View source for User:HarryAlffa/Solar System synopsis

{{main|Solar System}}

[[Image:Planets2008.jpg|450px|thumb| [[Sun]], [[Planet]]s and [[dwarf planet]]s. Spheres are to scale, distances between are not.]]

The '''Solar System''' (or '''solar system''') is the [[Sun]], the [[Earth|planet Earth]] and the [[Moon]], the other [[planets]] and their [[moons]], and the rest of the [[planetary system]]. It is most remarkable for the planet Earth as the only place in the [[Universe]] known to [[evolve]] and harbour <!--(intelligent)-->[[life]].

The Sun determines the Solar System, it makes up 99.86% of the [[Mass versus weight|mass]] of the system<ref>{{cite web  |title=Solar Data Sheet |publisher= [[Space.com]] |url=http://www.space.com/scienceastronomy/solarsystem/sun-ez.html |accessdate=2009-08-18 }}</ref>, and the gravity of this mass dominates the rest. It emits a continual but fluctuating, low [[density]] wind of charged particles called the [[solar wind]] which encapsulates much of the system in a "bubble", of even less density, called the [[heliosphere]].

All '''eight planets''' orbit the Sun at different [[Astronomical unit|distances from it]], but on roughly the same [[Plane (geometry)|plane]], so that if you could look at the Solar System "edge on" then all of the planets would roughly be in a horizontal plane around the Sun - the [[Ecliptic|plane of the ecliptic]]. No object orbits the Sun in a perfect circle, their paths are better described as oval shaped, or [[Elliptic orbit|elliptical orbit]]s, some more elliptical than others.

The [[Ecliptic|plane of the ecliptic]] is split into regions and sub-regions. Their structure, and defining components, in order of their distances from the Sun are:
[[Image:Solarsys.svg|thumb|450px| Incomplete, not to scale - illustration only, of plane of the ecliptic and regions (zones).]]
* <big>Inner Solar System</big>
:* <big>[[Terrestrial planet |Terrestrial Planets]]</big>
::* [[Mercury (planet)|Mercury]]
::* [[Venus]]
::* [[Earth]]
::* [[Mars]]
:* [[Asteroid Belt]]

* <big>Outer Solar System</big>
:* <big>[[gas giant|Gas Giants]]</big>
::* [[Jupiter]]
::* [[Saturn]]
::* [[Uranus]]
::* [[Neptune]]
:* <big>[[Trans-Neptunian object|Trans-Neptune]]</big>
::* [[Kuiper Belt]]
::* [[Scattered Disk]]

The [[asteroid belt]] contains rocky & metallic [[small Solar System bodies]] called [[asteroids]]; plus one of the [[dwarf planets]] - the others populate the [[Trans-Neptunian object|trans-Neptune region]] and are considerably smaller than any planet, with none reaching 25% the [[mass]] of the [[Moon]]. The Kuiper Belt & Scattered Disk contain [[small Solar System bodies]] with lesser density than those closer to the Sun, whose surface composition is largely ices, such as water and methane, but whose internal make-up is difficult to determine due to limitations of current observations.

One thousand times farther from the Sun than the Kuiper Belt, where long period [[comets]] spend most of their time, is the theoretical, spherical [[Oort Cloud]], encapsulating the whole of the Solar System.

Some controversy continues after the [[International Astronomical Union]] 2006 [[Pluto#Classification|reclassification of Pluto]] from a planet to a dwarf planet.
----
'''Did you know?'''

Mercury & Venus are the only two planets without orbiting [[natural satellite]]s, or "moons".

Earth is the only planet not named after a deity from [[Greek mythology|Greco]]-[[Roman mythology|Roman]] [[mythology]].

The gas giants are each encircled by [[planetary ring]]s of dust and other particles, Saturn's being the most visible and famous.

Dwarf planets are the only category of objects that populate more than one region of the Solar System; the [[Asteroid Belt]]; the [[Kuiper Belt]]; and the [[scattered disc |Scattered Disc]].

The largest dwarf planet is less than one fifth the diameter of Earth.

The Heliopause is the only region not named for any population of celestial objects.

{{TOClimit|limit=3}}

==Definition==
The '''Solar System''' is the [[Sun]] and those [[astronomical object|celestial objects]] bound to it by [[gravity]].

=Structure=
The [[hub]] in the Solar System's structure is the [[Sun]], which determines the system and makes up 99.86% of its [[Mass versus weight|mass]]<ref>{{cite web  |title=Solar Data Sheet |publisher= [[Space.com]] |url=http://www.space.com/scienceastronomy/solarsystem/sun-ez.html |accessdate=2009-08-18 }}</ref>. The [[gravity]] of this mass dominates the rest, as demonstrated by the other objects of the system orbiting it.

