The Solar System in the Milky Way galaxy
Distances from the
Sun are not to scale
Introduction
The Solar
System consists of the Sun and
the astronomical objects gravitationally bound in orbit around
it, all of which formed from
the collapse of a giant molecular cloud approximately 4.6 billion years ago.
The vast majority of the system's mass is in the Sun. Of the many objects that orbit the
Sun, most of the mass is
contained within eight relatively solitary planets whose
orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. 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, the gas giants, are
substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen
and helium; the two outermost planets, Uranus and Neptune, are
composed largely of ices, such as
water, ammonia and methane,
and are often referred to separately as "ice giants".
The Solar System is also home to a
number of regions populated by smaller objects. The asteroid belt,
which lies between Mars and Jupiter, is similar to the terrestrial planets as
it is composed mainly of rock and metal. Beyond Neptune 's
orbit lie the Kuiper belt and scattered disc;
linked populations of trans-Neptunian objects composed mostly of ices such as water, ammonia and methane.
Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded
by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, several dozen of which are
considered dwarf-planet candidates, various other small body populations
including comets, centaurs and interplanetary dust freely
travel between regions. 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.
The solar wind, a flow
of plasma from the Sun, creates a bubble in the interstellar medium known
as the heliosphere, which
extends out to the edge of the scattered disc. The Oort cloud, which
is believed to be the source for long-period comets,
may also exist at a distance roughly a thousand times further than the
heliosphere. The heliopause is the point at which pressure from
the solar wind is equal to the opposing pressure of interstellar wind.
The Solar System is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion
stars.
Discovery and exploration
-
For many thousands of years, humanity,
with a few notable exceptions, did not recognize the existence of the Solar
System. People believed the Earth to be stationary at the centre of the universe and categorically different from the
divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had
speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was
the first to develop a mathematically predictive heliocentric system.
His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton,
developed an understanding of physics that led to the gradual acceptance of
the idea that the Earth moves around the Sun and that the planets are governed
by the same physical laws that governed the Earth. Additionally, the invention
of the telescope led to the discovery of further planets and moons. In more
recent times, improvements in the telescope and the use of unmanned spacecraft have
enabled the investigation of geological phenomena such as mountains and craters, and
seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.
Structure
The principal component of the Solar
System is the Sun, amain-sequence G2 star that
contains 99.86 percent of the system's known mass and dominates it
gravitationally. The Sun's four largest orbiting bodies, the gas giants, account
for 99% of the remaining mass, with Jupiter and Saturn together comprising more
than 90%.
Most
large objects in orbit around the Sun lie near the plane of Earth's orbit,
known as the ecliptic. The
planets are very close to the ecliptic while comets and Kuiper belt objects are frequently at significantly
greater angles to it. All the planets and most other objects orbit the Sun in
the same direction that the Sun is rotating (counter-clockwise, as viewed from
above the Sun's north pole). There are exceptions, such as Halley's Comet.
The overall structure of the charted
regions of the Solar System consists of the Sun, four relatively small inner
planets surrounded by a belt of rocky asteroids, and four gas giants surrounded
by the Kuiper belt of icy objects. Astronomers sometimes informally divide this
structure into separate regions. The inner
Solar System includes the
four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the
four gas giants. Since the
discovery of the Kuiper belt, the outermost parts of the Solar System are
considered a distinct region consisting of the objects beyond Neptune .
Most of the planets in the Solar System
possess secondary systems of their own, being orbited by planetary objects
called natural satellites,
or moons (two of which are larger than the planet Mercury), or, in
the case of the four gas giants, by planetary rings;
thin bands of tiny particles that orbit them in unison. Most of the largest
natural satellites are insynchronous rotation, with one face permanently turned toward
their parent.
Kepler's laws of planetary motion describe the orbits of objects about
the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects
closer to the Sun (with smallersemi-major axes)
travel more quickly, as they are more affected by the Sun's gravity. On an
elliptical orbit, a body's distance from the Sun varies over the course of its
year. A body's closest approach to the Sun is called its perihelion,
while its most distant point from the Sun is called its aphelion. The
orbits of the planets are nearly circular, but many comets, asteroids and
Kuiper belt objects follow highly elliptical orbits. The positions of the
bodies in the Solar System can be predicted using numerical models.
