The 'Solar System' or 'solar system' consists of the
Sun and the other
celestial objects gravitationally bound to it: the eight
planets, their 165 known
moons,
[1] three
dwarf planets (
Ceres,
Pluto, and
Eris and their four known moons), and billions of
small bodies. This last category includes
asteroids,
Kuiper belt objects,
comets,
meteoroids, and
interplanetary dust.
In broad terms, the charted regions of the Solar System consist of the Sun, four
terrestrial 'inner planets', an
asteroid belt composed of small rocky bodies, four
gas giant 'outer planets', and a second belt, called the
Kuiper belt, composed of icy objects. Beyond the Kuiper belt lies the
scattered disc, the
heliopause, and ultimately the hypothetical
Oort cloud.
In order of their distances from the Sun, the planets are
Mercury,
Venus,
Earth,
Mars,
Jupiter,
Saturn,
Uranus, and
Neptune. Six of the eight planets are in turn orbited by
natural satellites, usually termed "moons" after Earth's
Moon, and each of the outer planets is encircled by
planetary rings of dust and other particles. All the planets except Earth are named after gods and goddesses from
Greco-
Roman mythology. The three dwarf planets are
Pluto, the largest known Kuiper belt object;
Ceres, the largest object in the asteroid belt; and
Eris, which lies in the scattered disc.
Terminology
Planets and dwarf planets of the Solar System; while the size is to scale, the relative distances from the Sun are not.
Objects
orbiting the Sun are divided into three classes: planets, dwarf planets, and small Solar System bodies.
A
planet is any body in orbit around the Sun that a) has enough
mass to form itself into a
spherical shape and b) has
cleared its immediate neighbourhood of all smaller objects. There are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
On
August 24 2006 the
International Astronomical Union defined the term "planet" for the first time, excluding Pluto and reclassifying it under the new category of
dwarf planet along with
Eris and
Ceres.
[2]
A dwarf planet is not required to clear its neighbourhood of other celestial bodies. Other objects that may become classified as dwarf planets are
Sedna,
Orcus, and
Quaoar.
From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. But in the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, most notably Eris, which is slightly larger than Pluto.
The remainder of the objects in orbit around the Sun are
small Solar System bodies (SSSBs).
[3]
Natural satellites, or moons, are those objects in orbit around planets, dwarf planets and SSSBs, rather than the Sun itself.
A planet's distance from the Sun varies in the course of its
year. Its closest approach to the Sun is called its
perihelion, while its farthest distance from the Sun is called its
aphelion.
Astronomers usually measure distances within the Solar System in
astronomical units (AU). One AU is the approximate distance between the Earth and the Sun, or roughly 149,598,000
km (93,000,000
mi). Pluto is roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One
light year, the best known unit of interstellar distance, is roughly 63,240 AU.
Informally, the Solar System is sometimes divided into separate zones. The 'inner Solar System' includes the four
terrestrial planets and the main asteroid belt. Some define the 'outer Solar System' as comprising everything beyond the asteroids.
[4] Others define it as the region beyond Neptune, with the four
gas giants considered a separate "middle zone".
[5]
Layout and structure
The principal component of the Solar System is the Sun, a
main sequence G2 star that contains 99.86% of the system's known
mass and dominates it
gravitationally.
[6] Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's 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 usually at significantly greater angles to it.
All of the planets and most other objects also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There are exceptions, such as
Halley's Comet.
Objects travel around the Sun following
Kepler's laws of planetary motion. Each object orbits along an approximate ellipse with the Sun at one focus of the ellipse. The closer an object is to the Sun, the faster it moves. The orbits of the planets are nearly circular, but many comets, asteroids and objects of the Kuiper belt 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 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.
Formation
Main articles: Formation and evolution of the Solar System,
Stellar evolution,
Nebular hypothesis
The Solar System is believed to have formed according to the
nebular hypothesis, first proposed in 1755 by
Immanuel Kant and independently formulated by
Pierre-Simon Laplace.
[7] This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant
molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars.
[8] Studies of ancient
meteorites reveal traces of
elements only formed in the hearts of very large exploding stars, indicating that the Sun formed within a
star cluster, and in range of a number of nearby
supernovae explosions. The
shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal
gas pressures and cause collapse.
[9]
The region that would become the Solar System, known as the
pre-solar nebula,
[10] had a diameter of between 7000 and 20,000 AU
and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).
