User:HarryAlffa/Solar System synopsis

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Sun, Planets and dwarf planets. Spheres are to scale, distances between are not.

The Solar System (or solar system) is the Sun, the 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 life.

The Sun determines the Solar System, it makes up 99.86% of the mass of the system[1], 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 distances from it, but on roughly the same 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 plane of the ecliptic. No object orbits the Sun in a perfect circle, their paths are better described as oval shaped, or elliptical orbits, some more elliptical than others.

The 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:

Incomplete, not to scale - illustration only, of plane of the ecliptic and regions (zones).
  • Inner Solar System
  • Outer Solar System

The asteroid belt contains rocky & metallic small Solar System bodies called asteroids; plus one of the dwarf planets - the others populate the 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 reclassification of Pluto from a planet to a dwarf planet.

Did you know?

Mercury & Venus are the only two planets without orbiting natural satellites, or "moons".

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

The gas giants are each encircled by planetary rings 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.

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.


The Solar System is the Sun and those celestial objects bound to it by gravity.


The hub in the Solar System's structure is the Sun, which determines the system and makes up 99.86% of its mass[2]. 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 elliptical orbits, at different distances from it, but on roughly the same plane, so that if you could look at the Solar System "edge on" then all the planets would roughly be in this ecliptic plane around the Sun.

Travelling out from the Sun, the ecliptic plane can be split into regions;

Incomplete, not to scale - illustration only, of ecliptic plane and regions (zones).
  • Inner Solar System
  • Outer Solar System

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-Neptune region, or "third zone", is now a region equal in the hierarchy to the inner and outer Solar System regions.[3]

  • Inner Solar System
  • Terrestrial planets, Asteroid belt
  • Outer Solar System
  • Gas Giants

Ecliptic plane & orbital inclination[edit]

Not to scale - illustration only. Notice the tilt of the closest orbit - Mercury, the most inclined orbit.

The 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 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[edit]

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 plane. The jauntiness of the angle to the ecliptic plane is the degree the planet's orbit is inclined, with all the planet's orbits inclined to this plane, except the Earth's because that defines it.

Thickness of ecliptic plane[edit]

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[edit]

The orbits of objects around the Sun are accurately described as elliptical orbits. The eccentricity of a perfect circle is zero, the eccentricity of an 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.


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, in either its orbit of the Sun or other primary - which is rare, or its own rotation - which is uncommon.

Rounded body axial tilt[edit]

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.


The ecliptic plane can be split into regions and sub-regions. Their structure, and defining components, in order of their distances from the Sun are:

Incomplete, not to scale - illustration only, of ecliptic plane and regions (zones).
  • Inner Solar System
  • Outer Solar System

The asteroid belt contains asteroids; plus one of the dwarf planets - the others populate the 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 planets are the only category to populate more than one region.

Inner Solar System[edit]

The inner Solar System is the terrestrial planets and asteroid belt, and is sometimes called the terrestrial region[4] 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 disks[5].

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[6]. In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,[7] preventing the formation of planet-sized bodies.

Outer Solar System[edit]

Sometimes the Outer Solar System is taken to mean; the Gas Giant Planets, the Kuiper Belt, and the Scattered Disk, but with more recent discoveries of objects in the Kuiper Belt and the Scattered Disk, and with more dwarf planet discoveries, the Outer Solar System is now usually synonymous with just the Gas Giants.

Trans-Neptune region[edit]

With the discovery of more dwarf planets and the realisation of the extend of the Kuiper Belt, and the Scattered Disk, the trans-Neptune region has been recognised.

Note that the dwarf planets 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[edit]

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 km, and from the Sun to the planet Neptune, is 4,495,100,000 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.

The mass of the Earth is 5,973,600,000,000,000,000,000,000 kg, and of Neptune is 102,430,000,000,000,000,000,000,000 kg. Rounding off to the nearest million kilograms doesn't help much. Even using scientific notation: Earth mass = 5.9736×1024 kg; Neptune mass = 1.0243×1026 kg, can be confusing when different value exponents are used.