Due to 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 from the Sun than
Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune
lies 10.5 AU out from Uranus. Attempts have been made to determine a
relationship between these orbital distances (for example, the Titius–Bode
law). but no such
theory has been accepted.
A number of Solar System models on
Earth attempt to convey the relative scales involved in the Solar System on
human terms. Some models are mechanical - called orreries -
while others extend across cities or regional areas. The largest such scale
model, theSweden Solar System, uses the 110-metre Ericsson Globe in Stockholm as its substitute Sun, and, following
the scale, Jupiter is a 7.5 metre sphere at Arlanda International Airport, 40 km away,
while the farthest current object, Sedna, is a 10-cm sphere in Luleå, 912 km away.
Composition
The Sun, which comprises nearly all the
matter in the Solar System, is composed of roughly 98% hydrogen and helium. Jupiter andSaturn, which comprise nearly all the remaining matter,
possess atmospheres composed of roughly 99% of those same elements. A composition gradient exists in the Solar System,
created by heat and light
pressure from the Sun; those objects
closer to the Sun, which are more affected by heat and light pressure, are
composed of elements with high melting points. Objects farther from the Sun are
composed largely of materials with lower melting points. The boundary in the
Solar System beyond which those volatile substances could condense is known as
the frost line, and it lies at roughly 4 AU
from the Sun.
The objects of the inner Solar System
are composed mostly of rock, the collective name for compounds
with high melting points, such as silicates,
iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low
melting points and high vapor
pressure such asmolecular hydrogen, helium, and neon, which were
always in the gaseous phase in the nebula.[19] 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
The Sun is the Solar System's star, and by far its chief component. Its
large mass (332,900 Earth masses) produces temperatures and densities in its core great
enough to sustain nuclear fusion, which
releases enormous amounts of energy, mostly radiated into space aselectromagnetic radiation, peaking in the 400-700 nm band
of visible light.
The
Sun is classified as a type G2 yellow dwarf, but
this name is misleading as, compared to the majority of stars in our galaxy, the Sun
is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph that plots
the brightness of stars with their surfacetemperatures.
Generally, hotter stars are brighter. Stars following this pattern are said to
be on themain
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.
Evidence suggests 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% 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 of "metals".
Interplanetary medium
Along with light, the Sun
radiates a continuous stream of charged particles (a plasma) known as
the solar wind. This
stream of particles spreads outwards at roughly 1.5 million kilometres per hour,
creating a tenuous atmosphere (the heliosphere) that permeates the Solar System
out to at least 100 AU (see heliopause). This
is known as the interplanetary medium. Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturb the heliosphere, creating space weather and causing geomagnetic storms.
The largest structure within the heliosphere is the heliospheric current sheet, a spiral form
created by the actions of the Sun's rotating magnetic field on the
interplanetary medium.
Earth's magnetic field stops its atmosphere from
being stripped away by the solar wind. Venus and Mars do not have magnetic
fields, and as a result, the solar wind causes their atmospheres to gradually
bleed away into space. Coronal mass ejections and
similar events blow a magnetic field and huge quantities of material from the surface
of the Sun. The interaction of this magnetic field and material with Earth's
magnetic field funnels charged particles into the Earth's upper atmosphere,
where its interactions create auroraeseen 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 ofsilicates and metals, the objects of the inner
Solar System are relatively close to the Sun; the radius of this entire region
is shorter than the distance between Jupiter and Saturn.
Inner planets
(sizes to scale,
interplanetary distances not)
The four inner or terrestrial planets
have dense, rocky compositions, few or no moons, and no ring systems. They
are composed largely of refractory minerals,
such as the silicates, which
form their crusts and mantles, and metals
such as iron and nickel, which form theircores. Three of the
four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features such asrift valleys and volcanoes. The term inner planet should not be confused with inferior planet,
which designates those planets that are closer to the Sun than Earth is (i.e.
Mercury and Venus).
Mercury
Mercury (0.4 AU from the Sun) is the closest planet to
the Sun and the smallest planet in the Solar System (0.055 Earth masses).