[12] As the nebula collapsed, conservation of
angular momentum made it rotate faster. As the material within the nebula
condensed, the
atoms within it began to collide with increasing frequency. The center, where most of the mass collected, became increasingly hotter than the surrounding disc.
As gravity, gas pressure,
magnetic fields, and rotation acted on the contracting nebula, it began to flatten into a spinning
protoplanetary disk with a diameter of roughly 200 AU
and a hot, dense
protostar at the center.
[13][14]
Studies of
T Tauri stars, young, pre-fusing solar mass stars believed to be similar to the Sun at this point in its evolution, show that they are often accompanied by discs of pre-planetary matter.
12 These discs extend to several hundred AU and reach only a thousand
kelvins at their hottest.
[15]
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.
After 100 million years, the pressure and density of
hydrogen in the center of the collapsing nebula became great enough for the
protosun to begin
thermonuclear fusion. This 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 star.
[16]
From the remaining cloud of gas and dust (the "
solar nebula"), the various planets formed. They are believed to have formed by
accretion: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten metres in diameter; then collided to form larger bodies (
planetesimals) of roughly 5 km in size; then gradually increased by further collisions at roughly 15
cm per year over the course of the next few million years.
[17]
The inner Solar System was too warm for volatile
molecules like
water and
methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc)
8 and composed largely of
compounds with high
melting points, such as
silicates and
metals. These rocky bodies eventually became the
terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the
asteroid belt.
[18]
Farther out still, beyond the
frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn became the
gas giants. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).
[19][20]
Once the young Sun began producing energy, the
solar wind (
see below) blew the gas and dust in the protoplanetary disk into interstellar space and ended the growth of the planets. T Tauri stars have far stronger
stellar winds than more stable, older stars.
[21][ Disc-Protoplanet interactions, Heng Hao, , , Astronomy and Astrophysics, 1979 ]
Sun
Main articles: Sun
The Sun as seen from Earth
The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior
density high enough to sustain
nuclear fusion, which releases enormous amounts of
energy, mostly
radiated into
space as
electromagnetic radiation such as
visible light.
The Sun is classified as a moderately large
yellow dwarf, but this name is misleading as, compared to stars in
our galaxy, the Sun is rather large and bright. Stars 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; the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common.
[22]
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 75 percent as bright as it is today.
[23]
Calculations of the ratios of hydrogen and
helium within the Sun suggest it is halfway through its life cycle. It will eventually move off the main sequence and become larger, brighter, cooler and redder, becoming a
red giant in about five billion years.
[24] At that point its
luminosity will be several thousand times its present value.
The Sun is a
population I star; it was born in the later stages of the
universe's evolution. It contains more elements heavier than hydrogen and helium ("
metals" in astronomical parlance) than older population II stars.
[25] 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.
[26]
Interplanetary medium
Main articles: 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,
[27] 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. The Sun's 11-year
sunspot cycle and frequent
solar flares and
coronal mass ejections disturb the heliosphere, creating
space weather.
[28] The Sun's rotating magnetic field acts on the interplanetary medium to create the
heliospheric current sheet, the largest structure in the solar system.
[29]
Earth's magnetic field protects
its atmosphere from interacting with the solar wind. Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.
[ ] The interaction of the solar wind with Earth's magnetic field creates the
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 planets which 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.
[30]
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.
[31] The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.
[32][33]
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. This region was, in old parlance, denoted inner space; the area outside the asteroid belt was denoted outer space.
Inner planets
Main articles: Terrestrial planet
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 solid
crusts and semi-liquid
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
:
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 "
wrinkle-ridges", probably produced by a period of contraction early in its history.
[34] Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.
[35] 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.
[36][37]
; Venus
:
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.
[13] 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.
[39]
; Earth
:
Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have
life. 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.
[40] It has one satellite, the
Moon, the only large satellite of a terrestrial planet in the Solar System.
; Mars
:
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.
[41] Mars has two tiny natural satellites (
Deimos and
Phobos) thought to be captured
asteroids.
[42]
Asteroid belt
Main articles: Asteroid belt
Image of the main asteroid belt and the Trojan asteroids
s are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals.
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.
[43] Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.
[ Hidden Mass in the Asteroid Belt, , G. A., Krasinsky, Icarus, 2002 ] The main belt is very sparsely populated;
spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10
-4 m are called
meteoroids.
[44]
Ceres
; Ceres
:
Ceres (2.77 AU) is the largest body in the asteroid belt and its only 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.