All eight planets circle the Sun in anticlockwise, oval shaped or [[Elliptic orbit|elliptical orbit]]s, at different [[Astronomical unit|distances from it]], but on roughly the same [[Plane (geometry)|plane]], so that if you could look at the Solar System "edge on" then all the planets would roughly be in this [[Ecliptic|ecliptic plane]] around the Sun.

Travelling out from the Sun, the [[Ecliptic|ecliptic plane]] can be split into regions;

[[Image:Solarsys.svg|thumb|450px| Incomplete, not to scale - illustration only, of ecliptic plane and regions (zones).]]
* <big>Inner Solar System</big>
:* [[Mercury (planet)|Mercury]]
:* [[Venus]]
:* [[Earth]]
:* [[Mars]]
:* [[Asteroid Belt]]

* <big>Outer Solar System</big>
:* [[Jupiter]]
:* [[Saturn]]
:* [[Uranus]]
:* [[Neptune]]
:* [[Kuiper Belt]]
:* [[Scattered Disk]]

One thousand times farther from the Sun than the [[Kuiper Belt]], where long period [[comets]] spend most of their time, is the theoretical, spherical [[Oort Cloud]], encapsulating the whole of the Solar System.

The definition of the outer Solar System may be evolving, so that it ends at [[Neptune]], and the [[Trans-Neptunian object|Trans-Neptune region]], or "third zone", is now a region equal in the hierarchy to the inner and outer Solar System regions.<ref>{{cite web 
|title=New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt
|author=Amir Alexander
|work=The Planetary Society year=2006
|url=http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html
|accessdate=2009-05-27}}</ref>

* Inner Solar System
:* Terrestrial planets, Asteroid belt
* Outer Solar System
:* Gas Giants
* [[Trans-Neptunian object|Trans-Neptune]] region
:* [[Kuiper Belt]]
:* [[Scattered Disk]]
:* [[Oort Cloud]]

==Ecliptic plane & orbital inclination==
[[Image:Solarsys.svg|thumb|centre|560px|Not to scale - illustration only. Notice the tilt of the closest orbit - Mercury, the most [[inclined orbit]].]]

The [[Ecliptic|ecliptic plane]] is a [[plane]], visualized as an imaginary disk with the Sun at its centre. This plane is used as a reference plane for the Solar System.

The Earth's orbital path describes an [[ellipse]] in space around the Sun, so a pattern of [[concentric]] ellipses is described by the planets. Imagining the Earth's ellipse was in fact the edge of a disk with the Sun at the centre, this imaginary disk, bisecting the Sun, is how the [[Ecliptic|ecliptic plane]] is arrived at. It is then imagined that the disk radiates out to the edge of the Solar System.

If you could look at the Solar System with the ecliptic plane "edge on" then all the planets would orbit the Sun roughly in this horizontal plane around a centre line of the Sun, but not around the Sun's equator.

===Orbital inclination===
Just as a hat can be worn at a jaunty angle, so the ellipses described by the planets are at a bit of an angle to that described by the Earth, and therefore to the [[Ecliptic|ecliptic plane]]. The jauntiness of the angle to the ecliptic plane is the degree the planet's orbit is [[Inclined orbit|inclined]], with all the planet's orbits inclined to this plane, except the Earth's because that defines it.

====Thickness of ecliptic plane====
;Height of Mercury above ecliptic plane
:X = 7.005 = angle from Mercury to ecliptic plane at Sun
:Hypot =69,816,900 km
:Opposite = Sin(x) * Hypot
:Opposite = Sin(7.005) * 69,816,900
:Opposite = 0.12195595893261418320170710813549 * 69,816,900
:Opposite = 8,514,586.9892024311671752649979849 km
:Height = (Opposite + Mercury radius) * 2
:Height = (Opposite + 2,439) * 2
;"Thickness" of ecliptic plane
:Thick = 8,517,025.9892024311671752649979849 * 2
:Thick = 17,034,051.97840486233435052999597 km

===Elliptical orbit's eccentricity, and year===
The orbits of objects around the Sun are accurately described as [[Elliptic orbit|elliptical orbit]]s. The [[Eccentricity (mathematics)|eccentricity]] of a perfect circle is zero, the eccentricity of an [[Orbital eccentricity| elliptical orbit]] is greater than zero and less than one.

All members of the [[planetary system]] have elliptical orbits, a body's closest approach to the Sun is called its [[perihelion]], while its farthest distance from the Sun is called its [[aphelion]].

The [[year]] on Earth is one [[Earth year]], the amount of time it takes the Earth to make one orbit of the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which [[Mars]] completes its own orbit.