Mercury has no natural satellites, and its only known geological features
besides impact craters are lobed ridges or rupes, probably produced by a period of
contraction early in its history.[41] Mercury's almost negligible atmosphere
consists of atoms blasted off its surface by the solar wind.[42] Its relatively large iron core and
thin mantle have not yet been adequately explained. Hypotheses include that its
outer layers were stripped off by a giant impact, and that it was prevented
from fully accreting by the young Sun's energy.Venus
Venus (0.7 AU
from the Sun) is close in size to Earth (0.815 Earth masses), and, like Earth,
has a thick silicate mantle around an iron core, a substantial atmosphere and
evidence of internal geological activity. However, it is much drier than Earth
and its atmosphere is ninety times as dense. Venus has no natural satellites.
It is the hottest planet, with surface temperatures over 400°C, most likely due
to the amount of greenhouse gases in the atmosphere. No definitive
evidence of current geological activity has been detected on Venus, but it has
no magnetic field that would prevent depletion of its substantial atmosphere,
which suggests that its atmosphere is regularly replenished by volcanic
eruptions.
Earth
Earth (1 AU
from the Sun) is the largest and densest of the inner planets, the only one
known to have current geological activity, and is the only place in the Solar
System where life is
known to exist. Its liquid hydrosphere is unique among the terrestrial
planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere
is radically different from those of the other planets, having been altered by
the presence of life to contain 21% free oxygen. It
has one natural satellite, the Moon, the only large satellite of a
terrestrial planet in the Solar System.
Mars
Mars (1.5 AU
from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It
possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1
millibars (roughly 0.6 percent that of the Earth's).[49] Its surface, peppered with vast
volcanoes such as Olympus Mons and rift valleys such as Valles Marineris,
shows geological activity that may have persisted until as recently as 2
million years ago.[50] Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos andPhobos) thought to
be captured asteroids.
Asteroid belt
Asteroids are small Solar System bodies[e] composed mainly of refractory rocky and metallic minerals, with some
ice.
The asteroid belt occupies the orbit
between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is
thought to be remnants from the Solar System's formation that failed to
coalesce because of the gravitational interference of Jupiter.
Asteroids range in size from hundreds
of kilometres across to microscopic. All asteroids except the largest, Ceres,
are classified as small Solar System bodies, but some asteroids such as Vesta and Hygiea may be reclassed asdwarf 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
asteroid belt is unlikely to be more than a thousandth of that of the Earth. The asteroid belt is very sparsely
populated; spacecraft routinely pass through without
incident. Asteroids with diameters between 10 and 10−4 m are
called meteoroids.
Ceres
Ceres (2.77 AU)
is the largest asteroid, a protoplanet, and a
dwarf planet. It has a diameter of slightly under 1000 km, and a mass
large enough for its own gravity to pull it into a spherical shape. Ceres was
considered a planet when it was discovered in the 19th century, but was
reclassified as an asteroid in the 1850s as further observations revealed
additional asteroids. It was classified in 2006 as a dwarf planet.
Asteroid groups
Asteroids in the asteroid belt are
divided into asteroid groups and families based
on their orbital characteristics. Asteroid moons are asteroids that orbit larger
asteroids. They are not as clearly distinguished as planetary moons, sometimes
being almost as large as their partners. The asteroid belt also contains main-belt comets,
which may have been the source of Earth's water.
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 resonancewith
Jupiter; that is, they go around the Sun three times for every two Jupiter
orbits.
The inner Solar System is also dusted
with rogue asteroids, many of which cross the orbits of the inner
planets.
Outer Solar System
The outer region of the Solar System is
home to the gas giants and their large moons. Many short-period comets,
including thecentaurs, also orbit in this region. Due to their greater
distance from the Sun, the solid objects in the outer Solar System contain a higher
proportion of volatiles such as water, ammonia and methane, than the rocky
denizens of the inner Solar System, as the colder temperatures allow these
compounds to remain solid.