[45] It was again reclassified in 2006 as a dwarf planet.
; Asteroid groups
: Asteroids in the main belt are divided into
asteroid groups and 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
[46] 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 orbits.
The inner Solar System is also dusted with
rogue asteroids, many of which cross the orbits of the inner planets.
Mid Solar System
The middle region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the
centaurs, also lie in this region. It has no traditional name; it is occasionally referred to as the "outer Solar System", although recently that term has been more often applied to the region beyond Neptune. The solid objects in this region are composed of a higher proportion of "ices" (water, ammonia, methane) than the rocky denizens of the inner Solar System.
Outer planets
Main articles: Gas giant
From top to bottom: Neptune, Uranus, Saturn, and Jupiter
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's atmospheres are largely hydrogen and helium. Uranus and Neptune's atmospheres have a higher percentage of “ices”, such as
water,
ammonia and
methane. Some astronomers suggest they belong in their own category, “ice giants.”
[47] 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 Mars).
; Jupiter
:
Jupiter (5.2 AU), at 318 Earth masses, masses 2.5 times 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 internal heating.
[48] Ganymede, the largest satellite in the Solar System, is larger than Mercury.
; Saturn
:
Saturn (9.5 AU), famous for its extensive
ring system, has similarities to Jupiter, such as its atmospheric composition. Saturn is far less massive, being only 95 Earth masses. Saturn has
sixty known satellites (and 3 unconfirmed); two of which,
Titan and
Enceladus, show signs of geological activity, though they are largely
made of ice.
[49] Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
; Uranus
:
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 into space.
[50] Uranus has
twenty-seven known satellites, the largest ones being
Titania,
Oberon,
Umbriel,
Ariel and
Miranda.
; Neptune
:
Neptune (30 AU), though slightly smaller than Uranus, is more
massive (equivalent to 17 Earths) and therefore
denser. It radiates more internal heat, but not as much as Jupiter or Saturn.
[51] Neptune has
thirteen known satellites. The largest,
Triton, is geologically active, with
geysers of
liquid nitrogen.
[52] Triton is the only large satellite with a
retrograde orbit. Neptune is accompanied in its orbit by a number of
minor planets in a 1:1
resonance with it, termed
Neptune Trojans.
Comets
Comet Hale-Bopp
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, such as
Halley's Comet, 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.
[53] Some comets with
hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.
[54] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.
[55]
; Centaurs
:The
centaurs, which extend from 9 to 30 AU, are icy comet-like bodies that orbit in the region between Jupiter and Neptune. The largest known centaur,
10199 Chariklo, has a diameter of between 200 and 250 km.
[56] The first centaur discovered,
2060 Chiron, has been called a comet since it develops a coma just as comets do when they approach the Sun.
[57] Some astronomers classify centaurs as inward-scattered
Kuiper belt objects along with the outward-scattered residents of the
scattered disc.
[58]
Trans-Neptunian region
The area beyond Neptune, often called the outer Solar System 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.
Kuiper belt
Main articles: Kuiper 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. This region is thought to be the source of short-period comets. 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.
[59] Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
Diagram showing the resonant and classical Kuiper belt
The Kuiper belt can be roughly divided into the "
resonant" belt and the "
classical" belt. The resonant belt consists of objects with orbits linked to that of Neptune (e.g. orbiting twice for every three Neptune orbits, or once for every two). The resonant belt 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.
[60] Members of the classical Kuiper belt are classified as
cubewanos, after the first of their kind to be discovered, .
[61]
; 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, having a 3:2
resonance with Neptune (it orbits twice round the Sun for every three Neptunian orbits). Kuiper belt objects whose orbits share this resonance are called
plutinos.
[62]
Scattered disc
Main articles: Scattered disc
Black: scattered; blue: classical; green: resonant
The scattered disc overlaps the Kuiper belt but extends much further outwards. Scattered disc objects are believed to come from the Kuiper belt, having 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."
[63]
; Eris
:
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.
[64] 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 solar wind is believed to surrender to the
interstellar medium at roughly four times Pluto's distance. However, the Sun's
Roche sphere, the effective range of its gravitational influence, is believed to extend up to a thousand times farther.
