three decades a myriad of space explorers have escaped the confines of
planet Earth and have set out to discover our planetary neighbors. This picture shows the
Sun and all nine planets of the solar system as seen by the space explorers. Starting at the
top-left corner is the Sun followed by the planets Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune, and Pluto. (Copyright 1998 by Calvin J. Hamilton)
Sun and Planets
This image shows the Sun and nine planets approximately to scale. The order of these
bodies are: Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and
Pluto. (Copyright Calvin J. Hamilton)
This image shows the Jovian planets Jupiter, Saturn, Uranus and Neptune approximately
to scale. The Jovian planets are named because of their gigantic Jupiter-like appearance.
(Copyright Calvin J. Hamilton)
The Largest Moons and Smallest Planets
This image shows the relative sizes of the largest moons and the smallest planets in the
solarsystem. The largest satellites pictured in this image are: Ganymede (5262 km), Titan
(5150 km), Callisto (4806 km), Io (3642 km), the Moon (3476 km), Europa (3138 km),
Triton (2706 km), and Titania (1580 km). Both Ganymede and Titan are larger than
planet Mercury followed by Io, the Moon, Europa, and Triton which are larger than the
planet Pluto. (Copyright Calvin J. Hamilton)
Diagram of Portrait Frames
On February 14, 1990, the cameras of Voyager 1 pointed back toward the Sun and took a
series of pictures of the Sun and the planets, making the first ever "portrait" of our solar
system as seen from the outside. This image is a diagram of how the frames for the solar
system portrait were taken. (Courtesy NASA/JPL)
All Frames from the Family Portrait
This image shows the series of pictures of the Sun and the planets taken on February 14,
1990, for the solar system family portrait as seen from the outside. In the course of taking
this mosaic consisting of a total of 60 frames, Voyager 1 made several images of the
inner solar system from a distance of approximately 6.4 billion kilometers (4 billion
miles) and about 32° above the ecliptic plane. Thirty-nine wide angle frames link together
six of the planets of our solar system in this mosaic. Outermost Neptune is 30 times
further from the Sun than Earth. Our Sun is seen as the bright object in the center of the
circle of frames. The insets show the planets magnified many times. (Courtesy
Portrait of the Solar System
These six narrow-angle color images were made from the first ever "portrait" of the solar
system taken by Voyager 1, which was more than 6.4 billion kilometers (4 billion miles)
from Earth and about 32° above the ecliptic. Mercury is too close to the Sun to be seen.
Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics,
and Pluto was not included in the mosaic because of its small size and distance from the
Sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter,
Saturn, Uranus, and Neptune. (Courtesy NASA/JPL)
Sun and Planet Summary
The following table lists statistical information for the Sun and planets:
Sun 0 109
25-36* 9 --- --- --- 1.410
0.39 0.38 0.05 58.8 0 7 0.2056 0.1° 5.43
Venus 0.72 0.95 0.89 244 0 3.394 0.0068 177.4° 5.25
Earth 1.0 1.00 1.00 1.00 1 0.000 0.0167 23.45° 5.52
Mars 1.5 0.53 0.11 1.029 2 1.850 0.0934 25.19° 3.95
Jupiter 5.2 11 318 0.411 16 1.308 0.0483 3.12° 1.33
Saturn 9.5 9 95 0.428 18 2.488 0.0560 26.73° 0.69
Uranus 19.2 4 17 0.748 15 0.774 0.0461 97.86° 1.29
30.1 4 17 0.802 8 1.774 0.0097 29.56° 1.64
Pluto 39.5 0.18 0.002 0.267 1 17.15 0.2482 119.6° 2.03
* The Sun's period of rotation at the surface varies from approximately 25 days at the
equator to 36 days at the poles. Deep down, below the convective zone, everything
appears to rotate with a period of 27 days