Outer planets
The four outer planets, or gas giants
(sometimes called Jovian planets), collectively make up 99 percent of the mass
known to orbit the Sun. Jupiter
and Saturn are each many tens of times the mass of the Earth and consist
overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive
(<20 Earth masses) and possess more ices in their makeup. For these reasons,
some astronomers suggest they belong in their own category, "ice
giants". All four gas giants
have rings, although
only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet,
which designates planets outside Earth's orbit and thus includes both the outer
planets and Mars.
Jupiter
Jupiter (5.2 AU), at 318 Earth masses, is
2.5 times the mass of all the other planets put together. It is composed
largely of hydrogen and helium. Jupiter's strong internal heat
creates a number of semi-permanent features in its atmosphere, such as cloud
bands and the Great Red Spot.
Jupiter has 66 known satellites.
The four largest, Ganymede, Callisto, Io, and Europa, show
similarities to the terrestrial planets, such as volcanism and internal heating.
Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn
Saturn (9.5 AU),
distinguished by its extensive ring system, has
several similarities to Jupiter, such as its atmospheric composition and
magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a
third as massive, at 95 Earth masses, making it the least dense planet in the
Solar System. The rings of Saturn are made up of small ice and rock particles.
Saturn has 62 confirmed satellites;
two of which, Titan and Enceladus, show
signs of geological activity, though they are largely made of ice. Titan, the second-largest moon in the
Solar System, is larger than Mercury and the only satellite in the Solar System
with a substantial atmosphere.
Uranus
Neptune
Comets
Comets are small Solar System bodies,[e] typically only a few kilometres
across, composed largely of volatile ices. They have highly eccentric orbits,
generally a perihelion within the orbits of the inner planets and an aphelion
far beyond Pluto. When a comet enters the inner Solar System, its proximity to
the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail
of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting
less than two hundred years. Long-period comets have orbits lasting thousands
of years. Short-period comets are 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.
Centaurs
Trans-Neptunian region
The area beyond Neptune ,
or the "trans-Neptunian region", is still largely unexplored. It appears to consist
overwhelmingly of small worlds (the largest having a diameter only a fifth that
of the Earth and a mass far smaller than that of the Moon) composed mainly of
rock and ice. This region is sometimes known as the "outer Solar
System", though others use that term to mean the region beyond the
asteroid belt.
Kuiper belt
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. Though it contains at least three dwarf
planets, it is composed mainly of small Solar System bodies. However, 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,[76] 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 theresonances. Resonances
are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first
resonance begins within the orbit of Neptune
itself. The classical belt consists of objects having no resonance with Neptune , and extends from roughly 39.4 AU to
47.7 AU.[78]Members
of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be
discovered, (15760) 1992 QB1, and
are still in near primordial, low-eccentricity orbits.
Pluto and Charon
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.
Charon, Pluto's
largest moon, is sometimes described as part of a binary system with
Pluto, as the two bodies orbit a barycenter of
gravity above their surfaces (i.e., they appear to "orbit each
other"). Beyond Charon, three much smaller moons, Nix, P4 andHydra, orbit within
the system.
Pluto has a 3:2 resonance with Neptune ,
meaning that Pluto orbits twice round the Sun for every three Neptunian orbits.
Kuiper belt objects whose orbits share this resonance are called plutinos.
Haumea and Makemake
P
Haumea (43.34 AU
average), and Makemake (45.79 AU
average), while smaller than Pluto, are the largest known objects in the classical Kuiper
belt (that is, they are not in a confirmed resonance with
Neptune). 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 given names and designated dwarf planets in 2008.[81] Their orbits are far more inclined
than Pluto's, at 28° and 29°.
Scattered disc
P
The scattered disc, which overlaps the
Kuiper belt but extends much further outwards, is thought to be the source of
short-period comets. Scattered disc objects are believed to have been ejected
into erratic orbits by the gravitational influence of Neptune's early outward migration. Most
scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia
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."[83]Some
astronomers also classify centaurs as inward-scattered Kuiper belt objects
along with the outward-scattered residents of the scattered disc.
Eris
P
Eris (68 AU
average) is the largest known scattered disc object, and caused a debate about
what constitutes a planet, since it is 25% more massive than Pluto[85] and about the same diameter. It is the
most massive 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.