==Anticlockwise==
Looking at the Solar System oriented with the Sun's north pole pointing upwards, anticlockwise is the general rule for everything: bodies orbiting the Sun; bodies turning about their own axis, like the Earth; moons orbiting the ''primary'' body, like the Earth's Moon.  Anything "disobeying" these "rules" is said to be [[retrograde orbit|retrograde]], in either its orbit of the Sun or other primary - which is rare, or its own rotation - which is uncommon.

===Rounded body axial tilt===
There is no rule of thumb for the tilt of the axis of rotation for rounded bodies, except to say that none is a [[perpendicular]] to the [[ecliptic plane]].

==Regions==
The [[Ecliptic|ecliptic plane]] can be split into regions and sub-regions. Their structure, and defining components, in order of their distances from the Sun are:
[[Image:Solarsys.svg|450px|thumb|right| Incomplete, not to scale - illustration only, of ecliptic plane and regions (zones).]]
* <big>Inner Solar System</big>
:* [[Terrestrial planet |Terrestrial Planets]]
::* [[Mercury (planet)|Mercury]]
::* [[Venus]]
::* [[Earth]]
::* [[Mars]]
:* [[Asteroid Belt]]

* <big>Outer Solar System</big>
:* [[gas giant|Gas Giants]]
::* [[Jupiter]]
::* [[Saturn]]
::* [[Uranus]]
::* [[Neptune]]
:* [[Trans-Neptunian object|Trans-Neptune]]
::* [[Kuiper Belt]]
::* [[Scattered Disk]]

The [[asteroid belt]] contains [[asteroids]]; plus one of the [[dwarf planets]] - the others populate the [[Trans-Neptunian object|trans-Neptune region]] (beyond Neptune) and are considerably smaller than any planet, with none reaching 25% the [[mass]] of the [[Moon]].

One thousand times farther from the Sun than the Kuiper Belt, where long period [[comets]] spend most of their time, is the theoretical, spherical [[Oort Cloud]], encapsulating the whole of the Solar System.

Note that the [[dwarf planet]]s are the only category to populate more than one region.

===Inner Solar System===
* [[Terrestrial planet|Terrestrial planet]]s are the four rocky planets;
:* [[Mercury (planet)|Mercury]]
:* [[Venus]]
:* [[Earth]]
:* [[Mars]]
* [[Asteroid belt|Asteroid Belt]] consists of;
:* [[Ceres]] (the only '''[[dwarf planet]]''' in the region)
:* [[Asteroid|Asteroids]]

The inner Solar System is the terrestrial planets and asteroid belt, and is sometimes called the ''terrestrial region''<ref>{{cite journal|title=New Dynamical Results in the Terrestrial Region of the Solar System
|author=Innanen, K. A.; Mikkola, S.
|journal=Nature
|url=http://www.ncbi.nlm.nih.gov/pubmed/15565147
|year=1995
|volume=432
|issue=7016
|pages=479–482
|doi=10.1038/nature03088
|pmid=15565147
|s2cid=4362887
|accessdate=2009-6-1}}</ref>
because the asteroids have a similar composition to the terrestrial planets, it is also a term commonly used for the inner rocky region of [[protoplanetary disk]]s<ref>{{cite journal|title=The building blocks of planets within the 'terrestrial' region of protoplanetary disks.|
author=van Boekel R, Min M, Leinert Ch et al.
| journal=Aas/Division for Planetary Sciences Meeting Abstracts #27 |url=http://adsabs.harvard.edu/abs/1995DPS....27.0807I
|year=2004
| volume=27 | bibcode=1995DPS....27.0807I |accessdate=2009-6-1}}</ref>.

Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth, but gravitational perturbations ejected most of the material from the belt within about a million years of formation, leaving behind less than 0.1% of the original mass<ref name="icarus153">{{cite journal
  | author=Petit, J.-M.; Morbidelli, A.; Chambers, J.
  | title=The Primordial Excitation and Clearing of the Asteroid Belt
  | journal=Icarus
  | year=2001
  | volume=153
  | issue=2
 | pages=338–347
  | url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
  | accessdate=2007-03-22  | doi = 10.1006/icar.2001.6702 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>. In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,<ref>{{cite journal
  | author=Edgar, R.; Artymowicz, P.
  | title=Pumping of a Planetesimal Disc by a Rapidly Migrating Planet
  | journal=Monthly Notices of the Royal Astronomical Society
  | year=2004
  | volume=354
  | issue=3
  | pages=769–772
  | url=http://www.astro.su.se/~pawel/edgar+artymowicz.pdf
  | accessdate=2007-04-16  | doi = 10.1111/j.1365-2966.2004.08238.x <!--Retrieved from CrossRef by DOI bot-->
| s2cid=18355985
 }}</ref> preventing the formation of planet-sized bodies.