Heliopause
The
heliosphere is divided into two separate regions. The solar wind travels at its maximum velocity out to about 95 AU, or three times the orbit of Pluto. The edge of this region is the
termination shock, the point at which the solar wind collides with the opposing winds of the
interstellar medium. Here the wind slows, 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 at its stellar-windward side, but tailing many times that distance in the opposite direction. 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 Enters Solar System's Final Frontier ]
The shape and form of the outer edge of the heliosphere is likely affected by the
fluid dynamics of interactions with the interstellar medium,
[65] 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.
[66]
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A dedicated mission beyond the heliosphere has been suggested.
[67][68]
Oort cloud
Main articles: Oort cloud
Artist's rendering of the Kuiper Belt and hypothetical
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 around 50,000 AU, and possibly to as far as 100,000 AU. 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.
[69][70]
Telescopic image of Sedna
'Sedna and the inner Oort cloud'
:
90377 Sedna 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 , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.
[71] 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.
[72] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.
Boundaries
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). The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU.
[73] 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.
[74] Objects may yet be discovered in the Solar System's uncharted regions.
Galactic context
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.
[75] Our Sun resides in one of the Milky Way's outer spiral arms, known as the
Orion Arm or Local Spur.
[76] The Sun lies between 25,000 and 28,000 light years from the
Galactic Center, 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.
[77]
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.
[ Galactic Habitable Zones Leslie Mullen ] The Solar System also lies well outside the star-crowded environs of the galactic center. Near the center, 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 center 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.
[78]
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
Vega.
[79]
Neighbourhood
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.
[80]
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 6 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
blue dwarf 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).
[81] Our closest solitary sunlike 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.
[82] The closest earth-like planet discovered,
Gliese 581c, is 20.40 light years away.
Discovery and exploration
Main articles: Geocentric model,
Heliocentrism
For many thousands of years, people, with a few notable exceptions, did not believe the Solar System existed. The Earth was believed not only to be stationary at the centre of the
universe, but to be categorically different from the divine or ethereal objects that moved through the sky. While
Nicolaus Copernicus and his predecessors, such as the
Indian mathematician-astronomer
Aryabhata and the Greek philosopher
Aristarchus of Samos, had speculated on a heliocentric reordering of the cosmos, it was the conceptual advances of the 17th century, led by
Galileo Galilei,
Johannes Kepler, and
Isaac Newton, which led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth, and therefore could be material worlds in their own right, with such earthly phenomena as craters, weather, geology, seasons and ice caps.
The five closest planets to Earth – Mercury, Venus, Mars, Jupiter, and Saturn – are amongst the brightest objects in the night sky and were called "πλανήτης" (''planētēs'', meaning "wanderer") by the
Ancient Greeks. They were known to move across the fixed stars; this is the origin of the word "planet". Uranus is also visible without optical aid at its brightest, but it is at the very limit of naked-eye detectability and therefore evaded discovery until 1781.
Telescopic observations
Main articles: Timeline of solar system astronomy
A replica of Isaac Newton's telescope
The first exploration of the Solar System was conducted by
telescope, when
astronomers first began to map those objects too faint to be seen with the naked eye.
Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the
Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.
[83] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon
Titan and the shape of the
rings of Saturn.
[84] Giovanni Domenico Cassini later discovered four more
moons of Saturn, the
Cassini division in Saturn's rings, and the
Great Red Spot of Jupiter.
[85]
Edmond Halley realised in 1705 that repeated sightings of
a comet were in fact recording the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun.
[86]
In 1781,
William Herschel was looking for
binary stars in the constellation of
Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.
[87]
Giuseppe Piazzi discovered
Ceres in 1801, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual reclassification as
asteroids.
[88]
By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out.
Urbain Le Verrier's calculations eventually led to the discovery of Neptune.
[89] The excess perihelion precession of
Mercury's orbit led Le Verrier to postulate the intra-Mercurian planet
Vulcan in 1859 – but that would turn out to be a red herring.
Further apparent discrepancies in the orbits of the outer planets led
Percival Lowell to conclude that yet another planet, "
Planet X," must still be out there. After his death, his
Lowell Observatory conducted a search which ultimately led to
Clyde Tombaugh's discovery of
Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was reclassified in 2006 as a
dwarf planet by the IAU.
89
In 1992, astronomers
David Jewitt of the
University of Hawaii and
Jane Luu of the
Massachusetts Institute of Technology discovered . This object proved to be the first of a new population, which came to be known as the
Kuiper belt; an icy analogue to the asteroid belt of which such objects as Pluto and Charon were deemed a part.
[90][91]
Mike Brown,
Chad Trujillo and
David Rabinowitz announced the discovery of
Eris in 2005, a
scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune.