Farthest regions
The point at which the Solar System
ends and interstellar space begins is not precisely defined, since its outer
boundaries are shaped by two separate forces: the solar wind and the Sun's
gravity. The outer limit of the solar wind's influence is roughly four times
Pluto's distance from the Sun; this heliopause is considered the beginning of the interstellar medium.[31] However, the Sun's Roche sphere, the
effective range of its gravitational dominance, is believed to extend up to a
thousand times farther.
Heliopause
The heliosphere is divided into two
separate regions. The solar wind travels at roughly 400 km/s until it
collides with the interstellar wind;
the flow of plasma in the interstellar medium. The collision occurs at the termination shock,
which is roughly 80–100 AU from the Sun upwind of the interstellar medium
and roughly 200 AU from the Sun downwind.[87] Here the wind slows dramatically,
condenses and becomes more turbulent,[87] forming a great oval structure known
as the heliosheath. This
structure is believed to look and behave very much like a comet's tail,
extending outward for a further 40 AU on the upwind side but tailing many
times that distance downwind; but evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is in
fact forced into a bubble shape by the constraining action of the interstellar
magnetic field. Both Voyager 1 and Voyager 2 are reported to have passed the
termination shock and entered the heliosheath, at 94 and 84 AU from the
Sun, respectively. The outer boundary of the heliosphere, the heliopause, is the
point at which the solar wind finally terminates and is the beginning of
interstellar space.
The shape and form of the outer edge of
the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar
medium as well as solar magnetic
fields prevailing to the south, e.g. it is bluntly shaped with the northern
hemisphere extending 9 AU farther than the southern hemisphere. Beyond the
heliopause, at around 230 AU, lies the bow shock, a plasma
"wake" left by the Sun as it travels through the Milky Way.
No spacecraft have yet passed beyond
the heliopause, so it is impossible to know for certain the conditions in local
interstellar space. It is expected that NASA's Voyager spacecraft will pass the heliopause some time in
the next decade and transmit valuable data on radiation levels and solar wind
back to the Earth.[92] How well the heliosphere shields the
Solar System from cosmic rays is poorly understood. A NASA-funded team has
developed a concept of a "Vision Mission" dedicated to sending a
probe to the heliosphere.
Oort cloud
The hypothetical Oort cloud is a
spherical cloud of up to a trillion icy objects that is believed to be the
source for all long-period comets and to surround the Solar System at roughly
50,000 AU (around 1 light-year (LY)), and possibly to as far as
100,000 AU (1.87 LY). It is believed to be composed of comets that
were ejected from the inner Solar System by gravitational interactions with the
outer planets. Oort cloud objects move very slowly, and can be perturbed by
infrequent events such as collisions, the gravitational effects of a passing
star, or the galactic tide, the tidal force exerted by theMilky Way.
Sedna
p
90377 Sedna (525.86 AU average) is a large,
reddish probable dwarf planet 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.[97] 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.[98] Sedna is very likely a dwarf planet,
though its shape has yet to be determined with certainty.
Boundaries
P
Much of the Solar System is still
unknown. The Sun's gravitational field is estimated to dominate the
gravitational forces of surrounding stars out
to about two light years (125,000 AU). Lower estimates for the radius of
the Oort cloud, by contrast, do not place it farther than 50,000 AU. Despite discoveries such as Sedna, the
region between the Kuiper belt and the Oort cloud, an area tens of thousands of
AU in radius, is still virtually unmapped. There are also ongoing studies of
the region between Mercury and the Sun. Objects may yet be discovered in the
Solar System's uncharted regions.
Galactic context
The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with
a diameter of about 100,000 light-years containing about 200 billion stars. The
Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur. The Sun lies between 25,000 and 28,000
light years from the Galactic Centre,[103] 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. Thesolar apex, the
direction of the Sun's path through interstellar space, is near the
constellation of Hercules in the
direction of the current location of the bright star Vega. The
plane of the ecliptic lies at an angle of about 60° to the galactic plane.