===Outer Solar System===
* [[gas giant|Gas Giant Planets]] are;
:* [[Jupiter]]
:* [[Saturn]]
:* [[Uranus]]
:* [[Neptune]]

Sometimes the ''Outer Solar System'' is taken to mean; the [[gas giant |Gas Giant Planets]], the [[Kuiper belt|Kuiper Belt]], and the [[scattered disc |Scattered Disk]], but with more recent discoveries of objects in the [[Kuiper belt|Kuiper Belt]] and the [[scattered disc |Scattered Disk]], and with more dwarf planet discoveries, the ''Outer Solar System'' is now usually synonymous with just the Gas Giants.

===Trans-Neptune region===
With the discovery of more dwarf planets and the realisation of the extend of the [[Kuiper belt|Kuiper Belt]], and the [[scattered disc |Scattered Disk]], the trans-Neptune region has been recognised.

* [[Kuiper belt|Kuiper Belt]] consists of;
:* [[Pluto]]
:* [[Haumea_(dwarf planet)|Haumea]]
:* [[Makemake (dwarf planet)|Makemake]]
:* [[Kuiper_belt |Kuiper Belt Objects]] and possibly other dwarf planets
:* short-period [[Comet|Comets]]
* [[scattered disc |Scattered Disk]] consists of;
:* [[Eris (dwarf planet)|Eris]]
:* [[scattered disc |Scattered Disk Objects]] and possibly other dwarf planets
* [[Oort cloud|Oort Cloud]] consists of;
:* [[Detached object|Oort Cloud Objects]]
:* long-period [[Comet|Comets]]

Note that the [[dwarf planet]]s are the only category to populate more than one region.

The '''[[Oort cloud]]''' is about a thousand times farther from the Sun than the Kuiper belt.

==Measures & masses==
The units normally used for distance and [[mass]] are the [[kilometre]] (km) and the [[kilogram]] (kg). However when conveying the scale of the Solar System the value of the numbers can be difficult to manage, or even get in the way of comprehension.

The distance between the centre of the Sun and the centre of the Earth is 149,597,871&nbsp;km, and from the Sun to the planet Neptune, is 4,495,100,000&nbsp;km. Rounding off the value to the nearest million kilometres gives you: Sun - Earth distance = 150 million km; Sun - Neptune distance = 4,495 million km. This is still quite awkward to use - inventing a new unit "million kilometre", which has been done with the little used [[Gigametre#SI_prefixed_forms_of_metre|Gigametre]].

The mass of the Earth is 5,973,600,000,000,000,000,000,000&nbsp;kg, and of Neptune is 102,430,000,000,000,000,000,000,000&nbsp;kg. Rounding off to the nearest million kilograms doesn't help much. Even using [[scientific notation]]: Earth mass = {{val|5.9736|e=24|u=kg}}; Neptune mass = {{val|1.0243|e=26|u=kg}}, can be confusing when different value exponents are used.

===Astronomical unit & Earth mass===
Astronomy decided on some comparative units of measurement eg. one based on the distance between the centre of the Sun and the centre of the Earth - an [[Astronomical Unit]], (AU), and one based on the mass of the Earth - an [[Earth mass]] (M<sub>🜨</sub>). This brings the value of the measurements into a much more human-comprehensible range

Using these units;
The Earth is 1&nbsp;AU from the Sun and has a mass of 1 M<sub>🜨</sub>
Neptune is 30&nbsp;AU from the Sun and has a mass of 17 M<sub>🜨</sub>

{| class="wikitable" border="1"
|+ Average Orbital Distance[http://nssdc.gsfc.nasa.gov/planetary/factsheet/]
! Planet !! [[kilometre]] !! [[Astronomical Unit]]
|-
| [[Mercury (planet)|Mercury]]|| {{val|5.791|e=6|u=km}}|| 0.387 AU
|-
| [[Venus]]||{{val|1.0821|e=8|u=km}}|| 0.723 AU
|-
| [[Earth]]|| {{val|1.49598|e=8|u=km}}|| 1.000 AU
|-
| [[Mars]]|| {{val|2.2792|e=8|u=km}}|| 1.524 AU
|-
| [[Jupiter]]|| {{val|7.7857|e=8|u=km}}|| 5.204 AU
|-
| [[Saturn]]|| {{val|1.43353|e=9|u=km}}|| 9.582 AU
|-
| [[Uranus]]|| {{val|2.87246|e=9|u=km}}|| 19.229 AU
|-
| [[Neptune]]|| {{val|4.49506|e=9|u=km}}|| 30.103 AU
|}

{| class="wikitable" border="1"
|+ Terrestrials Mass[http://nssdc.gsfc.nasa.gov/planetary/factsheet/]
! Planet !! [[kilogram]] !! [[Earth mass]]
|-
| [[Mercury (planet)|Mercury]]|| {{val|3.3022|e=23|u=kg}}|| 0.055 M<sub>🜨</sub>
|-
| [[Venus]]|| {{val|4.8685|e=24|u=kg}} || 0.815 M<sub>🜨</sub>
|-
| [[Earth]]|| {{val|5.9736|e=24|u=kg}} || 1.000 M<sub>🜨</sub>
|-
| [[Mars]]|| {{val|6.4185|e=23|u=kg}} || 0.107 M<sub>🜨</sub>
|}

The gas giant planets are substantially bigger than the other planets, so astronomy took the mass of the largest of these, Jupiter, and made a supplementary comparative unit of mass, calling it a [[Jupiter mass]], M<sub>J</sub>.