Astronomical unit & Earth mass[edit]

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). This brings the value of the measurements into a much more human-comprehensible range

Using these units; The Earth is 1 AU from the Sun and has a mass of 1 M Neptune is 30 AU from the Sun and has a mass of 17 M

Average Orbital Distance[1]
Planet kilometre Astronomical Unit
Mercury 5.791×106 km 0.387 AU
Venus 1.0821×108 km 0.723 AU
Earth 1.49598×108 km 1.000 AU
Mars 2.2792×108 km 1.524 AU
Jupiter 7.7857×108 km 5.204 AU
Saturn 1.43353×109 km 9.582 AU
Uranus 2.87246×109 km 19.229 AU
Neptune 4.49506×109 km 30.103 AU
Terrestrials Mass[2]
Planet kilogram Earth mass
Mercury 3.3022×1023 kg 0.055 M
Venus 4.8685×1024 kg 0.815 M
Earth 5.9736×1024 kg 1.000 M
Mars 6.4185×1023 kg 0.107 M

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, MJ.

Gas Giant Mass[3]
Planet kilogram Earth mass Jupiter mass
Jupiter 1.8986×1027 kg 317.8 M 1.000 MJ
Saturn 5.6846×1026 kg 95.159 M 0.299 MJ
Uranus 8.6832×1025 kg 14.536 M 0.046 MJ
Neptune 1.0243×1026 kg 17.147 M 0.054 MJ

One light-year, the best known unit of interstellar distance, is 63,241 AU[4]. Sun mass = 1.98892×1030 kg = 1048 MJ = 332,946 M. Solar mass (M) = the mass of the Sun, and is used in describing other stars.


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.


The Solar System is the Sun and the celestial objects bound to it by gravity.

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

The Sun[edit]

The Sun determines the Solar System, it makes up 99.86% of the mass of the system[8], 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.

 % of total mass[9]
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[10]. 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[edit]

A planetary system consists of the various objects orbiting a star such as planets, dwarf planets, moons, asteroids, meteoroids, comets, and cosmic dust.

Objects orbiting the Sun directly are divided into three classes: planets, dwarf planets, and small Solar System bodies.


On 24th August 2006 the International Astronomical Union (IAU), prompted by the discovery of Eris and subsequent discussions over its classification, defined the term planet for the first time as; a body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighbourhood of all smaller objects.[11] By this definition, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet[edit]

At the same meeting the IAU defined dwarf planet as a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite.[11] By this definition, the Solar System has five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Dwarf planets that orbit in the trans-Neptune region are called "plutoids".[12] Other objects that may become classified as dwarf planets are Sedna, Orcus, and Quaoar.

Natural satellites[edit]

Natural satellites or moons, are those objects in orbit around planets, dwarf planets and small Solar System bodies, rather than the Sun itself.

Small Solar System bodies[edit]

The remainder of the objects in orbit around the Sun are small Solar System bodies.[11]


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


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 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[edit]

These everyday terms are used a little differently in planetary science.

Gas is used to refer to hydrogen or helium.

On Earth, 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 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 silicates, which form the vast bulk of a terrestrial planet, and has a high boiling point.

The Eight Planets[edit]

All the planets except Earth are named after deities from Greco-Roman mythology.

Inner planets[edit]

The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)

The four inner or terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, plus nickel, surrounded by a silicate mantle in turn covered by a relatively thin crust. They have canyons, impact craters, mountains, and tectonic surface features such as rift valleys and volcanoes. Three of the four (Venus, Earth and Mars) possess secondary atmospheres — atmospheres generated through internal volcanism or comet impacts, as opposed to primary atmospheres — atmospheres captured directly from the original solar nebula.

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

Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.[13] Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.[14] 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.[15][16]
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.[17] 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.[18]
Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the universe where life is known to 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.[19] It has one natural satellite, the Moon (Latin: Luna), the only large satellite of a terrestrial planet in the Solar System.
Mars (1.5 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.[20] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.[21]

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[edit]

(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.[c] 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.”[22] 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 (5.2 AU), at 318 Earth masses, is 2.5 times all the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has sixty-three known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.[23] Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. Saturn has sixty known satellites (and three unconfirmed); two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.[24] Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.[25] Uranus has twenty-seven known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.
Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.[26] Neptune has thirteen known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[27] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.

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

Dwarf Planets[edit]

Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers 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.

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.


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[[Category:Solar System]] [[Category:Planetary science]] [[Category:Space science]] [[Category:Planetary systems]]