The Solar System's location in the
galaxy is a factor in the evolution of life on
Earth. Its orbit is close to circular, and orbits near the Sun are at roughly
the same speed as that of the spiral arms. Therefore, the Sun passes through
arms only rarely. Since spiral arms are home to a far larger concentration of supernovae,
gravitational instabilities, and radiation which could disrupt the Solar
System, this has given Earth long periods of stability for life to evolve.[106] 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.[106] Even at the Solar System's current
location, some scientists have hypothesised that recent supernovae may have
adversely affected life in the last 35,000 years by flinging pieces of expelled
stellar core towards the Sun as radioactive dust grains and larger, comet-like
bodies.
Neighbourhood
P
The immediate galactic neighbourhood of
the Solar System is known as the Local Interstellar Cloud or
Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble, an
hourglass-shaped cavity in the interstellar medium roughly
300 light years across. The bubble is suffused with high-temperature plasma
that suggests it is the product of several recent supernovae.
There are relatively few stars within ten light years (95
trillion km) 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 dwarfsBarnard's
Star (at 5.9 light
years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star
within ten light years is Sirius, a bright main-sequence star roughly twice the Sun's mass and
orbited by a white dwarf called Sirius B. It lies 8.6 light
years away. The remaining systems within ten light years are the binary red
dwarf system Luyten 726-8 (8.7 light years) and the solitary red
dwarf Ross 154 (9.7 light years). The Solar System's closest solitary
sun-like star is Tau Ceti, which
lies 11.9 light years away. It has roughly 80% the Sun's mass, but only 60% of
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.
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Formation and evolution
The Solar System formed from the
gravitational collapse of a giant molecular cloud 4.568 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,[114] 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. 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 beginthermonuclear fusion.
The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was
achieved: the thermal pressure equaled the force of gravity. At this point the
Sun became a main-sequence star.
The Nice model explains many otherwise puzzling
features of the history and structure of the Solar System. In this model, the
four giant planets (Jupiter, Saturn, Uranus and Neptune) originally formed in
orbits between ~5.5 and ~17 astronomical units (AU)
from the Sun, (inside the current orbit of Uranus). A disk of planetesimals, of
~35 Earth masses, extended beyond this to ~35 AU. Gravitational interactions
between these planets and the planetismal disc caused changes to the planets'
orbits. Over a period of several hundred million years, Saturn, Uranus and
Neptune migrated outwards, Neptune passing
Uranus, while Jupiter migrated a small distance inwards.
The Solar System will remain roughly as
we know it today until the hydrogen in the core of the Sun has been entirely
converted to helium, which will occur roughly 5.4 billion years from now.
This will mark the end of the Sun's main sequence life. At this time, the core
of the Sun will collapse, and the energy output will be much greater than at
present. The outer layers of the Sun will expand to roughly up to 260 times its
current diameter; 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).[120] Eventually, the core will be hot
enough for helium fusion to begin in the core; the Sun will burn helium for a
fraction of the time it burned hydrogen in the core. The Sun is not massive
enough to commence fusion of heavier elements, and nuclear reactions in the
core will dwindle. Its outer layers will fall away into space, leaving a white dwarf, an
extraordinarily dense object, half the original mass of the Sun but only the
size of the Earth. The ejected outer layers will form what is known as a planetary nebula,
returning some of the material that formed the Sun -- but now enriched with heavier elements like carbon -- to the interstellar
medium.
Size of planets in Solar system
Body
|
Type of object
|
Mean radius
(km) |
Volume
(109 km3) |
Mass
×1021 kg (Yg) |
Density
g/cm3 |
Surface gravity
(m/s2) |
|
696,000
|
1,412,000,000
|
1,989,100,000
|
1.409
|
274.0
|
|||
69,911
|
1,431,280
|
1,898,600
|
1.33
|
24.79
|
|||
58,232
(w/orings) |
827,130
|
568,460
|
0.70
|
10.445
|
|||
25,362
|
68,340
|
86,832
|
1.30
|
8.87
|
|||
24,622
|
62,540
|
102,430
|
1.76
|
11.15
|
|||
6,371.0
|
1,083.21
|
5,973.6
|
5.515
|
||||
6,051.8
(w/o gas) |
928.43
|
4,868.5
|
5.24
|
8.872
|
|||
3,390.0
|
163.18
|
641.85
|
3.94
|
3.7
|
|||
2,439.7
|
60.83
|
330.2
|
5.43
|
3.7
|
References
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