{| class="wikitable" border="1"
|+ Gas Giant Mass[http://nssdc.gsfc.nasa.gov/planetary/factsheet/]
! Planet !! [[kilogram]] !! [[Earth mass]] !! [[Jupiter mass]]
|-
| [[Jupiter]]|| {{val|1.8986|e=27|u=kg}}|| 317.8 M<sub>🜨</sub>|| 1.000 M<sub>J</sub>
|-
| [[Saturn]]|| {{val|5.6846|e=26|u=kg}}|| 95.159 M<sub>🜨</sub> || 0.299 M<sub>J</sub>
|-
| [[Uranus]]|| {{val|8.6832|e=25|u=kg}} || 14.536 M<sub>🜨</sub> || 0.046 M<sub>J</sub>
|-
| [[Neptune]]|| {{val|1.0243|e=26|u=kg}}|| 17.147 M<sub>🜨</sub> || 0.054 M<sub>J</sub>
|}

One [[light-year]], the best known unit of interstellar distance, is 63,241 AU[http://www.iau.org/public_press/themes/measuring/].
Sun mass = {{val|1.98892|e=30|ul=kg}} = 1048 M<sub>J</sub> = 332,946 M<sub>🜨</sub>.
[[Solar mass]] (M<sub>⊙</sub>) = the mass of the Sun, and is used in describing other stars.

==Heliosphere==
The Sun wafts a continual but fluctuating, low [[density]] wind of charged particles called the [[solar wind]] which has a fast and a slow component emerging from its visible surface ([[photosphere]]) and the top layer of its atmosphere ([[corona]]).

The [[heliosphere]] is created by the action of the solar wind blowing a bubble in the [[interstellar medium]], encapsulating much of the system.

==Components==
The Solar System is the [[Sun]] and the [[astronomical object|celestial objects]] bound to it by [[gravity]].

The 0.14% of the matter in the system not concentrated in the Sun has [[Self-organized criticality|self-organized]] into: eight spherical [[Accretion (astrophysics)|accretions]] called [[planets]], most with [[moons]]; a few, small, rounded bodies called [[dwarf planet]]s; and matter loosely described as rubble - [[small Solar System bodies]] including [[asteroids]] & [[comets]].

===The Sun===
The Sun determines the Solar System, it makes up 99.86% of the mass of the system<ref>{{cite web  |title=Solar Data Sheet |publisher= [[Space.com]] |url=http://www.space.com/scienceastronomy/solarsystem/sun-ez.html |accessdate=2009-08-18 }}</ref>, and the gravity of this mass dominates the rest.

By mass it is roughly 71% hydrogen, 27% helium and the remaining percentage of other elements, these being called "metals", which in astronomical terminology is any element more massive than helium.

{| class="wikitable" border="1"
|+ % of total mass<ref>{{cite web |title=Chemical Composition of Stars |publisher =[[NASA]] |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961112a.html }}</ref>
! Element!! Proportion
|-
| Hydrogen || 71.0
|-
| Helium || 27.1
|-
| Oxygen || 0.97
|-
| Carbon || 0.40
|-
| Nitrogen || 0.096
|-
| Silicon || 0.099
|-
| Magnesium || 0.076
|-
| Neon || 0.058
|-
| Iron || 0.014
|-
| Sulfur  || 0.040
|}

As well as generating heat and light from the nuclear fusion of hydrogen into helium at the core of the Sun, a continual but fluctuating, low density emission of charged particles called the solar wind "blows a bubble" called the heliosphere in the interstellar medium. This medium between the stars, most commonly thought of as outer space, is a very high vacuum but still has enough matter occupying it to interact with the high velocity (750 km/s) solar wind.

The huge mass of the Sun compresses the core to such an extent that the density and heat generated cause nuclear fusion of hydrogen into helium. The huge amounts of energy released further heats the core to 15 million Kelvin and radiates and convects out through the Sun's layers to be radiated off as heat, light and other [[electromagnetic radiation]] to outer space.

