Solar system basic

The Solar System is the astronomical name for the Sun, the Earth and the Moon, and the rest of the planetary system.
The Sun defines the solar system, it makes up 98.6 percent of the mass of the system, and the gravity of this mass dominates the rest. 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 "breeze" 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 Earth's orbit around the Sun is nearly a perfect circle, but when mapped it is found that the Earth moves around the Sun in a very slightly oval shaped, or elliptical orbit. Everything in the Solar System has an elliptical orbit, with some more elliptical than others.
The Eight Planets
In order of their distances from the Sun, the eight planets are:
* Mercury
* Venus
* Earth
* Mars
* Jupiter
* Saturn
* Uranus
* Neptune
The planets are the biggest objects that orbit the Sun. Astronomers think they are unlikely to find new planets in our solar system, but more small objects are found here every year.
Moons
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.
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 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.
* Ceres
* Pluto
* Haumea
* Eris
* 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.
Rocky and icy small Solar System bodies
All the other orbiting objects which can't be defined as a planet or a dwarf planet are called small Solar System bodies.
Gas, ice & rock
These everyday terms are used a little differently in planetary science.
Gas
Gas is used to refer to hydrogen or helium.
Ice, ices and volatiles
On Earth, 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
Rock refers to silicates, which form the vast bulk of a terrestrial planet, and have a high boiling point.
Asteroids
Asteroids are mostly small Solar System bodies (objects which can't be described as a planet or a dwarf planet) composed mainly of rocky, but sometimes metallic, minerals.
Comets
These 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.
Structure
The Solar System's structure can be described by a number of characteristics, including; informal regions, orbital characteristics of the planets, and the action of the solar wind.
Regions
Here they are with their constituents in order from the Sun, including the planets, and the dwarf planets in their proper order.
* <big>Terrestrial Planets</big> region contains the four rocky planets closest to the sun;
:* Mercury
:* Venus
:* Earth
:* Mars
* <big>Asteroid Belt</big> region contains;
:* Ceres (the only dwarf planet in this region)
:* Asteroids
* <big>Gas Giant Planets</big> region contains;
:* Jupiter
:* Saturn
:* Uranus
:* Neptune
* <big>Kuiper Belt</big> region contains;
:* Pluto
:* Haumea
:* Makemake
:* Kuiper Belt Objects and possibly other dwarf planets
:* short-period Comets
* <big>Scattered Disk</big> region contains;
:* Eris
:* Scattered Disk Objects and possibly other dwarf planets
* <big>Oort Cloud</big> region contains
:* long-period Comets
Note that the dwarf planets are the only category to populate more than one region.
Alternative region-names
Sometimes people use alternative names for similar regions defined above. These are even less formal, and less well defined.
* <big>Inner solar system</big> contains
:* Terrestrial Planets
:* Asteroid Belt
* <big>Outer solar system</big> contains
:* Gas Giant Planets
* <big>Trans-Neptune region</big> contains
:* Kuiper Belt
:* Scattered Disk
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 defined as above.
The Oort cloud is separate from the Trans-Neptune region, and much further out.
Orbital Characteristics

Elliptic orbit
Nothing orbits the Sun in a perfect circle, this imperfection is described as an oval shaped, or elliptical orbit. Everything in the Solar System has an elliptical orbit, with Mercury the most elliptical of the planets, while some of the small Solar System bodies have very eccentric orbits indeed.
Plane of the ecliptic
The Earth's orbit defines the plane of the ecliptic. All of the planets orbit the Sun roughly around this plane. If you could look at the solar system "edge on" then all the planets would be roughly in a horizontal plane around a centre line of the Sun, but not around the Sun's equator. The further "up" or "down" from this plane a planet orbits, the more inclined is it's orbit said to be. Mercury also has the most inclined orbit of the planets, and the small Solar System bodies can be very highly inclined as well as eccentric.

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