[92]
Observations by spacecraft
Main articles: Timeline of Solar System exploration
Artist's conception of ''
Pioneer 10'', which passed the orbit of Pluto in 1983. The last transmission was received in January 2003, sent from approximately 82 AU away. The 35-year-old space probe is now receding at over 43,400 km/h (27,000 mph) from the Sun.
[93]
Since the start of the
Space Age, a great deal of exploration has been performed by
robotic spacecraft missions that have been organized and executed by various space agencies.
All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. This achievement was realized just last year. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets and, in the case of
landers, perform tests of the soils and
atmospheres of some.
The first manmade object sent into space was the Soviet satellite ''
Sputnik 1'', launched in 1957, which successfully orbited the Earth for over a year. The American probe ''
Explorer 6'', launched in 1959, was the first satellite to image the Earth from space.
The first successful probe to fly by another Solar System body was ''
Luna 1'', which sped past the Moon in 1959. Originally meant to impact with the Moon, it instead missed its target and became the first manmade object to orbit the Sun. ''
Mariner 2'' was the first probe to fly by another planet, Venus, in 1962. The first successful flyby of Mars was made by ''
Mariner 4'' in 1964.
Mercury was first encountered by ''
Mariner 10'' in 1974.
The first probe to explore the outer planets was ''
Pioneer 10'', which flew by Jupiter in 1973. ''
Pioneer 11'' was the first to visit Saturn, in 1979. The
''Voyager'' probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980 – 1981. ''
Voyager 2'' then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The ''Voyager'' probes are now far beyond Neptune's orbit, and are on course to find and study the
termination shock, heliosheath, and heliopause. According to
NASA, both ''Voyager'' probes have encountered the termination shock at a distance of approximately 93 AU from the Sun.
No Kuiper belt object has yet been visited by a spacecraft. Launched on
January 19 2006, the ''
New Horizons'' probe is currently en route to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.
[95]
In 1966, the Moon became the first Solar System body beyond Earth to be orbited by an
artificial satellite (''
Luna 10''), followed by Mars in 1971 (''
Mariner 9''), Venus in 1975 (''
Venera 9''), Jupiter in 1995 (''
Galileo'', which also made the first asteroid flyby,
951 Gaspra, in 1991), the asteroid
433 Eros in 2000 (''
NEAR Shoemaker''), and Saturn in 2004 (''
Cassini–Huygens''). The
MESSENGER probe is currently en route to commence the first orbit of Mercury in 2011, while the ''
Dawn'' spacecraft is currently set to orbit the asteroid
Vesta in 2011 and the dwarf planet
Ceres in 2015.
The first probe to
land on another Solar System body was the
Soviet ''
Luna 2'' probe, which impacted the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966 (''
Venera 3''), Mars in 1971 (''
Mars 3'', although a fully successful landing didn't occur until ''
Viking 1'' in 1976), the asteroid
433 Eros in 2001 (''
NEAR Shoemaker''), and Saturn's moon
Titan in 2005 (''
Huygens''). The ''Galileo'' orbiter also dropped a probe into Jupiter's atmosphere in 1995; since Jupiter has no physical surface, it was destroyed by increasing temperature and pressure as it descended.
Manned exploration
Manned exploration of the Solar System is currently confined to Earth's immediate environs. The first human being to reach space (defined as an
altitude of over 100 km) and to orbit the Earth was
Yuri Gagarin, a
Soviet cosmonaut who was launched in ''
Vostok 1'' on
April 12,
1961. The first man to walk on the surface of another Solar System body was
Neil Armstrong, who stepped onto the
Moon on
July 21,
1969 during the
Apollo 11 mission. The
United States'
Space Shuttle is the only reusable spacecraft to successfully make multiple orbital flights. The first orbital
space station to host more than one crew was
NASA's
Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station
Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the
International Space Station, has maintained a continuous human presence in space since then. In 2004,
SpaceShipOne became the first privately funded vehicle to reach space on a suborbital flight. That same year,
President George W. Bush announced the
Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars.