The extremely dense solar core holds about 50% of the sun's mass, but occupies only about 1.5% of its total volume<ref>{{cite web |title= Genesis : Search for Origins, JPL, NASA |publisher =[[NASA]] |url=http://genesismission.jpl.nasa.gov/science/mod3_SunlightSolarHeat/SolarStructure/index.html }}</ref>. Physical conditions inside the sun's core are extreme. The temperature is thought to be around 15 million degrees Kelvin, a temperature so extreme that atoms are stripped of their electrons. Thus the sun's core is a mixture of protons, neutrons, nuclei, and free electrons. The radiative layer extends from the core about 70% of the way up to the surface.

===Planetary system===
{{seealso|Definition of planet}}
A [[planetary system]] consists of the various objects [[orbit]]ing a [[star]] such as [[planet]]s, [[dwarf planet]]s, [[natural satellite|moons]], [[asteroid]]s, [[meteoroid]]s, [[comet]]s, and [[cosmic dust]].

Objects [[orbit]]ing the Sun directly are divided into three classes: planets, dwarf planets, and small Solar System bodies.

====Planet====
On 24<sup>th</sup> August 2006 the [[International Astronomical Union]] (IAU), prompted by the discovery of [[Eris (dwarf planet)|Eris]] and subsequent discussions over its classification, defined [[2006 definition of planet|the term planet]] for the first time as; a body in orbit around the Sun that has enough [[mass]] to form itself into a [[sphere|spherical]] shape and has [[Cleared the neighbourhood|cleared its immediate neighbourhood]] of all smaller objects.<ref name="IAUPlanet">
{{cite web
|title=IAU 2006 General Assembly Result of the IAU Resolution votes
|publisher=IAU
|date=2006-08-24
|url=http://www.iau.org/public_press/news/release/iau0603/ 
|accessdate=2009-05-22}}</ref>
By this definition, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

====Dwarf planet====
At the same meeting the [[International Astronomical Union|IAU]] defined dwarf planet as a [[celestial body]] [[orbit]]ing the [[Sun]] that is massive enough to be rounded by its own [[gravity]] but which has not [[cleared the neighbourhood|cleared its neighbouring region]] of [[planetesimal]]s and is not a [[natural satellite|satellite]].<ref name="IAUPlanet"></ref> By this definition, the Solar System has five [[dwarf planet]]s: [[Ceres (dwarf planet)|Ceres]], [[Pluto]], [[Haumea (dwarf planet)|Haumea]], [[Makemake (dwarf planet)|Makemake]], and [[Eris (dwarf planet)|Eris]]. Dwarf planets that orbit in the trans-Neptune region are called "[[plutoid]]s".<ref name="IAU0804">{{cite web
|date=Jun 11, 2008, Paris
|title=Plutoid chosen as name for Solar System objects like Pluto
|publisher=[[International Astronomical Union]] (News Release - IAU0804)
|url=http://www.iau.org/public_press/news/release/iau0804
|accessdate=2008-06-11}}</ref>
Other objects that may become classified as dwarf planets are [[90377 Sedna|Sedna]], [[90482 Orcus|Orcus]], and [[50000 Quaoar|Quaoar]].

====Natural satellites====
[[Natural satellite]]s or [[moons]], are those objects in orbit around planets, dwarf planets and [[small Solar System body|small Solar System bodies]], rather than the Sun itself.

====Small Solar System bodies====
The remainder of the objects in orbit around the Sun are [[small Solar System body|small Solar System bodies]].<ref name="IAUPlanet"></ref>

=====Asteroids=====
[[Asteroids]] are [[small Solar System bodies]] composed mainly of rocky, but sometimes metallic, minerals.

=====Comets=====
[[Comets]] are often described as dirty snowballs, they are made up of rocky material mixed up with a large proportion of icy material. When their orbit takes them closer to the Sun they generate tails, or [[Coma (cometary)|comas]], by the action of the heat of the Sun boiling off the volatile icy material within. This tail always points away from the Sun due to the action of the [[solar wind]], even when the comet is travelling away from the Sun.

====Gas, ice & rock====
These everyday terms are used a little differently in [[planetary science]]. 

'''Gas''' is used to refer to hydrogen or helium.

On Earth, [[Ice|'''ice''']] is a word intrinsically bound with the substance water; the only common exception is the term [[dry ice]], which describes frozen carbon dioxide. However in planetary science, with the exception of hydrogen and helium, '''ice''', '''ices''' and '''[[volatiles]]''' refer to any substances with low boiling points which on Earth could only be liquefied or frozen by industrial processes. Regardless of the [[State_of_matter|state]] of the substance [[in situ]] ie. solid, liquid or vapour, the terms are still applied. Among the more common ices are water, ammonia, methane, carbon dioxide and nitrogen, as well as other [[organic compounds]].

'''Rock''' refers to [[silicate]]s, which form the vast bulk of a [[terrestrial planet]], and has a high boiling point.

===The Eight Planets===
All the planets except Earth are named after deities from [[Greek mythology|Greco]]-[[Roman mythology|Roman]] [[mythology]].

====Inner planets====
{{main|Terrestrial planet}}
[[Image:Terrestrial planet size comparisons.jpg|thumb|The inner planets. From left to right: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]] (sizes to scale)]]

The four inner or terrestrial planets all have roughly the same structure: a central metallic [[planetary core|core]], mostly [[iron]], plus [[nickel]], surrounded by a silicate [[Mantle (geology)|mantle]] in turn covered by a relatively thin  [[crust (geology)|crust]]. They have [[canyon]]s, [[impact craters]], [[mountain]]s, and [[tectonics|tectonic]] surface features such as [[rift valley]]s and [[volcano]]es. Three of the four (Venus, Earth and Mars) possess [[secondary atmosphere]]s — atmospheres generated through internal volcanism or comet impacts, as opposed to [[primary atmosphere]]s — atmospheres captured directly from the original [[solar nebula]].

Mercury and Venus are termed ''[[inferior planet]]s'', because their orbits are inside that of the Earth, while the other planets are ''[[superior planet]]s'' as they orbit outside Earth's orbit.

=====Mercury=====
: [[Mercury (planet)|Mercury]] (0.4&nbsp;AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or [[rupes]], probably produced by a period of contraction early in its history.<ref>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</ref> Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.<ref>{{cite web |year=2006 |author=Bill Arnett |title=Mercury |work=The Nine Planets |url=http://www.nineplanets.org/mercury.html |accessdate=2006-09-14}}</ref> 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.<ref>Benz, W., Slattery, W. L., Cameron, A. G. W. (1988), ''Collisional stripping of Mercury's mantle'', Icarus, v. 74, p. 516–528.</ref><ref>Cameron, A. G. W. (1985), ''The partial volatilization of Mercury'', Icarus, v. 64, p. 285–294.</ref>

=====Venus=====
: [[Venus]] (0.7&nbsp;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 [[Celsius|°C]], most likely due to the amount of [[greenhouse gas]]es in the atmosphere.<ref>{{cite document |author=Mark Alan Bullock |title=The Stability of Climate on Venus |publisher=Southwest Research Institute |year=1997 |url=http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |accessdate=2006-12-26 }}</ref> 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.<ref>{{cite web |year=1999 |author=Paul Rincon |title=Climate Change as a Regulator of Tectonics on Venus |work=Johnson Space Center Houston, TX, Institute of Meteoritics, University of New Mexico, Albuquerque, NM |url=http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |accessdate=2006-11-19}}</ref>

=====Earth=====
: [[Earth]] (1&nbsp;AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the [[universe]] where [[life]] is known to 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]].<ref>{{cite web |title=Earth's Atmosphere: Composition and Structure |author=Anne E. Egger, M.A./M.S. |work=VisionLearning.com |url=http://www.visionlearning.com/library/module_viewer.php?c3=&mid=107&l=|accessdate=2006-12-26}}</ref> It has one natural satellite, the Moon (Latin: Luna), the only large satellite of a terrestrial planet in the Solar System.

=====Mars=====
: [[Mars]] (1.5&nbsp;AU) is smaller than Earth or 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 recent epochs. Its red colour comes from rust in its iron-rich soil.<ref>{{cite web |year=2004 |title=Modern Martian Marvels: Volcanoes? |author=David Noever |work=NASA Astrobiology Magazine |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1360&mode=thread&order=0&thold=0 |accessdate=2006-07-23}}</ref> Mars has two tiny natural satellites ([[Deimos (moon)|Deimos]] and [[Phobos (moon)|Phobos]]) thought to be captured [[asteroid]]s.<ref>{{cite web |year=2004 |title=A Survey for Outer Satellites of Mars: Limits to Completeness |author=Scott S. Sheppard, David Jewitt, and Jan Kleyna |work=The Astronomical Journal |url=http://www.iop.org/EJ/article/1538-3881/128/5/2542/204263.html |accessdate=2006-12-26}}</ref>

=====Moons=====
[[Mercury (planet)|Mercury]] and [[Venus]] are the only planets without any [[moons]]. [[Mars]] has two small, rocky moons, while the other four, outer planets all have many rocky and icy moons each.