See also
★ List of Solar System objects:
★
★
By orbit
★
★
By mass
★
★
By radius
★
★
By name
★
★
By surface gravity
★
Attributes of the largest solar system bodies
★
Astronomical symbols
★
Geological features of the solar system
★
Numerical model of solar system
★
Table of planetary attributes
★
Timeline of discovery of Solar System planets and their natural satellites
★
Solar system model
★
Space colonization
★
Solar System in fiction
★
Celestia – Space-simulation on your computer (OpenGL)
★
Family Portrait (Voyager)
★
The Parable of the Solar System Model
Notes
- Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects ('Solar System'). However, the name is commonly rendered in lower case ('solar system') including in the ''Oxford English Dictionary'', ''Merriam-Webster's 11th Collegiate Dictionary'', and ''Encyclopædia Britannica''.
- The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),[96] the Kuiper Belt (estimated at roughly 0.1 Earth mass)
References
1. The Jupiter Satellite Page Scott S. Sheppard
2. Farewell Pluto?
3. The Final IAU Resolution on the definition of "planet" ready for voting
4. An Overview of the Solar System nineplanets.org
5. New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt Amir Alexander
6. The origin and evolution of the solar system M Woolfson
7. The Past History of the Earth as Inferred from the Mode of Formation of the Solar System, , T. J. J., See, Proceedings of the American Philosophical Society,
8. Lecture 13: The Nebular Theory of the origin of the Solar System
9. New Theory Proposed for Solar System Formation Jeff Hester
10. The chemical composition of the pre-solar nebula Irvine, W. M.
11.
12. Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a 1 Arcsecond Imaging Survey: Evolution and Diversity of the Disks in Their Accretion Stage, Yoshimi Kitamura, , , The Astrophysical Journal,
13.
14. Present Understanding of the Origin of Planetary Systems
15. Magnetic Star-Disk Coupling in Classical T Tauri Systems Manfred Küker, Thomas Henning and Günther Rüdiger
16. The formation of stars
17. The Formation of Planetesimals Peter Goldreich and William R. Ward
18. The Primordial Excitation and Clearing of the Asteroid Belt Jean-Marc Petit and Alessandro Morbidelli
19. Remote infrared observations of parent volatiles in comets: A window on the early solar system, , M. J., Mummma, Advances in Space Research, 2003
20. The formation of Uranus and Neptune in the Jupiter–Saturn region of the Solar System Edward W. Thommes, Martin J. Duncan and Harold F. Levison
21. On the disruption of a protoplanetary disk nebula by a T Tauri like solar wind, , B. G., Elmegreen, Astronomy and Astrophysics, 1979
22. The Second Guide Star Catalogue and Cool Stars Smart, R. L.; Carollo, D.; Lattanzi, M. G.; McLean, B.; Spagna, A.
23. Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere, , J.F., Kasting, Science, 1986
24. The Once and Future Sun Richard W. Pogge
25. On the Two Oosterhoff Groups of Globular Clusters, T. S. van Albada, Norman Baker, , , Astrophysical Journal, 1973
26. An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect Charles H. Lineweaver
27. Solar Physics: The Solar Wind
28. The Sun Does a Flip
29. Artist's Conception of the Heliospheric Current Sheet
30. Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays, , U. W., Langner, Advances in Space Research, 2005
31. Long-term Evolution of the Zodiacal Cloud
32. ESA scientist discovers a way to shortlist stars that might have planets
33. Origins of Solar System Dust beyond Jupiter, , M., Landgraf, The Astronomical Journal, 2002
34. Schenk P., Melosh H.J. (1994), ''Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere'', Abstracts of the 25th Lunar and Planetary Science Conference, 1994LPI....25.1203S
35. Mercury Bill Arnett
36. Benz, W., Slattery, W. L., Cameron, A. G. W. (1988), ''Collisional stripping of Mercury's mantle'', Icarus, v. 74, p. 516–528.
37. Cameron, A. G. W. (1985), ''The partial volatilization of Mercury'', Icarus, v. 64, p. 285–294.
38.
39. Climate Change as a Regulator of Tectonics on Venus Paul Rincon
40. Earth's Atmosphere: Composition and Structure Anne E. Egger, M.A./M.S.
41. Modern Martian Marvels: Volcanoes? David Noever
42. A Survey for Outer Satellites of Mars: Limits to Completeness Scott S. Sheppard, David Jewitt, and Jan Kleyna
43. New study reveals twice as many asteroids as previously believed
44. On the Definition of the Term Meteoroid, Beech, M., , , Quarterly Journal of the Royal Astronomical Society, 1995
45. History and Discovery of Asteroids
46. Main-Belt Comets May Have Been Source Of Earths Water Phil Berardelli
47. Formation of Giant Planets Jack J. Lissauer, David J. Stevenson
48. Geology of the Icy Galilean Satellites: A Framework for Compositional Studies Pappalardo, R T
49. Cryovolcanism on the icy satellites J. S. Kargel
50. 10 Mysteries of the Solar System Hawksett, David; Longstaff, Alan; Cooper, Keith; Clark, Stuart