====Outer planets====
{{main|Gas giant}}
[[Image:Gas giants in the solar system.jpg|thumb|(not to scale) 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.{{Ref label|C|c|none}} Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, “ice giants.”<ref>{{cite web |title=Formation of Giant Planets |author=Jack J. Lissauer, David J. Stevenson |work=NASA Ames Research Center; California Institute of Technology |year=2006 |url=http://www.gps.caltech.edu/uploads/File/People/djs/lissauer&stevenson(PPV).pdf|accessdate=2006-01-16}}</ref> All four gas giants have [[Planetary ring|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&nbsp;AU), at 318 Earth masses, is 2.5 times all the mass of all the other planets put together. It is composed largely of [[hydrogen]] and [[helium]]. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the [[Great Red Spot]]. Jupiter has [[Moons of Jupiter|sixty-three known satellites]]. The four largest, [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]], show similarities to the terrestrial planets, such as volcanism and internal heating.<ref>{{cite web |title=Geology of the Icy Galilean Satellites: A Framework for Compositional Studies |author=Pappalardo, R T |work=Brown University |year=1999 |url=http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |accessdate=2006-01-16}}</ref> Ganymede, the largest satellite in the Solar System, is larger than Mercury.

=====Saturn=====
: [[Saturn]] (9.5&nbsp;AU), distinguished by its extensive [[Rings of Saturn|ring system]], has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. Saturn has [[Moons of Saturn|sixty known satellites]] (and three unconfirmed); two of which, [[Titan (moon)|Titan]] and [[Enceladus (moon)|Enceladus]], show signs of geological activity, though they are largely [[Cryovolcano|made of ice]].<ref>{{cite web |title=Cryovolcanism on the icy satellites |author=J. S. Kargel |work=U.S. Geological Survey |year=1994 |url=http://www.springerlink.com/content/n7435h4506788p22/ |accessdate=2006-01-16}}</ref> Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

=====Uranus=====
: [[Uranus]] (19.6&nbsp;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.<ref>{{cite journal |title=10 Mysteries of the Solar System |author=Hawksett, David; Longstaff, Alan; Cooper, Keith; Clark, Stuart |journal=Astronomy Now |year=2005 |volume=19 |issue=8 |page=65 |bibcode=2005AsNow..19h..65H |url=http://adsabs.harvard.edu/abs/2005AsNow..19h..65H |accessdate=2006-01-16}}</ref> Uranus has [[Moons of Uranus|twenty-seven known satellites]], the largest ones being [[Titania (moon)|Titania]], [[Oberon (moon)|Oberon]], [[Umbriel (moon)|Umbriel]], [[Ariel (moon)|Ariel]] and [[Miranda (moon)|Miranda]].

=====Neptune=====
: [[Neptune]] (30&nbsp;AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more [[Density|dense]]. It radiates more internal heat, but not as much as Jupiter or Saturn.<ref>{{cite journal |title=Post Voyager comparisons of the interiors of Uranus and Neptune |author=Podolak, M.; Reynolds, R. T.; Young, R. |journal=NASA, Ames Research Center |year=1990 |volume=17 |issue=10 |page=1737 |doi=10.1029/GL017i010p01737 |bibcode=1990GeoRL..17.1737P |url=http://adsabs.harvard.edu/abs/1990GeoRL..17.1737P |accessdate=2006-01-16}}</ref> Neptune has [[Moons of Neptune|thirteen known satellites]]. The largest, [[Triton (moon)|Triton]], is geologically active, with [[geyser]]s of [[liquid nitrogen]].<ref>{{cite web |title=The Plausibility of Boiling Geysers on Triton |author=Duxbury, N.S., Brown, R.H. |work=Beacon eSpace |year=1995 |url=http://trs-new.jpl.nasa.gov/dspace/handle/2014/28034?mode=full |accessdate=2006-01-16 }}</ref> Triton is the only large satellite with a [[retrograde and direct motion|retrograde]] orbit. Neptune is accompanied in its orbit by a number of [[minor planet]]s, termed [[Neptune Trojan]]s, that are in 1:1 [[Orbital resonance|resonance]] with it.


As of September 2008 there are '''167 planetary moons''' in the Solar System.

===Dwarf Planets===
[[Pluto]] had been called a planet since it was discovered in 1930, but in 2006 [[astronomer]]s meeting at the [[International Astronomical Union]] decided for the first time on the definition of a planet, and Pluto didn't fit. Instead they defined a new category of [[dwarf planet]], into which Pluto did fit along with some other objects.

Pluto is now one of five dwarf planets, here they are in order of their distance from the Sun, but not where they orbit in relation to any of the planets.

* [[Ceres]]
* [[Pluto]]
* [[Haumea (dwarf planet)|Haumea]]
* [[Eris (dwarf planet)|Eris]]
* [[Makemake (dwarf planet)|Makemake]]

Astronomers expect to find more dwarf planets, and perhaps re-categorize known objects as dwarf planets.

There are '''6 moons''' orbiting three of the five dwarf planets.

==Notes==
<references/>

{{Solar system table}}
{{Earth's location}}
{{Systems}}

<nowiki>
[[Category:Solar System]]
[[Category:Planetary science]]
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[[Category:Planetary systems]]
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