51. Post Voyager comparisons of the interiors of Uranus and Neptune Podolak, M.; Reynolds, R. T.; Young, R.
52. The Plausibility of Boiling Geysers on Triton Duxbury, N.S., Brown, R.H.
53. Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration?, Sekanina, Zdenek, , , Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic, 2001
54. A study of the original orbits of ''hyperbolic'' comets, , M., Królikowska, Astronomy & Astrophysics, 2001
55. The activities of comets related to their aging and origin Fred L. Whipple
56. TNO/Centaur diameters and albedos
57. Chiron biography Patrick Vanouplines
58. List Of Centaurs and Scattered-Disk Objects
59. The Solar System Beyond The Planets Audrey Delsanti and David Jewitt
60. Procedures, Resources and Selected Results of the Deep Ecliptic Survey M. W. Buie, R. L. Millis, L. H. Wasserman, J. L. Elliot, S. D. Kern, K. B. Clancy, E. I. Chiang, A. B. Jordan, K. J. Meech, R. M. Wagner, D. E. Trilling
61. Beyond Neptune, the new frontier of the Solar System E. Dotto1, M.A. Barucci2, and M. Fulchignoni
62.
63. The 1000 km Scale KBOs David Jewitt
64. The discovery of 2003 UB313 Eris, the 10th planet largest known dwarf planet. Mike Brown
65. A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction Fahr, H. J.; Kausch, T.; Scherer, H.
66. The Sun's Heliosphere & Heliopause P. C. Frisch
67. Innovative Interstellar Explorer, R. L. McNutt, Jr. et al., , , AIP Conference Proceedings, 2006
68. Interstellar space, and step on it!
69. Rapid collisional evolution of comets during the formation of the Oort cloud. Stern SA, Weissman PR.
70. The Kuiper Belt and the Oort Cloud Bill Arnett
71. Sedna - 2003 VB12 David Jewitt
72. Sedna Mike Brown
73. The Solar System: Third edition, T. Encrenaz, JP. Bibring, M. Blanc, MA. Barucci, F. Roques, PH. Zarka, , , Springer, 2004,
74. A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Levison H.F.; Hassler D.M.
75. Magnetic fields in cosmology A.D. Dolgov
76. Three Dimensional Structure of the Milky Way Disk R. Drimmel, D. N. Spergel
77. Period of the Sun's Orbit around the Galaxy (Cosmic Year) Stacy Leong
78. Supernova Explosion May Have Caused Mammoth Extinction
79. Elementi di Astronomia e Astrofisica per il Corso di Ingegneria Aerospaziale V settimana C. Barbieri
80. Near-Earth Supernovas
81. Stars within 10 light years
82. Tau Ceti
83. Galileo Galilei (1564–1642) Eric W. Weisstein
84. Discoverer of Titan: Christiaan Huygens
85. Giovanni Domenico Cassini (June 8, 1625–September 14, 1712)
86. Comet Halley
87. Herschel, Sir William (1738–1822)
88. Discovery of Ceres: 2nd Centenary, 1 January 1801–1 January 2001
89. Mathematical discovery of planets J. J. O'Connor and E. F. Robertson
90. KUIPER BELT OBJECTS: Relics from the Accretion Disk of the Sun Jane X. Luu and David C. Jewitt
91. List of Trans-Neptunian Objects Minor Planet Center
92. Eris (2003 UB313
93. Farewell Pioneer 10 Donald Savage; Michael Mewhinney
94. Time Line of Space Exploration Randy Culp
95. New Horizons NASA's Pluto-Kuiper Belt Mission
96. ORIGIN AND DYNAMICAL EVOLUTION OF COMETS AND THEIR RESERVOIRS Alessandro Morbidelli
External links
★
Solar System Profile by
NASA's Solar System Exploration
★
NASA's Solar System Simulator
★
NASA/JPL Solar System main page
★
The Nine Planets - Comprehensive Solar System site by Bill Arnett
★
SPACE.com: All About the Solar System
★
Illustration of the distance between planets
★
Illustration comparing the sizes of the planets with each other, the sun, and other stars
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